tag:theconversation.com,2011:/uk/topics/white-dwarf-14827/articleswhite dwarf – The Conversation2022-12-08T19:24:10Ztag:theconversation.com,2011:article/1960522022-12-08T19:24:10Z2022-12-08T19:24:10Z‘Extreme stripping action’ led to the messy birth of the Southern Ring Nebula, Webb image reveals<figure><img src="https://images.theconversation.com/files/499746/original/file-20221208-22-s12rbl.png?ixlib=rb-1.1.0&rect=7%2C71%2C4764%2C3620&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">NASA</span></span></figcaption></figure><p>When the first five images from the James Webb Space Telescope (JWST) were unveiled, one of them stared at me with two eyes. It was an image of the Southern Ring Nebula, NGC3132, and smack in the middle were two bright stars.</p>
<p>Now, the fact that NGC3132 houses a binary star system (two stars orbiting one another) has been known since the days of the Hubble Space Telescope. </p>
<p>But in those early images, the central star that ejected the nebula – a tiny, hot white dwarf – was so dim it was almost invisible next to its bright Sun-like companion. In effect, the nebula had one eye almost closed. </p>
<p>But the JWST reveals more than Hubble did. It can collect “cooler” photons (light particles) in the infrared range of the electromagnetic spectrum. In this cooler light, we saw both stars in the binary system shining as bright as one another: two glaring eyes! </p>
<p>This was surprising to any astronomer who understands this type of nebula; super-hot white dwarfs typically don’t shine brightly in infrared light. It made sense for the cooler star to be shining this way, but observing the same brilliance from its partner was unexpected.</p>
<p>Emails started to bolt coast to coast and across oceans as astronomers pieced the puzzle together. The central white dwarf star of NGC3132, they realised, is enshrouded in dust. The dust is warmed up by the star’s heat and therefore shines in the infrared, producing the light we observed. </p>
<p>It was this that led us on the trail to find out what was really happening in the Southern Ring Nebula. Our findings from a team of nearly 70 astronomers are <a href="https://www.nature.com/articles/s41550-022-01845-2">published today</a> in Nature Astronomy. </p>
<h2>At the heart, a hot white dwarf</h2>
<p>The Southern Ring Nebula is a planetary nebula. That means it’s a gaseous nebula formed by a Sun-like star shedding most of its gas in the last act before its demise. </p>
<p>Once it shed much of its mass, the star became a hot white dwarf. This central star now sits in the middle of the nebula, cooling like a stellar ember, effectively dying. </p>
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<a href="https://images.theconversation.com/files/499740/original/file-20221208-11-k8glwk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/499740/original/file-20221208-11-k8glwk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/499740/original/file-20221208-11-k8glwk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=534&fit=crop&dpr=1 600w, https://images.theconversation.com/files/499740/original/file-20221208-11-k8glwk.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=534&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/499740/original/file-20221208-11-k8glwk.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=534&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/499740/original/file-20221208-11-k8glwk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=671&fit=crop&dpr=1 754w, https://images.theconversation.com/files/499740/original/file-20221208-11-k8glwk.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=671&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/499740/original/file-20221208-11-k8glwk.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=671&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">You can see the two central stars quite clearly in this image, the dust-enshrouded white dwarf in the red and its companion to its left.</span>
<span class="attribution"><span class="source">NASA</span>, <span class="license">Author provided</span></span>
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<p>The beauty of planetary nebulae is they can be looked at forensically: parts of the nebula farther from the middle were ejected earlier in time. In this way, the entire nebula functions a bit like a geological record.</p>
<p>With its dying white dwarf in the centre, our group approached NGC3132 like a crime scene.</p>
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<h2>Two unknown suspects emerge</h2>
<p>First, we quickly realised the dust making the central star shine so brightly was actually a disk wrapped closely around the central star that must have been forged by a companion. This orbiting companion star would have stripped gas away from the central star, hastening its demise. </p>
<p>We didn’t spot the companion, though. We think it’s either too faint to detect, or has potentially perished in the interaction and merged with the central star. </p>
<p>Then we noticed something else: broken concentric arches engraved in the extended halo of the nebula. These also betrayed the presence of an orbiting companion. Could this culprit be the same one that forged the disk of dust?</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/499743/original/file-20221208-24-8f4ak2.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/499743/original/file-20221208-24-8f4ak2.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/499743/original/file-20221208-24-8f4ak2.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=591&fit=crop&dpr=1 600w, https://images.theconversation.com/files/499743/original/file-20221208-24-8f4ak2.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=591&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/499743/original/file-20221208-24-8f4ak2.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=591&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/499743/original/file-20221208-24-8f4ak2.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=743&fit=crop&dpr=1 754w, https://images.theconversation.com/files/499743/original/file-20221208-24-8f4ak2.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=743&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/499743/original/file-20221208-24-8f4ak2.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=743&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Note the concentric arches on the edges of the nebula.</span>
<span class="attribution"><span class="source">NASA</span>, <span class="license">Author provided</span></span>
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<p>We don’t think so. Although the arches have suffered some “weathering”, our measurements of them betray the presence of yet another companion star. This one is placed a little too far from the central star to have created the dust disk. </p>
<p>And just like that, we had gathered evidence the Southern Ring Nebula contains not just two stars in a binary system, but four. </p>
<p>And we would gather more yet. </p>
<h2>Tied up in a bumpy, gassy bubble</h2>
<p>The “ring” that gives the nebula its name is actually the wall of an egg-shaped bubble containing hot gas, heated by the central star. This wall is marked with noticeable protuberances.</p>
<p>Combining the JWST image with data from the European Southern Observatory, our team created a 3D model that revealed these protuberances come in pairs, moving in opposite directions away from the central star. </p>
<p>One possible explanation is the interaction that created the dust disk didn’t involve just one close companion, but two. In other words, we’re looking at a potential fifth star in the mix – interacting chaotically with the central star to blow out jets that push out those protuberances.</p>
<p>This fifth star’s presence is still tentative. But we can say with a good degree of certainty the stellar system that created the Southern Ring Nebula comprises not just the binary star system (the two eyes of the nebula), but also a third star that ripped away the gas to form the disk, and another that inscribed a track of concentric arches in the gas bubble. </p>
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<img alt="" src="https://images.theconversation.com/files/499742/original/file-20221208-12-q7hy77.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/499742/original/file-20221208-12-q7hy77.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=325&fit=crop&dpr=1 600w, https://images.theconversation.com/files/499742/original/file-20221208-12-q7hy77.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=325&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/499742/original/file-20221208-12-q7hy77.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=325&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/499742/original/file-20221208-12-q7hy77.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=409&fit=crop&dpr=1 754w, https://images.theconversation.com/files/499742/original/file-20221208-12-q7hy77.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=409&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/499742/original/file-20221208-12-q7hy77.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=409&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">The panels above are a time-lapse cartoon of the formation of NGC3132. Star 1 is the central star (shown in the first panel before it becomes a white dwarf). Star 2 is the distant bystander companion. Star 3 and 4 are responsible for emitting jets that created protuberances in the shape of the gas bubble, and form the dusty disk around the white dwarf (shown in panel 4). Star 5 gave rise to the formation of the concentric arches.</span>
<span class="attribution"><span class="source">NASA, ESA, CSA, E. Wheatley (STScI)</span>, <span class="license">Author provided</span></span>
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<p>As for the second eye of the nebula – the one we’d always known about – it was definitely an innocent bystander. It’s too far from the central star to have participated in its demise.</p>
<h2>One case closed, more to come</h2>
<p>The case of the Southern Ring Nebula isn’t the only one demonstrating how stars work in packs. Much of stellar astrophysics is being revisited today in light of the realisation of just how gregarious stars can be. And we’re all the more excited for it. </p>
<p>A wealth of phenomena arise from stellar interactions, from supernova explosions, to the merging of black holes and neutron stars giving rise to gravitational wave events. </p>
<p>As the JWST delivers more detailed images of the universe, astronomers will be keenly dusting off their gloves to tackle more mysteries.</p>
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Read more:
<a href="https://theconversation.com/the-james-webb-space-telescope-has-taken-its-first-aligned-image-of-a-star-heres-how-it-was-done-178315">The James Webb Space Telescope has taken its first aligned image of a star. Here's how it was done</a>
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<p class="fine-print"><em><span>Orsola De Marco is affiliated with Astronomy Australia Limited (non-executive board director and chair of the board). This is a non-for-profit company that applies for and administers NCRIS grants to Australian Astronomy.</span></em></p>Astronomers have unpacked the mystery of how one star’s death created the nebula NGC3132 – never before seen in such detail.Orsola De Marco, Professor of Astrophysics, Macquarie UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1696312021-10-13T19:12:52Z2021-10-13T19:12:52ZA distant dead star shows a glimpse of our Solar System’s future<figure><img src="https://images.theconversation.com/files/425917/original/file-20211012-27-16co7nu.jpg?ixlib=rb-1.1.0&rect=0%2C5%2C3952%2C2419&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Artist's rendition of the Jupiter-like planet and its white dwarf star</span> <span class="attribution"><span class="source">W. M. Keck Observatory/Adam Makarenko</span></span></figcaption></figure><p>The golden age of discovery of planets around other stars (known as exoplanets) began in 1995. Since the first discoveries, more than 4,500 worlds have been found, most of them orbiting ordinary stars like our Sun. </p>
<p>The Sun is about 4.6 billion years old, and Earth and all the other planets formed at about the same time. But what will happen to the planets in another 5 billion years, when the Sun eventually dies? </p>
<p>In <a href="https://www.nature.com/articles/s41586-021-03869-6">a new study published in Nature</a>, we show a glimpse of the possible future of our Solar System, when the Sun burns through all its hydrogen fuel and becomes a dead star called a white dwarf.</p>
<p>This possible future is depicted in the form of a white dwarf thousands of light years away, which hosts a gas giant planet on a similar orbit to Jupiter, between 2.5 and 6 times as far from its star as Earth is from the Sun.</p>
<h2>Magnifying gravity</h2>
<p>The journey to this discovery began in 2010, when the white dwarf and its Jupiter-like companion aligned perfectly with a much more distant star in the dense star fields at the centre of the Milky Way. </p>
<p>The gravity of the white dwarf and its companion acted like a magnifying glass, bending the light from the distant star and making it appear brighter to observers here on Earth. This effect, known as “gravitational microlensing”, was predicted by Einstein in 1936.</p>
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Read more:
<a href="https://theconversation.com/how-we-found-a-white-dwarf-a-stellar-corpse-by-accident-114089">How we found a white dwarf – a stellar corpse – by accident</a>
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<p>While the background star was magnified, the small scale of this chance event meant we could not distinguish between the star in the foreground and the star in the background, let alone the planet.</p>
<p>But details in how the magnification of the background star changes over time can be used to reveal properties of the closer star and its planet. So an international team of astronomers led by those from the University of Tasmania and NASA Goddard headed to Hawai’i to use one of the largest telescopes in the world for a better look.</p>
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<img alt="" src="https://images.theconversation.com/files/425925/original/file-20211012-25-6hym0o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/425925/original/file-20211012-25-6hym0o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/425925/original/file-20211012-25-6hym0o.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/425925/original/file-20211012-25-6hym0o.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/425925/original/file-20211012-25-6hym0o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/425925/original/file-20211012-25-6hym0o.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/425925/original/file-20211012-25-6hym0o.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">The twin Keck telescopes of Mauna Kea, Hawai'i.</span>
<span class="attribution"><span class="source">Joshua Blackman</span></span>
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<p>The Keck-II telescope atop the dormant Mauna Kea volcano has a 10-metre interlocking array of hexagonal mirrors and “laser-guided adaptive optics” to filter out “twinkling” caused by changes in the atmosphere. We used it to obtain extremely high-resolution images of both the background and foreground star.</p>
<p>To our surprise, however, we could not see the foreground star at all. Predictions from the original magnification event in 2010 indicated that this star, weighing about half as much as the Sun, should be visible. But we could not detect it.</p>
<p>After a few years grappling with our data to ensure we weren’t making a mistake, we realised we could not see the star because it is a white dwarf, which in this case was too faint to detect.</p>
<h2>Dead stars</h2>
<p>White dwarfs are Earth-sized remnants of ordinary stars like our Sun. About 95% of the stars in the Milky Way will eventually become white dwarfs.</p>
<p>In about 5 billion years’ time, when the Sun burns through all its hydrogen fuel, it will balloon in size to become a red giant, likely obliterating Mercury and Venus in the process. Earth may also be destroyed, or at least severely disrupted; if by some miracle humankind still exists by then, our distant descendants will have to move off-world to survive.</p>
<p>In the red giant phase, the Sun can delay its inevitable collapse by burning heavier atoms such as helium. However, this reprieve will last only 100 million years or so. </p>
<p>When these heavier fuels run out, the Sun will collapse into its final white dwarf state. In the collapse, the Sun will blow off about half its mass as a cloud of hot gas and push the surviving planets into a wider orbit.</p>
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<span class="caption">An artist’s rendition of the system.</span>
<span class="attribution"><span class="source">W. M. Keck Observatory/Adam Makarenko</span></span>
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<p>For the planets, there is a fine balancing act between being swallowed up during the expansion of the red giant and possibly being ejected into deep space when the white dwarf forms. Our discovery shows what some theorists have predicted: that planets at wide enough orbits are likely to survive the death of their host star. </p>
<p>Because most stars end up as white dwarfs, we don’t have a very precise estimate of what this system looked like when it formed. However, the statistics favour an origin as a star not too different in mass from the Sun. </p>
<p>The Universe isn’t old enough for stars smaller than about 80% as big as the Sun to have evolved into white dwarfs, and stars more than about twice the size of the Sun are intrinsically rare and also more likely to experience more turbulent deaths that would destroy their planetary systems. </p>
<p>Using the Hubble Space Telescope or its successor, the James Webb Space Telescope (due to launch in December 2021), we hope to learn more about the system by directly measuring the incredibly faint residual light emitted by this dead sun.</p>
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Read more:
<a href="https://theconversation.com/james-webb-space-telescope-an-astronomer-on-the-team-explains-how-to-send-a-giant-telescope-to-space-and-why-167516">James Webb Space Telescope: An astronomer on the team explains how to send a giant telescope to space – and why</a>
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<p class="fine-print"><em><span>Joshua Blackman receives funding from the Australian Research Council. </span></em></p><p class="fine-print"><em><span>Andrew A. Cole receives funding from the Australian Research Council. </span></em></p>In 5 billion years the Sun will collapse. A new discovery suggest some planets may still survive afterwards.Joshua W. Blackman, Astronomer, University of TasmaniaAndrew A. Cole, Associate Professor in Astrophysics, University of TasmaniaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1695142021-10-13T15:22:27Z2021-10-13T15:22:27ZWe’ve spotted a planet surviving its dying star – here’s what it tells us about end of our Solar System<figure><img src="https://images.theconversation.com/files/425717/original/file-20211011-20-ki73i8.jpg?ixlib=rb-1.1.0&rect=125%2C71%2C3868%2C2520&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Scientists have spotted a Jupiter-like planet surviving the death of its star.</span> <span class="attribution"><span class="source">Credit: W. M. Keck Observatory/Adam Makarenko</span></span></figcaption></figure><p>How will the Solar System die? It’s a hugely important question that researchers have speculated a lot about, using our knowledge of physics to create complex theoretical models. We know that the Sun will eventually become a “<a href="https://www.space.com/42949-sun-crystal-white-dwarf-stars-lifecycle.html">white dwarf</a>”, a burnt stellar remnant whose dim light gradually fades into darkness. This transformation will involve a violent process that will destroy an unknown number of its planets.</p>
<p>So which planets in will survive the death of the Sun? One way to seek the answer is to look at the fates of other similar planetary systems. This has proven difficult, however. The feeble radiation from white dwarfs makes it difficult to spot exoplanets (planets around stars other than our Sun) which have survived this stellar transformation – they are literally in the dark.</p>
<p>In fact, of the over <a href="https://exoplanetarchive.ipac.caltech.edu/">4,500 exoplanets</a> that are currently known, just a handful have been found around white dwarfs – and the location of these planets suggests they arrived there after the death of the star. </p>
<p>This lack of data paints an incomplete picture of our own planetary fate. Fortunately, we are now filling in the gaps. In our new paper, published <a href="https://www.nature.com/articles/s41586-021-03869-6">in Nature</a>, we report the discovery of the first known exoplanet to survive the death of its star without having its orbit altered by other planets moving around – circling a distance comparable to those between the Sun and the Solar System planets.</p>
<h2>A Jupiter-like planet</h2>
<p>This new exoplanet, which we discovered with the <a href="https://www.keckobservatory.org">Keck Observatory</a> in Hawaii, is particularly similar to Jupiter in both mass and orbital separation, and provides us with a crucial snapshot into planetary survivors around dying stars. A star’s transformation into a white dwarf involves a violent phase in which it becomes a bloated “red giant”, also known as a “<a href="http://www.astronomy.ohio-state.edu/%7Ethompson/1101/lecture_evolution_low_mass_stars.html">giant branch</a>” star, hundreds of times bigger than before. We believe that this exoplanet only just survived: if it was initially closer to its parent star, it would have been engulfed by the star’s expansion.</p>
<p>When the Sun eventually becomes a red giant, its radius will actually reach outwards to Earth’s current orbit. That means the Sun will (probably) engulf Mercury and Venus, and possibly the Earth – but we are not sure.</p>
<p>Jupiter, and its moons, have been expected to survive, although we previously didn’t know for sure. But with our discovery of this new exoplanet, we can now be more certain that Jupiter really will make it. Moreover, the margin of error in the position of this exoplanet could mean that it is almost half as close to the white dwarf as Jupiter currently is to the Sun. If so, that is additional evidence for assuming that Jupiter, and Mars, will make it.</p>
<p>So could any life survive this transformation? A white dwarf could power life on moons or planets that end up being very close to it (about one-tenth the distance between the Sun and Mercury) for the first few billion years. After that, there wouldn’t be enough radiation to sustain anything. </p>
<h2>Asteroids and white dwarfs</h2>
<p>Although planets orbiting white dwarfs have been difficult to find, what has been much easier to detect are <a href="https://www.nature.com/articles/nature15527">asteroids breaking up</a> close to the white dwarf’s surface. For exoasteroids to get so close to a white dwarf, they need to have enough momentum imparted to them by surviving exoplanets. Hence, exoasteroids have been long assumed to be evidence that exoplanets are there too. </p>
<p>Our discovery finally provides confirmation of this. Although in the system being discussed in the paper, current technology does not allow us to see any exoasteroids, at least now we can piece together different parts of the puzzle of planetary fate by merging the evidence from different white dwarf systems.</p>
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<p>The link between exoasteroids and exoplanets also applies to our own Solar System. Individual objects in the asteroid main belt and Kuiper belt (a disc in the outer Solar System) are likely to survive the Sun’s demise, but some will be moved by gravity by one of the surviving planets towards the white dwarf’s surface.</p>
<h2>Future discovery prospects</h2>
<p>The new white dwarf exoplanet was found with what is known as the <a href="https://theconversation.com/explainer-how-to-find-an-exoplanet-part-2-60439">microlensing detection method</a>. This looks at how light bends due to a strong gravitational field, which happens when a star momentarily aligns with a more distant star, as seen from Earth. </p>
<p>The gravity from the foreground star magnifies the light from the star behind it. Any planets orbiting the star in the foreground will bend and warp this magnified light, which is how we can detect them. The white dwarf we investigated is one-quarter of the way towards the centre of the Milky Way galaxy, or about 6,500 light years away from our Solar System, and the more distant star is in the centre of the galaxy. </p>
<p>A key feature of the microlensing technique is that it is sensitive to planets which orbit stars at the Jupiter-Sun distance. The other known planets which orbit white dwarfs have been found with different techniques which are sensitive to different star-planet separations. Two examples relate to planets which have survived a star’s transformation into a white dwarf and have ended up closer to it than before. One was found by <a href="https://www.nature.com/articles/s41586-020-2713-y">transit photometry</a> – a method to detect planets as they pass in front of a white dwarf, which creates a dip in the light received by Earth – and the other was discovered through the detection of the <a href="https://www.nature.com/articles/s41586-019-1789-8">planet’s evaporating atmosphere</a>.</p>
<p>One further detection technique – <a href="https://www.planetary.org/articles/wobbly-stars-the-astrometry-method">astrometry</a>, which precisely measures the movement of white dwarfs in the sky – is also predicted to yield results. In a few years, astrometry from the <a href="https://sci.esa.int/web/gaia">Gaia mission</a> is expected to find about a dozen planets orbiting white dwarfs. Perhaps these could offer better evidence as to exactly how the Solar System will die. </p>
<p>This variety of discovery techniques bodes well for potential future detections, which may offer further insight into the fate of our own planet. But for now, the newly discovered Jupiter-like exoplanet provides the clearest glimpse into our future.</p><img src="https://counter.theconversation.com/content/169514/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Dimitri Veras receives funding from the Science and Technology Facilities Council, grant ST/P003850/1, which funds his Ernest Rutherford Fellowship.</span></em></p>For the first time ever, astronomers have astrophysical evidence that Jupiter and many other planets will survive the death of the Sun.Dimitri Veras, Associate Professor and STFC Ernest Rutherford Fellow of Astrophysics, University of WarwickLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1302012020-01-30T19:07:15Z2020-01-30T19:07:15ZWarp factor: we’ve observed a spinning star that drags the very fabric of space and time<figure><img src="https://images.theconversation.com/files/312748/original/file-20200130-41485-1gsbuz5.jpg?ixlib=rb-1.1.0&rect=5%2C0%2C1911%2C1072&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A white dward (centre) and its companion pulsar make for an excellent natural gravitational laboratory.</span> <span class="attribution"><span class="source">Mark Myers/OzGrav</span></span></figcaption></figure><p>One of the predictions of Einstein’s <a href="https://theconversation.com/au/topics/theory-of-general-relativity-1632">general theory of relativity</a> is that any spinning body drags the very fabric of space-time in its vicinity around with it. This is known as “frame-dragging”.</p>
<p>In everyday life, frame-dragging is both undetectable and inconsequential, as the effect is so ridiculously tiny. Detecting the frame-dragging caused by the entire Earth’s spin requires satellites such as the US$750 million Gravity Probe B, and the detection of angular changes in gyroscopes equivalent to just one degree every 100,000 years or so.</p>
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<strong>
Read more:
<a href="https://theconversation.com/explainer-einsteins-theory-of-general-relativity-3481">Explainer: Einstein's Theory of General Relativity</a>
</strong>
</em>
</p>
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<p>Luckily for us, the Universe contains many naturally occurring gravitational laboratories where physicists can observe Einstein’s predictions at work in exquisite detail. Our team’s research, <a href="https://science.sciencemag.org/cgi/doi/10.1126/science.aax7007">published today in Science</a>, reveals evidence of frame-dragging on a much more noticeable scale, using a radio telescope and a unique pair of compact stars whizzing around each other at dizzying speeds.</p>
<p>The motion of these stars would have perplexed astronomers in Newton’s time, as they clearly move in a warped space-time, and require Einstein’s general theory of relativity to explain their trajectories.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/311076/original/file-20200121-117938-cwhdgk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/311076/original/file-20200121-117938-cwhdgk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/311076/original/file-20200121-117938-cwhdgk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/311076/original/file-20200121-117938-cwhdgk.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/311076/original/file-20200121-117938-cwhdgk.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/311076/original/file-20200121-117938-cwhdgk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/311076/original/file-20200121-117938-cwhdgk.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/311076/original/file-20200121-117938-cwhdgk.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The white dwarf-pulsar binary system PSR J1141-6545 discovered by the CSIRO’s Parkes radio telescope. The pulsar orbits its white dwarf companion every 4.8 hours. The white dwarf’s rapid rotation drags space-time around it, causing the entire orbit to tumble in space.</span>
<span class="attribution"><span class="source">Mark Myers/ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav)</span></span>
</figcaption>
</figure>
<p>General relativity is the foundation of modern gravitational theory. It explains the precise motion of the stars, planets and satellites, and even the flow of time. One of its lesser-known predictions is that spinning bodies drag space-time around with them. The faster an object spins and the more massive it is, the more powerful the drag.</p>
<p>One type of object for which this is very relevant is called a <a href="https://theconversation.com/how-we-found-a-white-dwarf-a-stellar-corpse-by-accident-114089">white dwarf</a>. These are the leftover cores from dead stars that were once several times the mass of our Sun, but have since exhausted their hydrogen fuel. What remains is similar in size to Earth but hundreds of thousands of times more massive. White dwarfs can also spin very quickly, rotating every minute or two, rather than every 24 hours like Earth does. </p>
<p>The frame-dragging caused by such a white dwarf would be roughly 100 million times as powerful as Earth’s.</p>
<p>That is all well and good, but we can’t fly to a white dwarf and launch satellites around it. Fortunately, however, nature is kind to astronomers and has its own way of letting us observe them, via orbiting stars called pulsars.</p>
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<p>Twenty years ago, CSIRO’s Parkes radio telescope discovered a unique stellar pair consisting of a white dwarf (about the size of Earth but about 300,000 times heavier) and a radio pulsar (just the size of a city but 400,000 times heavier). </p>
<p>Compared with white dwarfs, pulsars are in another league altogether. They are made not of conventional atoms, but of neutrons packed tightly together, making them incredibly dense. What’s more, the pulsar in our study spins 150 times every minute. </p>
<p>This mean that, 150 times every minute, a “lighthouse beam” of radio waves emitted by this pulsar sweeps past our vantage point here on Earth. We can use this to map the path of the pulsar as it orbits the white dwarf, by timing when its pulse arrives at our telescope and knowing the speed of light. This method revealed that the two stars orbit one another in less than 5 hours.</p>
<p>This pair, officially called PSR J1141-6545, is an ideal gravitational laboratory. Since 2001 we have trekked to Parkes several times a year to map this system’s orbit, which exhibits a multitude of Einsteinian gravitational effects.</p>
<p>Mapping the evolution of orbits is not for the impatient, but our measurements are ridiculously precise. Although PSR J1141-6545 is several hundred quadrillion kilometres away (a quadrillion is a million billion), we know the pulsar rotates 2.5387230404 times per second, and that its orbit is tumbling in space. This means the plane of its orbit is not fixed, but instead is slowly rotating.</p>
<h2>How did this system form?</h2>
<p>When pairs of stars are born, the most massive one dies first, often creating a white dwarf. Before the second star dies it transfers matter to its white dwarf companion. A disk forms as this material falls towards the white dwarf, and over the course of tens of thousands of years it revs up the white dwarf, until it rotates every few minutes.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/311075/original/file-20200121-117943-14v6seu.PNG?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/311075/original/file-20200121-117943-14v6seu.PNG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/311075/original/file-20200121-117943-14v6seu.PNG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=431&fit=crop&dpr=1 600w, https://images.theconversation.com/files/311075/original/file-20200121-117943-14v6seu.PNG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=431&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/311075/original/file-20200121-117943-14v6seu.PNG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=431&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/311075/original/file-20200121-117943-14v6seu.PNG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=542&fit=crop&dpr=1 754w, https://images.theconversation.com/files/311075/original/file-20200121-117943-14v6seu.PNG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=542&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/311075/original/file-20200121-117943-14v6seu.PNG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=542&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Artist’s impression of a white dwarf being spun-up by the transfer of matter from its companion. Material at the surface of the swollen star falls towards the white dwarf and forms a disk of material travelling so quickly it causes the star to spin rapidly.</span>
<span class="attribution"><span class="source">ARC Centre of Excellence for Gravitational Wave Discovery</span></span>
</figcaption>
</figure>
<p>In rare cases such as this one, the second star can then detonate in a supernova, leaving behind a pulsar. The rapidly spinning white dwarf drags space-time around with it, making the pulsar’s orbital plane tilt as it is dragged along. This tilting is what we observed through our patient mapping of the pulsar’s orbit.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/weve-detected-new-gravitational-waves-we-just-dont-know-where-they-come-from-yet-116267">We've detected new gravitational waves, we just don't know where they come from (yet)</a>
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<p>Einstein himself thought many of his predictions about space and time would never be observable. But the past few years have seen a revolution in extreme astrophysics, including the <a href="https://theconversation.com/gravitational-waves-discovered-top-scientists-respond-53956">discovery of gravitational waves</a> and the <a href="https://theconversation.com/observing-the-invisible-the-long-journey-to-the-first-image-of-a-black-hole-115064">imaging of a black hole shadow</a> with a worldwide network of telescopes. These discoveries were made by billion-dollar facilities. </p>
<p>Fortunately there is still a role in exploring general relativity for 50-year-old radio telescopes like the one at Parkes, and for patient campaigns by generations of graduate students.</p><img src="https://counter.theconversation.com/content/130201/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Matthew Bailes receives funding from the Australian Research Council for the ARC Centre of Excellence for Gravitational Wave Discovery and is an ARC Laureate Fellow.</span></em></p><p class="fine-print"><em><span>Vivek Venkatraman Krishnan does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>One of Einstein’s weirder predictions is that massive, spinning objects exert a drag on space-time itself. Now an orbiting pair of unusual stars has revealed this effect in action.Matthew Bailes, ARC Laureate Fellow, Swinburne University of TechnologyVivek Venkatraman Krishnan, Scientific staff, Max Planck Institute for Radio AstronomyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1276992019-11-25T14:34:10Z2019-11-25T14:34:10ZNew type of star system? Mysterious radio signal puzzles astronomers<figure><img src="https://images.theconversation.com/files/303437/original/file-20191125-74599-sltvmi.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Meerkat telescope</span> <span class="attribution"><span class="source">Sotiris Sanidas</span>, <span class="license">Author provided</span></span></figcaption></figure><p>After observing a part of the sky near the <a href="https://www.universetoday.com/19574/ara/">Southern Constellation of Ara</a> for about two months using <a href="https://www.ska.ac.za/gallery/meerkat/">MeerKAT</a>, a radio telescope based in the Karoo desert in South Africa, <a href="http://www.thunderkat.uct.ac.za/">our team of scientists</a> noticed something strange. The radio emission of an object brightened by a factor of three over roughly three weeks. </p>
<p>Intrigued, we continued watching the object and followed this up with observations from other telescopes. We discovered that the unusual flare came from a <a href="https://www.space.com/22509-binary-stars.html">binary star system </a> – two stars orbiting each other – in our own galaxy. The finding, <a href="https://academic.oup.com/mnras/article/491/1/560/5610241">published in the Monthly Notices of the Royal Astronomical Society</a>, has, however, turned out to be very difficult to explain.</p>
<p>This is MeerKAT’s first discovery of a “transient source” – an object that is not constant, either undergoing a significant change in brightness or coming in and out of view altogether. Given the catchy name “MKT J170456.2-482100”, it was found in the first field observed with the telescope, which means it is likely to be the tip of an iceberg of transients waiting to be discovered.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/303500/original/file-20191125-74580-16dnqjj.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/303500/original/file-20191125-74580-16dnqjj.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=514&fit=crop&dpr=1 600w, https://images.theconversation.com/files/303500/original/file-20191125-74580-16dnqjj.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=514&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/303500/original/file-20191125-74580-16dnqjj.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=514&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/303500/original/file-20191125-74580-16dnqjj.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=646&fit=crop&dpr=1 754w, https://images.theconversation.com/files/303500/original/file-20191125-74580-16dnqjj.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=646&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/303500/original/file-20191125-74580-16dnqjj.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=646&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="source">Radio emission detected during the measurement, with the flare circled.</span>, <span class="license">Author provided</span></span>
</figcaption>
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<p>To understand our discovery, we started by matching our source with the position of a star, called TYC 8332-2529-1, about 1,800 light years from Earth. Because this star is relativity bright, we anticipated that a number of different optical telescopes – detecting visible light rather than radio waves – would have observed this star in the past. </p>
<p>Luckily, this turned out to be the case, allowing us to use such data to find out more about the star. It is a giant – about two and a half times the mass of the Sun. Some of the optical telescopes, including <a href="http://www.astronomy.ohio-state.edu/%7Eassassin/index.shtml">ASAS</a>, <a href="https://en.wikipedia.org/wiki/Kilodegree_Extremely_Little_Telescope">KELT</a> and <a href="http://www.astronomy.ohio-state.edu/%7Eassassin/index.shtml">ASAS-SN</a>, provided us with over 18 years of observations of the star. These helped us discover that the brightness of the star changes over a period of 21 days. We think this is because the star has large spots on it, just like sunspots. </p>
<p>We used the <a href="https://www.salt.ac.za/">SALT telescope</a> to obtain optical spectra of the star – similar to using a prism to split white light into its constituent wavelengths. This can be used to determine the chemical elements present in the star, as well as the presence of a magnetic field. What’s more, they enable scientists to tell if a star is moving, as movement causes these spectral lines to shift (Doppler shift). </p>
<p>The spectra revealed that the star has a magnetic field, and that it orbits a companion star every 21 days. However, we can only see a very faint, possible signature of the companion star in our observations so far. This tells us that the companion must be much fainter than the giant star. We also found, however, that the companion is likely to have at least 1.5 times the mass of the Sun. </p>
<p>So what could the companion be? A <a href="https://imagine.gsfc.nasa.gov/science/objects/dwarfs2.html">white dwarf</a> (a cold, dead star) may seem likely, as they are often part of binary star systems like this. However, most white dwarfs have a smaller mass than the companion we spotted – with a maximum mass of 1.6 times the mass of the Sun. So it is unlikely to be such a star.</p>
<h2>The plot thickens</h2>
<p>The radio flare itself could be caused by magnetic activity of the giant star, similar to solar flares but much brighter and more energetic. However, such flares are usually observed on dwarf stars rather than giant stars. </p>
<p><a href="https://en.wikipedia.org/wiki/RS_Canum_Venaticorum_variable">Known star systems</a> involving a giant star and a Sun-like star could explain the findings – with the magnetic activity of the giant star giving rise to flares. However, this doesn’t fit, as there is no sign in the spectra that the binary companion is actually a Sun-like star.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/303408/original/file-20191125-74542-10rtzae.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/303408/original/file-20191125-74542-10rtzae.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=398&fit=crop&dpr=1 600w, https://images.theconversation.com/files/303408/original/file-20191125-74542-10rtzae.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=398&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/303408/original/file-20191125-74542-10rtzae.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=398&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/303408/original/file-20191125-74542-10rtzae.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=500&fit=crop&dpr=1 754w, https://images.theconversation.com/files/303408/original/file-20191125-74542-10rtzae.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=500&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/303408/original/file-20191125-74542-10rtzae.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=500&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">MeerKAT radio telescope.</span>
<span class="attribution"><span class="source">wikipedia</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p><a href="http://www.jodrellbank.manchester.ac.uk/people/staff/profile/?ea=Ben.Stappers">Ben Stappers</a>, principal investigator of <a href="https://www.MeerTRAP.org">MeerTRAP</a>, one of the teams working on the project, said that because the properties of the system don’t easily fit into our current knowledge of binary or flaring stars, it “may represent an entirely new source class”. We suspect that this might be some sort of exotic system that we have never seen before involving a radio-flaring giant star orbiting a neutron star (the dense remnant of a supernova star explosion) or a black hole.</p>
<p>MeerKAT is going to continue observing this source every week for the next four years, with the <a href="http://www.astronomy.ohio-state.edu/%7Eassassin/index.shtml">ASAS-SN optical telescope</a> continuing to observe the giant star. This means we will be able to explore the physics and nature of this source and its flares for many years to come.</p>
<p>This will tell us about the dynamics of this system, how flares occur and ultimately help us investigate how it formed. As MeerKAT continues to search the sky, we hope that this is the first of many new and unusual sources waiting to be discovered.</p><img src="https://counter.theconversation.com/content/127699/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Laura Nicole Driessen is part of MeerTRAP, which is based at the University of Manchester and receives funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No 694745). </span></em></p>Radio flare may be the result of a giant star orbiting some unusual object – a combination we have never seen before.Laura Nicole Driessen, PhD candidate in Radio Astronomy, University of ManchesterLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1140892019-04-30T03:34:26Z2019-04-30T03:34:26ZHow we found a white dwarf – a stellar corpse – by accident<figure><img src="https://images.theconversation.com/files/270571/original/file-20190423-175548-g8mn17.jpg?ixlib=rb-1.1.0&rect=122%2C6%2C1818%2C1020&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Searching for planets around nearby stars is like searching for a needle in a field of haystacks. </span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/trevor_dobson_inefekt69/25626842713">Trevor Dobson/Flikr</a>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span></figcaption></figure><p>One of the great things about science is that, when you start to observe a new object in space, you can never be sure quite what you’ll find. </p>
<p>We received a fantastic reminder of this during observations designed to check whether nearby stars had planetary companions. Our observations confirmed the discovery of a couple of planets, but also yielded an unexpected surprise.</p>
<p>Buried among our candidates was the corpse of a star – <a href="http://astronomy.swin.edu.au/cosmos/W/White+Dwarf">a white dwarf</a> – a discovery we announced this month in <a href="https://doi.org/10.3847/1538-4357/ab0e74" title="Discovery of a Compact Companion to a Nearby Star">The Astrophysical Journal</a>.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/why-pluto-is-losing-its-atmosphere-winter-is-coming-115567">Why Pluto is losing its atmosphere: winter is coming</a>
</strong>
</em>
</p>
<hr>
<h2>The search for stellar wobbles</h2>
<p>Our story begins with a survey called the Anglo-Australian Planet Search (<a href="http://newt.phys.unsw.edu.au/%7Ecgt/planet/AAPS_Home.html">AAPS</a>), which spent 17 years looking for alien worlds using the 3.9-metre <a href="https://www.aao.gov.au/about-us/anglo-australian-telescope">Anglo-Australian Telescope</a> at <a href="https://www.sidingspringobservatory.com.au/">Siding Spring Observatory</a> in New South Wales.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/270197/original/file-20190421-1403-1xhz7ak.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/270197/original/file-20190421-1403-1xhz7ak.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/270197/original/file-20190421-1403-1xhz7ak.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/270197/original/file-20190421-1403-1xhz7ak.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/270197/original/file-20190421-1403-1xhz7ak.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/270197/original/file-20190421-1403-1xhz7ak.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/270197/original/file-20190421-1403-1xhz7ak.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/270197/original/file-20190421-1403-1xhz7ak.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Anglo-Australian Telescope, at Siding Spring Observatory, offers spectacular views of the southern sky.</span>
<span class="attribution"><span class="source">Jonti Horner</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>We often say a planet orbits a star (Earth orbits the Sun, for example), but the truth is slightly more complicated. Instead, the two orbit around their common centre of mass. As a result, a star that hosts a planet will wobble, rocking back and forth over time.</p>
<p><a href="https://theconversation.com/explainer-how-to-find-an-exoplanet-part-1-56682">Radial velocity surveys</a> search for planets by attempting to detect that telltale wobble. Over its lifetime, the AAPS discovered more than 40 planets in this manner. </p>
<p>But it is almost certain that more planets remained undiscovered in the AAPS data. So we began searching for those hidden worlds.</p>
<p>In several cases we found stars that exhibited distinct signs of a wobble, but for which less than a full orbit had been completed. Without observing a full orbit, we don’t know whether the companions causing the wobble are planets, or other stars.</p>
<p>So how can we work out what we’ve found?</p>
<h2>Direct imaging – a new trick</h2>
<p>We identified 21 stars around which there could be a planet, but to be sure, we needed more data. Unfortunately, the AAPS had ended, so we needed to do something innovative.</p>
<p>For each of our stars, there were two possibilities: either the wobble is caused by a planet, or by something bigger (such as a <a href="http://coolcosmos.ipac.caltech.edu/cosmic_classroom/cosmic_reference/brown_dwarfs.html">brown dwarf</a> or <a href="https://www.atnf.csiro.au/outreach/education/senior/astrophysics/binary_types.html">an unseen stellar companion</a>).</p>
<p>Recent advances in astronomical imaging techniques mean we can now use the world’s largest telescopes to look at nearby stars and see objects very close to them – closer than has ever been possible before. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/QIadRr0QX_Q?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Astronomical imaging showing the four giant planets orbiting HR 8799.</span></figcaption>
</figure>
<p>We used the <a href="https://www.gemini.edu/sciops/telescopes-and-sites">8.1m Gemini-South telescope</a> in Chile to obtain high-resolution images of our target stars, to see whether we could see any previously hidden companions. </p>
<p>Despite the power of the technique, any planets around our targets would remain invisible. But if the observed wobbles were caused by more massive objects, we should be able to see those objects and hence rule out the planetary hypothesis.</p>
<h2>The peculiar case of HD 118473</h2>
<p>For 20 of our targets, things went as we expected. In some cases, we detected a previously undiscovered stellar companion. In others, we could rule out massive companions, giving us confidence in the presence of planets around those stars.</p>
<p>But for one star, things got weird. On the basis of the wobble data, we knew that the <em>lowest possible mass</em> the companion could have is around 0.44 times the mass of the Sun. That’s <a href="http://blogs.discovermagazine.com/outthere/2017/08/04/how-big-is-the-biggest-possible-planet/">much too massive to be a planet</a>.</p>
<p>With that much mass, we would <a href="https://slate.com/technology/2014/06/the-brown-dwarf-limit-astronomers-have-found-the-smallest-star-known.html">expect the companion to be a star</a>, fainter and cooler than the Sun, but easily visible with Gemini-South.</p>
<p>But when we looked at our images, no companion star was visible.</p>
<h2>A macabre twist</h2>
<p>The radial velocity data is clear – there is a massive companion orbiting HD118473, causing that star to wobble back and forth with a period of 5.67 years.</p>
<p>But it can’t be a planet (it’s far too massive), and it can’t be a star (we’d be able to see it). So what could it be?</p>
<p>The answer comes down to the way stars live and die.</p>
<p>Vast as stars are, their supply of fuel is not unlimited. Eventually the fuel runs out and the end of the star’s life is imminent. The more massive the star, the more spectacular that end will be.</p>
<p>A star like the Sun <a href="https://theconversation.com/curious-kids-whats-going-to-happen-to-the-sun-in-the-future-will-it-explode-78029">will eventually swell to become a red giant</a>, then will puff off its outer layers, creating a spectacular planetary nebula, and leaving behind a glowing ember – its core, bare and exposed to space.</p>
<p>That core is a <a href="http://astronomy.swin.edu.au/cosmos/W/White+Dwarf">white dwarf</a> – around the size of Earth, but with the mass of a star. Tiny, compared with the star from which it came, the white dwarf gradually cools and fades to obscurity over billions of years.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/270572/original/file-20190423-175528-curwg7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/270572/original/file-20190423-175528-curwg7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/270572/original/file-20190423-175528-curwg7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/270572/original/file-20190423-175528-curwg7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/270572/original/file-20190423-175528-curwg7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/270572/original/file-20190423-175528-curwg7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/270572/original/file-20190423-175528-curwg7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/270572/original/file-20190423-175528-curwg7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Artist’s impression of Sirius B, the closest known white dwarf.</span>
<span class="attribution"><span class="source">NASA, ESA and G. Bacon (STScI)</span></span>
</figcaption>
</figure>
<p>More massive stars die violently – as <a href="http://astronomy.swin.edu.au/cosmos/s/supernova">supernovae</a> that outshine whole galaxies. But they also leave behind corpses that are faint and hard to spot. <a href="http://astronomy.swin.edu.au/cosmos/n/neutron+star">Neutron stars</a> – the size of a city, but with a mass greater than the Sun – and <a href="http://astronomy.swin.edu.au/cosmos/B/Black+Hole">black holes</a> – tiny and invisible, except when they’re devouring something.</p>
<p>All this brings us back to our hidden companion to HD118473 – the mass of a star, but too faint to see. What could it be?</p>
<h2>An unexpected ancient relic</h2>
<p>By far the most likely answer is that the hidden companion is a white dwarf. In the distant past, HD118473 was a <a href="http://astronomy.swin.edu.au/cosmos/b/binary+star">binary star</a> with the two components shining bright as they orbited their common centre of mass.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/observing-the-invisible-the-long-journey-to-the-first-image-of-a-black-hole-115064">Observing the invisible: the long journey to the first image of a black hole</a>
</strong>
</em>
</p>
<hr>
<p>For a few billion years, nothing changed, until the more massive of the stars reached the end of its life. It swelled to become a <a href="http://astronomy.swin.edu.au/cosmos/R/Red+giant+stars">red giant</a> then shed its outer layers, leaving behind a white dwarf, too dim for us to detect.</p>
<p>The white dwarf’s companion continues through space as we speak, still whirling in a celestial waltz with what remains of its companion. A dim, hidden relic to deceive exoplanet hunters, and a reminder of how science always has another surprise waiting around the corner.</p><img src="https://counter.theconversation.com/content/114089/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>The authors do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Science is full of surprises. While searching for planets orbiting nearby stars, researchers stumbled across the remains of a star that once outshone the Sun.Jonti Horner, Professor (Astrophysics), University of Southern QueenslandStephen Kane, Associate Professor, University of California, RiversideLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1153292019-04-15T12:03:35Z2019-04-15T12:03:35ZCurious Kids: what would happen if the sun exploded?<figure><img src="https://images.theconversation.com/files/269294/original/file-20190415-147499-7uzj0m.jpg?ixlib=rb-1.1.0&rect=35%2C17%2C5955%2C4760&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">What's left after a star explodes. </span> <span class="attribution"><a class="source" href="https://upload.wikimedia.org/wikipedia/commons/d/d4/Keplers_supernova.jpg">NASA/ESA/JHU/R.Sankrit & W.Blair via Wikimedia Commons. </a></span></figcaption></figure><figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/165749/original/image-20170419-32713-1kyojyz.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/165749/original/image-20170419-32713-1kyojyz.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=376&fit=crop&dpr=1 600w, https://images.theconversation.com/files/165749/original/image-20170419-32713-1kyojyz.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=376&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/165749/original/image-20170419-32713-1kyojyz.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=376&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/165749/original/image-20170419-32713-1kyojyz.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=472&fit=crop&dpr=1 754w, https://images.theconversation.com/files/165749/original/image-20170419-32713-1kyojyz.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=472&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/165749/original/image-20170419-32713-1kyojyz.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=472&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption"></span>
<span class="attribution"><a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p><em><a href="https://theconversation.com/au/topics/curious-kids-36782">Curious Kids</a> is a series by <a href="https://theconversation.com/uk">The Conversation</a>, which gives children of all ages the chance to have their questions about the world answered by experts. All questions are welcome: send them – along with your name, age and the town or city where you live – to curiouskids@theconversation.com. We won’t be able to answer every question, but we’ll do our best.</em></p>
<hr>
<blockquote>
<p><strong><em>What would happen if the sun exploded? – Lizey, aged 12, Australia.</em></strong></p>
</blockquote>
<p>The sun is a star, and when a star explodes it’s called <a href="https://www.esa.int/kids/en/learn/Our_Universe/Stars_and_galaxies/Supernovas">a supernova</a>. These types of explosions are very bright, and very powerful. They release lots of dust into space, which is used to make more stars and planets. Our solar system was made using stuff from these explosions. Even humans are <a href="https://www.youtube.com/watch?v=tLPkpBN6bEI">made of star stuff</a>! </p>
<p>If the sun suddenly exploded like this, the whole solar system would be destroyed. You don’t have to worry though – only stars ten times the size of our sun, or bigger, can explode like this. Our sun will end its life in a different way. </p>
<p>A supernova is like bursting a balloon. But when our sun dies, it will happen slowly, like when you gradually let the air out of a balloon.</p>
<h2>The death of the sun</h2>
<p>The sun will start to die when it runs out of fuel in <a href="http://thescienceexplorer.com/universe/what-will-happen-when-sun-eventually-dies">about 5,000,000,000 years</a> (that’s five billion years). This is 77 times longer than the Tyrannosaurus-Rex has been extinct … a very, very long time. </p>
<p>When the sun starts to die, it will get bigger and slightly colder, turning into what astronomers call a “red giant”. It will get so big, that it will eat Mercury, Venus and even Earth. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/269298/original/file-20190415-147480-1ae7x5u.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/269298/original/file-20190415-147480-1ae7x5u.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=375&fit=crop&dpr=1 600w, https://images.theconversation.com/files/269298/original/file-20190415-147480-1ae7x5u.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=375&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/269298/original/file-20190415-147480-1ae7x5u.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=375&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/269298/original/file-20190415-147480-1ae7x5u.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=471&fit=crop&dpr=1 754w, https://images.theconversation.com/files/269298/original/file-20190415-147480-1ae7x5u.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=471&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/269298/original/file-20190415-147480-1ae7x5u.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=471&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Earth could be in big trouble.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-illustration/solar-system-sun-red-giant-star-1039317385?src=Sl-qDevfHdfgoqMHTlyAyA-1-7">Shutterstock.</a></span>
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</figure>
<p>When the sun is a red giant, it will be big and puffy, and start to blow off its outer layers out of the solar system. It will get smaller and smaller, eventually becoming what we then call a white dwarf. </p>
<h2>The sun as a white dwarf</h2>
<p>A white dwarf is the core of a dead star. They are super heavy, weighing almost as much as the sun, while being only the size of the Earth. A teaspoon of white dwarf would weigh <a href="https://www.nasa.gov/topics/universe/features/whitedwarf_pulsar.html">somewhere around 6,000 kilograms</a> – as much as an adult elephant!</p>
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</figure>
<p>When the sun is a white dwarf, most of the solar system will still be around. Mercury, Venus and Earth will be gone, but Mars, Jupiter, Saturn, Uranus and Neptune will survive and continue to go around the sun. So will the asteroid belt, Kuiper belt and dwarf planets like Pluto. </p>
<p>Because a white dwarf is small, it doesn’t produce as much light. A white dwarf doesn’t have any fuel to give it energy, so it also gets colder and colder over time. Eventually it will become very dark.</p>
<h2>Life after the sun</h2>
<p>The light from the sun is what keeps our planet warm. Without it, the planets in the solar system will get very cold. This would make it harder for life to stay alive in the solar system. </p>
<p>A white dwarf doesn’t produce much light. But in the future, humans might build spaceships that will allow us to leave Earth. Humans might even build something to move the Earth. This would let the planet survive being eaten by the sun as a red giant. </p>
<p>The sun will become a red giant and then a white dwarf over billions of years. This is a very long time. We cannot watch a star do all of this, but we can learn how stars are born and die by looking at the stars in our galaxy - the Milky Way. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/the-fate-of-the-earth-we-discovered-the-remains-of-a-planet-following-the-violent-death-of-its-parent-star-114848">The fate of the Earth? We discovered the remains of a planet following the violent death of its parent star</a>
</strong>
</em>
</p>
<hr>
<p>The Milky Way has stars of all ages, and over time astronomers have worked out which ones are young, old or dead. By <a href="https://theconversation.com/the-fate-of-the-earth-we-discovered-the-remains-of-a-planet-following-the-violent-death-of-its-parent-star-114848">studying the old and dead stars</a>, we can discover what will happen to our sun in the far, far future.</p>
<hr>
<p><em>More <a href="https://theconversation.com/topics/curious-kids-36782?utm_source=TCUK&utm_medium=linkback&utm_campaign=TCUKengagement&utm_content=CuriousKidsUK">Curious Kids</a> articles, written by academic experts:</em></p>
<ul>
<li><p><em><a href="https://theconversation.com/curious-kids-is-water-blue-or-is-it-just-reflecting-off-the-sky-113199?utm_source=TCUK&utm_medium=linkback&utm_campaign=TCUKengagement&utm_content=CuriousKidsUK">Is water blue or is it just reflecting off the sky? – The students of Ms Brown’s class, Neerim South Public School, Victoria, Australia</a></em></p></li>
<li><p><em><a href="https://theconversation.com/curious-kids-who-is-siri-114940?utm_source=TCUK&utm_medium=linkback&utm_campaign=TCUKengagement&utm_content=CuriousKidsUK">Who is Siri? – Miles, aged four, London, UK.</a></em></p></li>
<li><p><em><a href="https://theconversation.com/curious-kids-how-did-the-months-get-their-names-113558?utm_source=TCUK&utm_medium=linkback&utm_campaign=TCUKengagement&utm_content=CuriousKidsUK">How did the months get their names? - Sylvie, aged eight, Brisbane, Australia.</a></em></p></li>
</ul><img src="https://counter.theconversation.com/content/115329/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Christopher Manser does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>By studying old and dead stars, we can discover what will happen to our sun in the far, far future. And it won’t end with a big explosion.Christopher Manser, Postdoctoral Researcher of Astrophysics, University of WarwickLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1148482019-04-04T18:04:48Z2019-04-04T18:04:48ZThe fate of the Earth? We discovered the remains of a planet following the violent death of its parent star<figure><img src="https://images.theconversation.com/files/267364/original/file-20190403-177190-45tk6b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">University of Warwick/Mark Garlick</span></span></figcaption></figure><p>If it weren’t for the sun constantly showering us with energy, there would be no life on Earth. But eventually stars like it run out of fuel, expand into red giants and finally collapse into small, faint objects called <a href="https://imagine.gsfc.nasa.gov/science/objects/dwarfs2.html">white dwarfs</a>. So what will happen to us and the other planets in our solar system when the sun dies? It’s not been entirely clear.</p>
<p>Now my colleagues and I have spotted the possible core remnant of a planet orbiting the white dwarf <a href="https://www.eso.org/public/archives/releases/sciencepapers/eso1544/eso1544a.pdf">SDSSJ122859.93+104032.9</a>, residing some 410 light years away. Our results, <a href="http://science.sciencemag.org/cgi/doi/10.1126/science.aat5330">published in Science</a>, offer important clues about the fate of the planets in our own solar system. </p>
<p>Scientists have identified <a href="https://theconversation.com/the-five-most-earth-like-exoplanets-so-far-50669">thousands of “exoplanets”</a> orbiting stars other than the sun, many of them very similar to our own. They can do this by measuring the small movement a star makes as it responds to the gravitational tug of an orbiting planet. They have also discovered discs of gas and debris surrounding white dwarfs, but <a href="https://www.cfa.harvard.edu/%7Eavanderb/wd1145_017.pdf">only one case</a> of a planetary fragment. The reason such remnants are so hard to spot is that they are so tiny that they have a very small gravitational pull on their parent star.</p>
<p>We came up with a completely new approach to probe the gas around white dwarfs. The technique, called spectroscopy, splits the light we see from an object into its separate colours, producing a spectrum in a similar way to a prism. The observations were made using the <a href="http://www.gtc.iac.es/gtc/gtc.php">Gran Telescopio Canarias</a> – the largest telescope in the world with a 10.4-metre diameter mirror.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/267391/original/file-20190403-177171-5it4kj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/267391/original/file-20190403-177171-5it4kj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/267391/original/file-20190403-177171-5it4kj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/267391/original/file-20190403-177171-5it4kj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/267391/original/file-20190403-177171-5it4kj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/267391/original/file-20190403-177171-5it4kj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/267391/original/file-20190403-177171-5it4kj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Gran Telescopio Canarias.</span>
<span class="attribution"><span class="source">Pixabay</span></span>
</figcaption>
</figure>
<p>The planetary fragment we found produced a stream of gas that could be detected by our spectrometer. We could spot it orbiting the star by looking at how its spectrum shifted in colour as the body moved towards and away from us. This change in colour is <a href="https://imagine.gsfc.nasa.gov/features/yba/M31_velocity/spectrum/doppler_more.htm">called a doppler shift</a>, which is essentially a stretching or squashing of waves because of motion. It is similar to the pitch of the sound of an ambulance being higher when it is coming towards you, and lower when it is moving away. </p>
<p>The body completed one passage around its host star in just over two hours, orbiting at a distance that is smaller than the radius of the sun in a disc of gas and dust. </p>
<h2>Mystery body</h2>
<p>The discovery is surprising, as we didn’t think anything could survive so close to a white dwarf. A white dwarf is only about the size of the Earth but it contains around 60-70% of the sun’s mass – making it extremely dense. If a body orbits too close to a white dwarf, its immense gravity will tear it apart. This was the fate of the material that formed the disc around it.</p>
<p>So how did it survive without getting ripped apart? It would have to be very dense or have some amount of internal strength holding it together. We calculated that it would have a maximum diameter of 720km, which is the size of a small minor planet. To compare, the <a href="https://theconversation.com/dawn-breaks-over-distant-ceres-and-perhaps-reveals-signs-of-habitability-38967">dwarf planet Ceres</a> in our own solar system has a diameter of 946km.</p>
<p>The origin of this body remains a mystery. One possibility is that this is the core of a minor planet that was pushed close to the white dwarf by a larger planet further out in the remnant planetary system – like a Jupiter. As the minor planet passed close to the white dwarf, its crust and mantle layers would have been ripped apart.</p>
<p>All that would be left of the body would be its dense, iron-dominated core. This kind of object is quite common, with one famous resident in our own solar system: the asteroid <a href="https://theconversation.com/metal-asteroid-psyche-is-all-set-for-an-early-visit-from-nasa-88044">16-Psyche</a>.</p>
<h2>The end of Earth?</h2>
<p>Systems such as the one we’ve just discovered can help us understand the future of our own planetary system. In about five billion years, the sun will eventually start to expand into a red giant. At this point, it will engulf Mercury, Venus and most likely Earth – unless we manage to move our planet into a wider orbit, which <a href="https://www.newscientist.com/article/dn14983-moving-the-earth-a-planetary-survival-guide/">should be possible in theory</a>. However, Mars, the asteroid belt and the rest of the solar system will survive engulfment and continue orbiting it as the sun collapses into a white dwarf. </p>
<p>During this process, planets like Jupiter could also scatter asteroids, comets or even minor planets towards the white dwarf. There they would undergo partial or complete disruption, forming a disc like the one we’ve just investigated. It is unlikely that any living organisms on planetary or moon fragments could survive this process. And even if they did, they would struggle to live on in the faint light of a white dwarf.</p>
<p>This is not only the solar system’s fate, but that of practically all known exoplanet systems. In the much much closer future, we hope to use the method we have developed here to find more planetary bodies around other white dwarfs. We know of six candidate white dwarfs that are orbited by discs made of dust and gas, and we want to test whether these discs are the “smoking gun” for the presence of minor planets. The more such planets we find, the more we can learn about what happens to a planetary system as its star dies.</p>
<p>But our technique could also help us discover more about the composition of exoplanets – something that is <a href="https://theconversation.com/more-than-1-000-new-exoplanets-discovered-but-still-no-earth-twin-59274">extremely hard to do</a> with exoplanets orbiting stars like our sun. The atmosphere of a standard white dwarf is very pure, made of either hydrogen or helium. But while it is consuming planetary material, its atmosphere gets polluted – enabling us to calculate how much of each element there is. This could tell us what the disintegrating planet was made of and even if it had water, helping us to build a better picture of what kind of exoplanets are the most likely to host life.</p><img src="https://counter.theconversation.com/content/114848/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Christopher Manser was supported by the European Research Council (ERC) under the EU's research and innovation programme (grant agreement 320964) during this project.</span></em></p>Exoplanet discovery can help us work out how the Earth will end its days.Christopher Manser, Postdoctoral Researcher of Astrophysics, University of WarwickLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1046092018-10-10T09:04:37Z2018-10-10T09:04:37ZHow we solved a centuries-old mystery by discovering a rare form of star collision<figure><img src="https://images.theconversation.com/files/239858/original/file-20181009-72100-kg229j.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The ALMA telescope has seen tantalising hints of a violent event.</span> <span class="attribution"><span class="source">ESO/B. Tafreshi/TWAN (twanight.org)</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span></figcaption></figure><p>A bright new star appeared in the sky in June, 1670. It was seen by the Carthusian monk Père Dom Anthelme in Dijon, France, and astronomer <a href="https://en.wikipedia.org/wiki/Johannes_Hevelius">Johannes Hevelius</a> in Gdansk, Poland. Over the next few months, it slowly faded to invisibility. But in March 1671, it reappeared – now even more luminous and among the 100 brightest stars in the sky. Again it faded, and by the end of the summer it was gone. Then in 1672, it put in a third appearance, now only barely visible to the naked eye. After a few months it was gone again and hasn’t been seen since. </p>
<p>This has always seemed to be an odd event. For centuries, astronomers regarded it as the <a href="https://en.wikipedia.org/wiki/CK_Vulpeculae">oldest known nova</a> – a type of star explosion. But this explanation became untenable in the 20th century. A nova is a fairly common event, when hydrogen ignites in an otherwise extinct star causing a thermonuclear runaway reaction. Stars can also explode as supernovae, following an implosion of their core. However, we know now that neither would give the kind of repeated appearance seen in this event. </p>
<p>So what was it? Our new research, published in the <a href="https://doi.org/10.1093/mnras/sty2554">Monthly Notices of the Royal Astronomical Society</a>, offers a whole new explanation. </p>
<p>In 1982, the American astronomer <a href="http://www.astro.columbia.edu/profile?uid=mshara">Mike Shara</a> found a nebula – an interstellar cloud of dust, hydrogen, helium and other gases – at the position of the missing star, which had since acquired the name <a href="https://en.wikipedia.org/wiki/CK_Vulpeculae">CK Vul</a> in between. This proved that something had indeed happened here. Astronomers later noted that the nebula was expanding, and that the expansion had started around 300 years ago. But the star itself couldn’t be seen.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/239793/original/file-20181008-72106-a7p3kt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/239793/original/file-20181008-72106-a7p3kt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=593&fit=crop&dpr=1 600w, https://images.theconversation.com/files/239793/original/file-20181008-72106-a7p3kt.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=593&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/239793/original/file-20181008-72106-a7p3kt.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=593&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/239793/original/file-20181008-72106-a7p3kt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=745&fit=crop&dpr=1 754w, https://images.theconversation.com/files/239793/original/file-20181008-72106-a7p3kt.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=745&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/239793/original/file-20181008-72106-a7p3kt.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=745&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">This chart of the position of the ‘nova’ (marked in red) was recorded by the famous astronomer Hevelius and was published by the Royal Society in England.</span>
<span class="attribution"><a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Things became even stranger when the astronomer <a href="https://www.researchgate.net/profile/Tomasz_Kaminski2">Tomasz Kamiński</a> <a href="https://arxiv.org/abs/1807.10647">discovered</a> that the nebula contained a most unusual mix of elements, being very abundant in two isotopes (elements with a different number of neutrons in their nucleus compared to the “normal” atom): a type of nitrogen (15N) and radioactive aluminium (26Al). These require very high temperatures to form. Whatever happened, this had been a high-energy event. </p>
<h2>New observations</h2>
<p>We observed the location of the star with <a href="https://www.almaobservatory.org/en/home/">ALMA observatory in Chile.</a>. This spectacular-looking telescope uses 64 separate dishes, and observes in the microwave region of light. It is particularly good at detecting radiation from molecules in space. What we found is that the debris from the event is visible as two rings of dust, resembling an hourglass. This hourglass is embedded within a larger hourglass seen in previous observations, and itself contains other structures – nested like a Russian doll. </p>
<p>Such hourglass lobes indicate the presence of jets coming from the centre, which blow out the opposing bubbles. But the hourglasses are at slightly different angles. This suggests that the originating structure was spinning, and this requires a protracted process. Whatever happened, it was not just a single explosion. The ejection must have taken some time. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/239791/original/file-20181008-72100-1tqlccm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/239791/original/file-20181008-72100-1tqlccm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=325&fit=crop&dpr=1 600w, https://images.theconversation.com/files/239791/original/file-20181008-72100-1tqlccm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=325&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/239791/original/file-20181008-72100-1tqlccm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=325&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/239791/original/file-20181008-72100-1tqlccm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=408&fit=crop&dpr=1 754w, https://images.theconversation.com/files/239791/original/file-20181008-72100-1tqlccm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=408&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/239791/original/file-20181008-72100-1tqlccm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=408&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Dust rings seen by ALMA.</span>
<span class="attribution"><a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>But if it wasn’t an explosion, what happened? The alternative to a stellar explosion is a collision between two stars. These are rare events which have caught much attention in recent years. In 2008, a collision <a href="https://www.aanda.org/articles/aa/abs/2011/04/aa16221-10/aa16221-10.html">was caught near the centre of our galaxy</a>. The colliding stars circled each other closely, before finally merging.</p>
<p>During the event, the stars became 100 times brighter than before, and over the next two years they faded again. A similar event may have happened in the year 2000, when a star called <a href="https://en.wikipedia.org/wiki/V838_Monocerotis">V838 Mon</a> suddenly brightened and then slowly faded. </p>
<p>CK Vul could be the result of a merger between two normal stars. But this didn’t seem to fit. Luckily, though, there is a complete zoo of possible collisions, as stars come in many types. We have now worked out that two stars from the opposite side of the stellar spectrum could have produced the pattern seen in the sky. </p>
<p>The main actor would have been a <a href="https://imagine.gsfc.nasa.gov/science/objects/dwarfs2.html">white dwarf</a>, a dense remnant left after a star like the sun reaches the end of its life. The supporting actor would have been a <a href="https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question62.html">brown dwarf</a>, an object in the twilight zone between stars and planets: too light to produce the hydrogen fusion which normally takes place at the centre of a stars, but too heavy to be a planet. They are 10 to 80 times heavier than Jupiter. Brown dwarfs are probably quite common, but they are hard to find because they are so faint. </p>
<p>A collision between a white dwarf and a brown dwarf would be spectacular. The brown dwarf would be shredded by the much heavier and denser white dwarf. Some of the shredded dwarf would rain down on the white dwarf and provide the fuel for a thermonuclear reaction. The rest of the brown dwarf would be swept up in the debris from the outburst. </p>
<p>Unlike a normal star, white dwarfs can be extremely faint, and after the merger and thermonuclear explosion, would eventually have returned to this brightness. The remaining dust shells may also have contributed, making it opaque to visible light. A merger of normal stars would have left a star of normal luminosity, and even if obscured could still have been seen in the infrared.</p>
<p>Is this what actually happened? We have made a plausible model but further tests would be required to produce conclusive evidence. For example, would this collision provide the right conditions to form radioactive aluminium? Upcoming observations could look at the details of the innermost region of the hourglass structure to find out.</p>
<p>Our discovery represents the first ever detection of a collision between a white and a brown dwarf. Once confirmed, we can use it to look for other events like it. Astronomy is an adventure: a beautiful mix of physics and discovery. We are still learning.</p><img src="https://counter.theconversation.com/content/104609/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Albert Zijlstra receives funding from the UK Science and Technology Facility Council (STFC) </span></em></p>The ‘oldest known nova’ (a star explosion) in the sky was actually not a nova, astronomers show.Albert Zijlstra, Professor of Astrophysics, University of ManchesterLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/495202015-10-21T17:03:07Z2015-10-21T17:03:07ZDead star demolishes planet – offering a glimpse into how the Earth could end its days<figure><img src="https://images.theconversation.com/files/99158/original/image-20151021-15449-1aur6ac.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A disintergating asteroid caught in the gravitational pull of a white dwarf star: could this be the future fate of the Earth?</span> <span class="attribution"><span class="source">Mark A. Garlick </span></span></figcaption></figure><p>Astronomers have made <a href="http://nature.com/articles/doi:10.1038/nature15527">the first direct discovery</a> of a <a href="http://science.nationalgeographic.com/science/space/universe/white-dwarfs-article/">white dwarf star</a> being orbited by a disintegrating minor planet that will ultimately collide into it. The observation, made by the <a href="http://kepler.nasa.gov/">Kepler space telescope</a>, offers a glimpse into what could happen to the Earth in a few billion years as the Sun, like most stars, becomes a white dwarf.</p>
<p>The study, published in <a href="http://www.nature.com/">Nature</a>, also adds to a growing number of studies reporting that dwarf stars can have atmospheres polluted with heavy elements – in some cases the constituents of water. Knowing that planets can be the source of such contamination gives weight to a hypothesis that says water on Earth was <a href="https://theconversation.com/watery-asteroid-gobbled-up-by-a-white-dwarf-implications-for-life-18987">deposited by rocky bodies</a> from distant regions in our solar system.</p>
<h2>Strange pollution</h2>
<p>Most stars, including our Sun, will <a href="http://science.nationalgeographic.com/science/space/universe/white-dwarfs-article/">become white dwarfs</a> as they die – before going out as a black dwarf or supernova – when they have exhausted their nuclear fuel. Our standard model for white dwarfs would predict white dwarfs to have no elements heavier than helium in the atmosphere, but a growing number of measurements of white dwarf atmospheres show the <a href="https://theconversation.com/polluted-dwarf-star-could-hold-the-key-to-the-origin-of-water-on-earth-41468">presence of heavier elements</a> such as oxygen, magnesium, silicon and iron. This is surprising because the star’s strong surface gravity is expected to cause heavy elements to sink quickly to the centre – leaving simple atmospheres of hydrogen and helium. </p>
<p>One explanation for this atmospheric pollution is that rocky bodies with similar properties to those in our solar system collide with a white dwarf and replenish its atmosphere with new heavy elements, including water. However, until now, there was no direct evidence that such rocky bodies exist or could fall onto a white dwarf. </p>
<p>The authors of the new study discovered the planet by noticing a dip in the brightness – a transit-like signal – from the white dwarf <a href="http://simbad.u-strasbg.fr/simbad/sim-id?Ident=WD+1145%2B017">WD 1145+017</a> (also known as EPIC 201563164). They also detected similar but weaker signals – all with periods ranging between 4.5 and five hours. Using additional data from a range of Earth-based telescopes, they interpreted these dips as times when low-mass objects orbit in front of the white dwarf, blocking out some of its light. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/99174/original/image-20151021-15410-17049wm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/99174/original/image-20151021-15410-17049wm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=328&fit=crop&dpr=1 600w, https://images.theconversation.com/files/99174/original/image-20151021-15410-17049wm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=328&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/99174/original/image-20151021-15410-17049wm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=328&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/99174/original/image-20151021-15410-17049wm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=412&fit=crop&dpr=1 754w, https://images.theconversation.com/files/99174/original/image-20151021-15410-17049wm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=412&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/99174/original/image-20151021-15410-17049wm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=412&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Kepler and the area it is investigating.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/nasablueshift/4797426132">NASA Blueshift/Flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Although the behaviour of the dips with time is complex, the team thinks that at least six minor planetary objects with masses comparable to or smaller than <a href="https://theconversation.com/dawn-breaks-over-distant-ceres-and-perhaps-reveals-signs-of-habitability-38967">dwarf planet Ceres</a> orbit the WD 1145+017 white dwarf every 4.5 or 4.9 hours. The bodies appear to be rocky with <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/dens.html">densities</a> greater than Pluto – at least two grammes per cubic centimetre. They also have comet-like dust tails produced when the incident radiation from the white dwarf heats up their surfaces, causing minerals such as <a href="http://www.minerals.net/mineral/orthoclase.aspx">orthoclase</a> and <a href="fayalite">fayalite</a> to form streaming metal vapours.</p>
<p>The astronomers also analysed the light from the star itself through <a href="https://theconversation.com/explainer-seeing-the-universe-through-spectroscopic-eyes-37759">spectroscopy</a> and could confirm that it is indeed polluted with elements including magnesium, aluminium, silicon, calcium, iron and nickel. These elements most likely ended up there in the past million years – very recently given the dwarf formed 175 million years ago.</p>
<h2>What does it mean for the Earth?</h2>
<p>When our own Sun dies it will initially expand to become <a href="http://www.universetoday.com/18847/life-of-the-sun/">a huge red-giant star</a> and engulf Mercury and Venus. Whether it will expand to reach Earth is still a <a href="http://www.scientificamerican.com/article/the-sun-will-eventually-engulf-earth-maybe/">matter of debate</a>. When its nuclear fuel is exhausted gravity will then cause the Sun to shrink down to about the size of the Earth itself but with a density so high that a teaspoon worth of this star would <a href="http://www.nasa.gov/topics/universe/features/whitedwarf_pulsar.html">have a mass</a> of nearly 15 tons. This evolutionary cycle could perturb the orbits of other planets in the solar system, increasing the chance that they would collide with each other and ultimately disintegrate and fall into the Sun, just like in the new study.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/99182/original/image-20151021-15421-1vurrti.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/99182/original/image-20151021-15421-1vurrti.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=453&fit=crop&dpr=1 600w, https://images.theconversation.com/files/99182/original/image-20151021-15421-1vurrti.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=453&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/99182/original/image-20151021-15421-1vurrti.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=453&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/99182/original/image-20151021-15421-1vurrti.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=570&fit=crop&dpr=1 754w, https://images.theconversation.com/files/99182/original/image-20151021-15421-1vurrti.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=570&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/99182/original/image-20151021-15421-1vurrti.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=570&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Internal structure of a Sun-like star and a red giant.</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/Giant_star#/media/File:Structure_of_Stars_%28artist%E2%80%99s_impression%29.jpg">ESO/wikipedia</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>The research could also help explain another major question in planetary science: whether the water on Earth was already present in the primordial material that formed our planet or whether it was planted here by collisions with other bodies. </p>
<p>Earlier this year another white dwarf, SDSS J1242, was found to have an atmosphere polluted with <a href="https://theconversation.com/polluted-dwarf-star-could-hold-the-key-to-the-origin-of-water-on-earth-41468">a large amount of oxygen</a>, which raised the possibility that a water-carrying asteroid might have planted it there. But it was not possible to directly detect the rocky debris that caused the pollution in this system. The new study provides compelling evidence for the connection between disintegrating rocky bodies and polluted white dwarf atmospheres.</p>
<p>Four billion years ago, the Earth and other rocky planets are thought to have been bombarded by comets and asteroids. While the asteroids in our solar system are today barren objects, the two studies indicate that they can indeed carry a number of heavy elements. Perhaps 4 billion years ago some of them contained water and perhaps even the complex organic molecules that provided the <a href="https://theconversation.com/explainer-what-philae-did-in-its-60-hours-on-comet-67p-34289">building blocks of life</a>. </p>
<p>The authors hope that future studies of WD 1145+017, and perhaps other systems, with transit spectroscopy might detect the presence of more complex molecules in the dust tails of the ill-fated debris.</p><img src="https://counter.theconversation.com/content/49520/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Carole Mundell receives funding from the Science and Technology Facilities Council, the Royal Society and the Wolfson Foundation. However, the views expressed here are her own and not those of the research council.</span></em></p>A study into a distant white dwarf could help us learn more about the future fate of the Earth – and it could be a violent one.Carole Mundell, Head of Astrophysics, University of BathLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/414682015-05-07T15:55:31Z2015-05-07T15:55:31ZPolluted dwarf star could hold the key to the origin of water on Earth<figure><img src="https://images.theconversation.com/files/80852/original/image-20150507-1210-13z4b8j.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Artist's view of a watery asteroid heading to a white dwarf star.</span> <span class="attribution"><a class="source" href="http://en.wikipedia.org/wiki/Asteroid#/media/File:Artist%27s_view_of_watery_asteroid_in_white_dwarf_star_system_GD_61.jpg">ESA/Hubble</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>Astronomers have discovered a <a href="http://mnras.oxfordjournals.org/lookup/doi/10.1093/mnras/stv701">white dwarf star</a> with a polluted atmosphere that may shed light on where the water on Earth comes from and how much water there is outside our own solar system. </p>
<p>A major question in planetary science is whether the water on Earth was already present in the primordial material that formed our planet or whether it was planted here by collisions with bodies such as <a href="https://solarsystem.nasa.gov/planets/profile.cfm?Object=Asteroids">asteroids</a>, <a href="https://solarsystem.nasa.gov/planets/profile.cfm?Object=Comets">comets</a> and <a href="https://solarsystem.nasa.gov/planets/profile.cfm?Object=Dwarf">proto-planets</a>.</p>
<h2>Oxygen in the atmospere</h2>
<p>New research by a team of British and German astronomers suggests that water delivery by collision may be common in other star systems outside our solar system. They came to this conclusion by measuring the chemical composition of the atmosphere of a white dwarf star, dubbed SDSS J1242. </p>
<p><a href="http://science.nationalgeographic.com/science/space/universe/white-dwarfs-article/">White dwarfs</a> are essentially corpses of former stars. Most low or medium-sized suns will become white dwarfs at the end of their lifetime. The strong surface gravity within these stars causes heavier elements, such as carbon and oxygen, to sink to their centres, leaving simple atmospheres of hydrogen and helium. </p>
<p>The atmosphere of SDSS J1242 is dominated by helium but the researchers also found large amounts of oxygen and hydrogen, along with rock-forming elements magnesium, silicon and iron. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/80860/original/image-20150507-1254-15h7tzz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/80860/original/image-20150507-1254-15h7tzz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/80860/original/image-20150507-1254-15h7tzz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/80860/original/image-20150507-1254-15h7tzz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/80860/original/image-20150507-1254-15h7tzz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/80860/original/image-20150507-1254-15h7tzz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/80860/original/image-20150507-1254-15h7tzz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">We still don’t know where the oceans on Earth come from.</span>
<span class="attribution"><a class="source" href="http://www.esa.int/spaceinimages/Images/2015/04/MSG-3_image_of_Earth_April_2015">ESA</a></span>
</figcaption>
</figure>
<p>The new measurements suggest SDSS J1242 has accreted at least an exatonne (which is about 10<sup>18</sup> tonnes) of material in its life time – similar to the mass of the dwarf planet Ceres in our solar system. This has happened at a mind-boggling rate of of 20,000 tonnes per second, which is higher than for any other known metal-polluted white dwarf. The large amount of oxygen suggests that nearly 40% of the mass of the planetary debris is water – possibly in the form of ice delivered by a water-rich asteroid.</p>
<p>A similar mechanism has been suggested for the origin of oceans on Earth. Four billion years ago, the Earth and other rocky planets are thought to have been bombarded by comets and asteroids, which were scattered from the asteroid belts into the path of the inner planets as the gas giants migrated outwards, delivering water and possibly the complex organic molecules that <a href="https://theconversation.com/explainer-what-philae-did-in-its-60-hours-on-comet-67p-34289">provided the building blocks of life</a>. </p>
<h2>Prime candidates</h2>
<p>Comets are known to contain water and, for some time, seemed the most promising candidates for transferring water to Earth. However, a growing body of measurements has suggested the water in comets are of <a href="https://theconversation.com/rosetta-is-making-a-splash-again-but-results-show-comets-water-not-the-same-as-earths-35411">a different kind to that found on Earth</a>. This is because water on comets contain more deuterium – a heavy isotope of hydrogen – than water on Earth. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/80858/original/image-20150507-1258-1fng7q2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/80858/original/image-20150507-1258-1fng7q2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=599&fit=crop&dpr=1 600w, https://images.theconversation.com/files/80858/original/image-20150507-1258-1fng7q2.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=599&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/80858/original/image-20150507-1258-1fng7q2.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=599&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/80858/original/image-20150507-1258-1fng7q2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=753&fit=crop&dpr=1 754w, https://images.theconversation.com/files/80858/original/image-20150507-1258-1fng7q2.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=753&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/80858/original/image-20150507-1258-1fng7q2.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=753&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Comet 67P/Churyumov-Gerasimenko has the wrong kind of water.</span>
<span class="attribution"><a class="source" href="http://www.esa.int/spaceinimages/Images/2015/05/Comet_on_26_April_2015_NavCam">ESA/Rosetta/NavCam</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>The record holder was the water in comet 67P/Churyumov–Gerasimenko, measured by the Rosetta spacecraft in 2014, which has a <a href="http://www.sciencemag.org/content/347/6220/1261952">deuterium level 3.4 times higher</a> than that of water on Earth. Researchers therefore now pin their hopes on asteroids. While they are today dry, barren objects, they have a similar chemistry to the Earth and may have contained much more water when the solar system formed.</p>
<p>The discovery of asteroid-donated water in white dwarf SDSS J1242 appears to add weight to the hypothesis. But, as the authors of this study emphasise, if the amount of carbon in J1242 turns out to be the same as that in our sun, all of the oxygen detected could have been delivered in the form of carbon dioxide rather than water. While the authors argue this is unlikely, higher quality observations at optical and ultraviolet wavelengths could provide a definitive answer. </p>
<p>Future observations of other planetary systems and more detailed study of polluted white dwarfs will be important in establishing the role of asteroids as a source of water – and perhaps life – on Earth and other worlds.</p><img src="https://counter.theconversation.com/content/41468/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Carole Mundell receives funding from the Science and Technology Facilities Council, the Royal Society and the Wolfson Foundation. However, the views expressed here are her own and not those of the research council.</span></em></p>The discovery of a white dwarf star with hydrogen and oxygen in its atmosphere suggests water could be planted on stars and planets by bodies like asteroids.Carole Mundell, Head of Astrophysics, University of BathLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/372722015-02-09T19:17:01Z2015-02-09T19:17:01ZWhite dwarf merger is set to prove supernova theory<figure><img src="https://images.theconversation.com/files/71399/original/image-20150208-28615-dpzggf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Artist’s impression of two white dwarf stars destined to merge and create a Type Ia supernova in 700-million years time.</span> <span class="attribution"><a class="source" href="http://www.eso.org/public/australia/images/eso1505a/">ESO/L. Calçada</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Two white dwarfs found orbiting each other at the centre of a planetary nebula are now known to have enough mass that they will eventually trigger a special kind of supernova, according to research published today in <a href="http://www.eso.org/public/archives/releases/sciencepapers/eso1505/eso1505a.pdf">Nature</a>. </p>
<p>Finding a stellar pair in a planetary nebula is not too surprising. In fact the study, led by Miguel Santander-Garcia from the <a href="http://www.fomento.es/MFOM/LANG_CASTELLANO/DIRECCIONES_GENERALES/INSTITUTO_GEOGRAFICO/Astronomia/">Observatorio Astronomico Nacional</a>, Spain, was aimed at investigating asymmetric planetary nebulae to test whether their odd-shapes could be caused by the presence of two central stars.</p>
<p>Taking just four hours to orbit each other, the stars are close enough that within the next 700 million years the two are set to merge and combine their mass into a single star.</p>
<p>But what sets this discovery apart is that the combined mass of the stars is at least 1.5 solar masses, and more likely even higher at 1.76 solar masses. This value exceeds the maximum mass that can be contained in a white dwarf and remain stable. </p>
<p>Hence, when these two stars merge, the new star must collapse, triggering what is categorised by astronomers as a <a href="http://astronomy.swin.edu.au/cosmos/T/Type+Ia+supernova">Type Ia supernova</a>.</p>
<h2>Burnt out suns</h2>
<p>The stellar pair was found in the planetary nebula Henize 2-428 from observations taken with the European Southern Observatory’s <a href="http://www.eso.org/public/australia/teles-instr/vlt/">Very Large Telescope</a> at the Paranal Observatory in Chile. </p>
<p>They are the remains of stars like our sun. As old age approaches, an average-sized star will puff off its outer layers sending out intricate shells of gas. </p>
<p>What is left behind is called a <a href="https://theconversation.com/a-stellar-mid-life-crisis-why-do-some-cluster-stars-die-early-14598">white dwarf</a>. It lights up the gas surrounding it to create the beautiful <a href="http://hubblesite.org/gallery/album/nebula/planetary/">planetary nebula</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/71400/original/image-20150208-28605-1cwhil0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/71400/original/image-20150208-28605-1cwhil0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/71400/original/image-20150208-28605-1cwhil0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=565&fit=crop&dpr=1 600w, https://images.theconversation.com/files/71400/original/image-20150208-28605-1cwhil0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=565&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/71400/original/image-20150208-28605-1cwhil0.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=565&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/71400/original/image-20150208-28605-1cwhil0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=710&fit=crop&dpr=1 754w, https://images.theconversation.com/files/71400/original/image-20150208-28605-1cwhil0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=710&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/71400/original/image-20150208-28605-1cwhil0.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=710&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The strangely shaped planetary nebula Henize 2-428. The light from the two white dwarfs is combined to form the bright central star.</span>
<span class="attribution"><span class="source">ESO</span></span>
</figcaption>
</figure>
<p>A white dwarf is highly compressed and very dense. It has shrunk down to about the size of the Earth (for comparison, the sun’s diameter is currently more than 100 times that of the Earth).</p>
<p>The star is prevented from collapsing any further by pressure produced by what is known as <a href="http://astronomy.swin.edu.au/cosmos/E/Electron+Degeneracy+Pressure">electron degeneracy</a>. In other words, the star is so compressed that the electrons themselves cannot find the space to move. This pressure balances the star’s inward pull of gravity.</p>
<h2>Tipping point</h2>
<p>But there is a limit to how much mass a white dwarf can contain and remain supported by electron degeneracy.</p>
<p>That limit is 1.4 solar masses, and it was determined by Indian-born American astrophysicist <a href="http://www.britannica.com/EBchecked/topic/105462/Subrahmanyan-Chandrasekhar">Subrahmanyan Chandrasekhar</a> in 1930, and is now known as the <a href="http://astronomy.swin.edu.au/cosmos/C/Chandrasekhar+Limit">Chandrasekhar-limit</a>. </p>
<p>It was met with some <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2012/01/the-chandrasekhar-limit-the-threshold-that-makes-life-possible/">controversy at the time</a>. For it meant that if a star tipped over this limit then there was no way to prevent the star from completely collapsing in on itself. </p>
<p>This made it inevitable that bizarre objects such as <a href="http://astronomy.swin.edu.au/cosmos/N/Neutron+Star">neutron stars</a> and <a href="https://theconversation.com/explainer-black-holes-7431">black holes</a> could actually exist.</p>
<h2>Stellar explosions</h2>
<p>Today, we have no such qualms imagining black holes and the events that trigger them. The most common is when a massive star (more than eight times the mass of our sun) can no longer sustain thermonuclear reactions, and hence it runs out of energy and is crushed by gravity.</p>
<p>The resulting supernova explosion is known as a core-collapse supernova. But a Type Ia supernova occurs when a white dwarf is pushed over the Chrandrasekhar-limit. </p>
<p>In the simplest scenario the white dwarf accretes gas from a companion star. When the limit is reached, the white dwarf explodes as a supernova, and the companion is left behind.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/TwYSzpoItic?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
</figure>
<p>But a number of Type Ia supernovae do not fit this scenario. For example, the supernova remnant <a href="http://chandra.harvard.edu/photo/2010/snr0509/">SNR 0509-67.5</a>, contains <a href="http://www.nasa.gov/mission_pages/hubble/science/supernova-source.html">no sign of a secondary star</a>. It prompted the scenario that perhaps a supernova could be triggered by two white dwarfs colliding, which would destroy any evidence of their existence.</p>
<h2>Smoking gun</h2>
<p>Astronomers have so far identified five other white dwarf pairs. Follow-up observations of the stars in Henize 2-428 with the <a href="http://www.mercator.iac.es/general/presentation">Mercator telescope</a> in the Canary Islands, Spain, determined that these are the most massive pair of white dwarfs currently known. </p>
<p>What’s more, each star is massive, containing around 0.9 solar masses. This is backed up by their similar luminosities and temperatures.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/71403/original/image-20150208-28601-b11ilg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/71403/original/image-20150208-28601-b11ilg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/71403/original/image-20150208-28601-b11ilg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=690&fit=crop&dpr=1 600w, https://images.theconversation.com/files/71403/original/image-20150208-28601-b11ilg.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=690&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/71403/original/image-20150208-28601-b11ilg.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=690&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/71403/original/image-20150208-28601-b11ilg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=867&fit=crop&dpr=1 754w, https://images.theconversation.com/files/71403/original/image-20150208-28601-b11ilg.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=867&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/71403/original/image-20150208-28601-b11ilg.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=867&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Has the mystery of SNR 0509-67.5 in the Large Magellanic Cloud now been solved?</span>
<span class="attribution"><span class="source">NASA, ESA, CXC, SAO, B. Schaefer and A. Pagnotta, Hubble Heritage, J. Hughes</span></span>
</figcaption>
</figure>
<p>These observations give us confidence that white dwarf mergers should now be considered more than just theoretical. </p>
<h2>Brightest supernova explores dark energy</h2>
<p>Type Ia supernovae are the brightest of all supernovae. Such a supernova can outshine its entire galaxy and is easily seen across vast distances.</p>
<p>Furthermore, because they rely on the demise of a white dwarf, the flash of the explosion produces a burst of light that can be carefully determined.</p>
<p>It is these characteristics that make Type Ia supernovae so important for <a href="http://hubblesite.org/hubble_discoveries/dark_energy/de-type_ia_supernovae.php">measuring cosmic distances</a> and they have been crucial in uncovering <a href="https://theconversation.com/brian-schmidt-in-conversation-8383">dark energy</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/71404/original/image-20150208-28598-o86an4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/71404/original/image-20150208-28598-o86an4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/71404/original/image-20150208-28598-o86an4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/71404/original/image-20150208-28598-o86an4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/71404/original/image-20150208-28598-o86an4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/71404/original/image-20150208-28598-o86an4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/71404/original/image-20150208-28598-o86an4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/71404/original/image-20150208-28598-o86an4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Type Ia supernovae like SN 1994D can be seen across vast distances.</span>
<span class="attribution"><span class="source">High-Z Supernova Search Team, HST, NASA</span></span>
</figcaption>
</figure>
<p>This interesting discovery announced today not only helps improve our knowledge of how planetary nebulae obtain their structure, it could also lead to a better determination of <a href="http://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/">dark energy</a>, the mysterious force that is causing the expansion of the universe to accelerate.</p><img src="https://counter.theconversation.com/content/37272/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Tanya Hill is the Australian representative of the European Southern Observatory's Science Outreach Network.</span></em></p><p class="fine-print"><em><span>Orsola De Marco receives funding from the ARC for work in stellar astrophysics</span></em></p>Two white dwarfs found orbiting each other at the centre of a planetary nebula are now known to have enough mass that they will eventually trigger a special kind of supernova, according to research published…Tanya Hill, Honorary Fellow of the University of Melbourne and Senior Curator (Astronomy), Museums Victoria Research InstituteOrsola De Marco, Professor and ARC Future Fellow, Macquarie UniversityLicensed as Creative Commons – attribution, no derivatives.