tag:theconversation.com,2011:/au/topics/elliptical-galaxies-24333/articlesElliptical galaxies – The Conversation2021-12-26T20:24:30Ztag:theconversation.com,2011:article/1722332021-12-26T20:24:30Z2021-12-26T20:24:30ZHunting galaxies far far away – here’s how anyone can explore the universe<figure><img src="https://images.theconversation.com/files/434739/original/file-20211130-16-17byu9f.png?ixlib=rb-1.1.0&rect=58%2C2%2C1296%2C723&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption"></span> <span class="attribution"><span class="license">Author provided</span></span></figcaption></figure><p><em>This article is part of a <a href="https://theconversation.com/au/topics/how-to-guides-113946">series</a> explaining how readers can learn the skills to take part in activities that academics love doing as part of their work.</em></p>
<hr>
<p>By far my favourite thing about my job as an astronomer is those rare moments when I get to see beautiful distant galaxies, whose light left them millions to billions of years ago. It’s a combination of pure awe and scientific curiosity that excites me about “galaxy hunting”.</p>
<p>In astronomy today, much of our work is handling enormous amounts of data by writing and running programs to work with images of the sky. A downside to this is that we don’t always have that “hands-on” experience of looking at every square inch of the universe while we study it. I’m going to show you, though, how I get my fix of wonder by looking at galaxies that only a select few people will ever have seen, until now. </p>
<p>In just our <a href="https://science.nasa.gov/observable-universe">observable universe</a> we estimate there are over 2 trillion galaxies! </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/curious-kids-how-are-galaxies-formed-171907">Curious Kids: how are galaxies formed?</a>
</strong>
</em>
</p>
<hr>
<h2>Galaxies at your fingertips</h2>
<p>Only a few decades ago astronomers had to tediously examine photographic plates after a long, cold and lonely night of observing. In the 21st century we have access to information any time, anywhere via the internet. </p>
<p>Automatic telescopes and surveys now provide us with so much data we require machines to help us analyse it. In some cases human eyes will only ever look at what the computers have deemed is interesting! Massive amounts of data are hosted online, just waiting to be admired, for free. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/new-powerful-telescopes-allow-direct-imaging-of-nascent-galaxies-12-billion-light-years-away-74910">New powerful telescopes allow direct imaging of nascent galaxies 12 billion light years away</a>
</strong>
</em>
</p>
<hr>
<h2>Go online for a universe atlas</h2>
<p><a href="https://aladin.u-strasbg.fr/AladinLite/">Aladin Lite</a> is one of the greatest online tools available to look at our universe through the eyes of many different telescopes. Here we can scan the entire sky for hidden galaxies, and even decipher information about their stellar populations and evolution. </p>
<p>Let’s start our universal tour by searching for one of the most visually stunning galaxies out there, the Cartwheel Galaxy. In the Aladin interface, you can search for both the popular name of an object (like “cartwheel galaxy”) or known co-ordinates. The location will be centred in the interface. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/434683/original/file-20211130-13-16pqn2r.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/434683/original/file-20211130-13-16pqn2r.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=328&fit=crop&dpr=1 600w, https://images.theconversation.com/files/434683/original/file-20211130-13-16pqn2r.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=328&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/434683/original/file-20211130-13-16pqn2r.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=328&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/434683/original/file-20211130-13-16pqn2r.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=412&fit=crop&dpr=1 754w, https://images.theconversation.com/files/434683/original/file-20211130-13-16pqn2r.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=412&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/434683/original/file-20211130-13-16pqn2r.png?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">Online view in Aladin Lite of the Cartwheel Galaxy, a lenticular/ring galaxy 500 million light years away from Earth discovered in 1941 by iconic astronomer Fritz Zwicky.</span>
</figcaption>
</figure>
<p>The first image of the Cartwheel Galaxy we see is from optical imaging by the Digitised Sky Survey. The colours we see represent different filters from this telescope. However, these are fairly representative of what the galaxy would look like with our own eyes. </p>
<p>A general rule of thumb as an astronomer is that “colour” differences within galaxies are because of physically different environments. It’s important to note that things that look blue (shorter wavelengths) are generally hotter than things that look red (longer wavelengths). </p>
<p>In this galaxy, the outer ring appears to be more blue then the centre red section. This might hint at star formation and stellar activity happening in the outer ring, but less so in the centre.</p>
<p>To confirm our suspicions of star formation we can select to look at data from different surveys, in different wavelengths. When young stars are forming, vast amounts of UV radiation are emitted. By changing the survey to GALEXGR6/AIS, we are now looking at only UV wavelengths, and what a difference that makes! </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/434735/original/file-20211130-17-h8oqro.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/434735/original/file-20211130-17-h8oqro.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=337&fit=crop&dpr=1 600w, https://images.theconversation.com/files/434735/original/file-20211130-17-h8oqro.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=337&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/434735/original/file-20211130-17-h8oqro.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=337&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/434735/original/file-20211130-17-h8oqro.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/434735/original/file-20211130-17-h8oqro.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/434735/original/file-20211130-17-h8oqro.png?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">Online view in Aladin Lite of the Cartwheel Galaxy in GALEX UV wavelengths.</span>
</figcaption>
</figure>
<p>The whole centre section of the galaxy seems to “disappear” from our image. This suggests that section is likely home to older stars, with less active <a href="https://link.springer.com/chapter/10.1007%2F978-1-4612-2232-3_15">stellar nurseries</a>. </p>
<p>Aladin is home to 20 different surveys. They provide imaging of the sky from optical, UV, infrared, X and gamma rays. </p>
<p>When I am wandering the universe looking for interesting galaxies here, I generally start out in optical and find ones that look interesting to me. I then use the different surveys to see how the images change when looking at specific wavelengths. </p>
<h2>Universal Where’s Wally</h2>
<p>Now you’ve had a crash course in galaxy hunting, let the game begin! You can spend hours exploring the incredible images and finding interesting-looking galaxies. I recommend looking at images from DECalS/DR3 for the highest resolution and detail when zooming further in. </p>
<p>The best method is to just drag the sky atlas around. If you find something interesting, you can find out any information we have on it by selecting the target icon and clicking on the object.</p>
<p>To help you on your galactic expedition here are my favourite finds of the different types of objects you might see. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/434733/original/file-20211130-20-7ub9nq.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/434733/original/file-20211130-20-7ub9nq.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/434733/original/file-20211130-20-7ub9nq.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/434733/original/file-20211130-20-7ub9nq.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/434733/original/file-20211130-20-7ub9nq.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/434733/original/file-20211130-20-7ub9nq.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/434733/original/file-20211130-20-7ub9nq.png?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">Examples of spiral galaxies found using Aladin online. Spirals are the most iconic galaxy shape and include many of the brightest galaxies in the nearby universe, like the Andromeda Galaxy.</span>
</figcaption>
</figure>
<p><strong><a href="https://astronomy.swin.edu.au/cosmos/S/spiral+galaxy">Spiral galaxies</a></strong> typically have a central rotating disc with large spiral “arms” curving out from the denser central regions. They are incredibly beautiful. Our own Milky Way is a spiral galaxy. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/is-our-milky-way-galaxy-a-zombie-already-dead-and-we-dont-know-it-52732">Is our Milky Way galaxy a zombie, already dead and we don't know it?</a>
</strong>
</em>
</p>
<hr>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/434737/original/file-20211130-15-1sgwvgj.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/434737/original/file-20211130-15-1sgwvgj.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=286&fit=crop&dpr=1 600w, https://images.theconversation.com/files/434737/original/file-20211130-15-1sgwvgj.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=286&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/434737/original/file-20211130-15-1sgwvgj.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=286&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/434737/original/file-20211130-15-1sgwvgj.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=359&fit=crop&dpr=1 754w, https://images.theconversation.com/files/434737/original/file-20211130-15-1sgwvgj.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=359&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/434737/original/file-20211130-15-1sgwvgj.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=359&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Examples of elliptical galaxies. This type of galaxy has an approximately ellipsoidal shape and a smooth, nearly featureless image.</span>
</figcaption>
</figure>
<p><strong><a href="https://astronomy.swin.edu.au/cosmos/E/Elliptical+Galaxy">Elliptical galaxies</a></strong> are largely featureless and less “flat” then spirals, with stars occupying almost a 3D ellipse at times. These type of galaxies tend to have older stars and less active star-forming regions compared to spiral galaxies.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/434741/original/file-20211130-13-339x0w.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/434741/original/file-20211130-13-339x0w.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=273&fit=crop&dpr=1 600w, https://images.theconversation.com/files/434741/original/file-20211130-13-339x0w.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=273&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/434741/original/file-20211130-13-339x0w.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=273&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/434741/original/file-20211130-13-339x0w.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=342&fit=crop&dpr=1 754w, https://images.theconversation.com/files/434741/original/file-20211130-13-339x0w.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=342&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/434741/original/file-20211130-13-339x0w.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=342&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Examples of lenticular galaxies. These are a type of galaxy intermediate between elliptical and a spiral galaxies.</span>
</figcaption>
</figure>
<p><strong><a href="https://astronomy.swin.edu.au/cosmos/L/Lenticular+Galaxy">Lenticular galaxies</a></strong> appear like cosmic pancakes, fairly flat and featureless in the night sky. These galaxies can be thought of as the “in between” of spiral and elliptical galaxies. The majority of star formation has stopped but lenticular galaxies can still have significant amounts of dust in them. </p>
<p>There are also other amazing types of galaxies, including <a href="https://pweb.cfa.harvard.edu/research/topic/galaxies-merging-and-interacting">mergers</a> and <a href="https://public.nrao.edu/gallery/what-is-a-lensed-galaxy/">lenses</a>, which are just waiting for you to find them. I’d love to see what amazing things you find over on Twitter at @sarawebbscience. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/5-ways-families-can-enjoy-astronomy-during-the-pandemic-144647">5 ways families can enjoy astronomy during the pandemic</a>
</strong>
</em>
</p>
<hr>
<p><em>You can read other articles in this series <a href="https://theconversation.com/au/topics/how-to-guides-113946">here</a>.</em></p><img src="https://counter.theconversation.com/content/172233/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Sara Webb 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>Anyone with an internet connection can search the universe and possibly discover never-before-seen galaxies.Sara Webb, Postdoctoral Research Fellow, Centre for Astrophysics and Supercomputing, Swinburne University of TechnologyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1627592021-06-22T20:02:02Z2021-06-22T20:02:02ZTo find out how galaxies grow, we’re zooming in on the night sky and capturing cosmic explosions<figure><img src="https://images.theconversation.com/files/407605/original/file-20210622-28-12cigvu.png?ixlib=rb-1.1.0&rect=16%2C0%2C1338%2C725&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Sara Webb</span>, <span class="license">Author provided</span></span></figcaption></figure><p><em>Scientists see flora, fauna and phenomena the rest of us rarely do. In this series, we’ve invited them to share their unique <a href="https://theconversation.com/au/topics/photos-from-the-field-92499">photos</a>.</em></p>
<hr>
<p>Across Australia, astronomers are using cutting-edge technologies to capture the night sky, hoping to eventually tackle some of our biggest questions about the universe.</p>
<p>As we and our colleagues delve deeper into the cosmos, looking for cosmic explosions, our observations are helping shed light on longstanding mysteries — and making way for entirely new paths of inquiry. </p>
<h2>Cosmic eruptions fill the sky</h2>
<p>Swinburne’s Deeper, Wider, Faster (DWF) program — which one of us (Sara Webb) worked on throughout her PhD — was developed to hunt for the fastest and most mysterious explosions in the universe.</p>
<p>But to understand what causes cosmic explosions, we must “look” at these events with multiple eyes, through different telescopes around the world. Today we’ll take you on a journey using data from one of these telescopes, the Blanco 4m, at Chile’s Cerro Tololo Inter-American Observatory.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/406404/original/file-20210615-3629-3u495x.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/406404/original/file-20210615-3629-3u495x.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=380&fit=crop&dpr=1 600w, https://images.theconversation.com/files/406404/original/file-20210615-3629-3u495x.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=380&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/406404/original/file-20210615-3629-3u495x.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=380&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/406404/original/file-20210615-3629-3u495x.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=478&fit=crop&dpr=1 754w, https://images.theconversation.com/files/406404/original/file-20210615-3629-3u495x.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=478&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/406404/original/file-20210615-3629-3u495x.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=478&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The Blanco 4m telescope in Chile. This telescope has a dark energy camera attached to it.</span>
<span class="attribution"><span class="source">Rebecca Allen</span></span>
</figcaption>
</figure>
<p>First, all 60+ individual images taken of the field of view from this telescope are combined into a mosaic. Within them we see the thousands of bright sources. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/406470/original/file-20210615-3598-1klpd3k.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/406470/original/file-20210615-3598-1klpd3k.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/406470/original/file-20210615-3598-1klpd3k.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/406470/original/file-20210615-3598-1klpd3k.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/406470/original/file-20210615-3598-1klpd3k.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/406470/original/file-20210615-3598-1klpd3k.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/406470/original/file-20210615-3598-1klpd3k.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/406470/original/file-20210615-3598-1klpd3k.png?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">This is an example of dark energy camera data taken by the DWF program. This image is of an enormous section of the sky.</span>
<span class="attribution"><span class="source">Sara Webb</span></span>
</figcaption>
</figure>
<p>These images are transferred across the Pacific to be processed on Swinburne’s OzStar supercomputer — which is more powerful than 10,000 personal laptops and can handle thousands of different jobs at once.</p>
<p>Once uploaded, the images are broken down into smaller chunks. This is when we start to see details.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/406480/original/file-20210615-3738-690lbf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/406480/original/file-20210615-3738-690lbf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/406480/original/file-20210615-3738-690lbf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/406480/original/file-20210615-3738-690lbf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/406480/original/file-20210615-3738-690lbf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/406480/original/file-20210615-3738-690lbf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/406480/original/file-20210615-3738-690lbf.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">Pictured are some of the galaxies visible within smaller cutouts of data sent to the DWF program from the Blanco 4m.</span>
<span class="attribution"><span class="source">Sara Webb</span></span>
</figcaption>
</figure>
<p>But the galaxies above, spectacular as they are, still aren’t what we’re looking for. We want to capture new “sources” resulting from dying stars and cosmic explosions, which we can identify by having our computers search for light in places it wasn’t previously detected.</p>
<p>A source could be many different things including a flaring star, a dying star or an asteroid. To find out we have to collect continuous information about its brightness and the different wavelengths of light it emits, such as radio, x-ray, gamma-ray and so forth. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/406500/original/file-20210615-3582-t27jos.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/406500/original/file-20210615-3582-t27jos.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=254&fit=crop&dpr=1 600w, https://images.theconversation.com/files/406500/original/file-20210615-3582-t27jos.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=254&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/406500/original/file-20210615-3582-t27jos.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=254&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/406500/original/file-20210615-3582-t27jos.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=319&fit=crop&dpr=1 754w, https://images.theconversation.com/files/406500/original/file-20210615-3582-t27jos.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=319&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/406500/original/file-20210615-3582-t27jos.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=319&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">To the left is an old image of a patch of sky and to the right is a updated image with a new source having just occurred. This one is likely a flare star or an asteroid.</span>
<span class="attribution"><span class="source">Sara Webb</span></span>
</figcaption>
</figure>
<p>Once we spot a source, we monitor changes in its brightness over the coming hours and days. If we think it may represent a rare cosmic explosions, we trigger other telescopes to collect additional data. </p>
<h2>Peering into the distant past</h2>
<p>Galaxies are vast collections of stars, gas, dust and dark matter. They vary in shape, size and colour, but the two main types we see in the universe today are blue spirals and red ellipticals. But how do they form? And why are there different types? </p>
<p>Astronomers know the shapes and colours of a galaxy are linked to its evolution, but they’re still trying figure out exactly which shapes and colours are linked to specific growth pathways.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/weve-mapped-a-million-previously-undiscovered-galaxies-beyond-the-milky-way-take-the-virtual-tour-here-148442">We've mapped a million previously undiscovered galaxies beyond the Milky Way. Take the virtual tour here.</a>
</strong>
</em>
</p>
<hr>
<p>We think galaxies grow in size and mass through two main channels. They produce stars when their vast hydrogen clouds collapse under gravity. As more gas is transformed into stars, they grow in size. </p>
<p>Thanks to space-based technology such as the Hubble Space Telescope and powerful on-ground telescopes, astronomers can now peer back in time to study galaxy growth over the history of the universe.</p>
<p>This is possible since the further away a galaxy is, the longer its light travelled to reach us. Because the speed of light is constant, we can determine when the light was emitted — as long as we know the galaxy’s distance from Earth (called its “redshift”).</p>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/407571/original/file-20210622-15-1gzt54p.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/407571/original/file-20210622-15-1gzt54p.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=595&fit=crop&dpr=1 600w, https://images.theconversation.com/files/407571/original/file-20210622-15-1gzt54p.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=595&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/407571/original/file-20210622-15-1gzt54p.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=595&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/407571/original/file-20210622-15-1gzt54p.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=748&fit=crop&dpr=1 754w, https://images.theconversation.com/files/407571/original/file-20210622-15-1gzt54p.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=748&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/407571/original/file-20210622-15-1gzt54p.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=748&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A selection of distant galaxies spotted in my study of galaxy growth over time. These appear very different to nearby galaxies.</span>
<span class="attribution"><span class="source">Rebecca Allen</span></span>
</figcaption>
</figure>
<p>I measured this growth as part of my PhD, by taking images of galaxies that exist at different redshifts from as far back as when the universe was only one billion years old, and comparing their sizes. </p>
<h2>When galaxies merge</h2>
<p>Looking around the universe today, we mostly see galaxies clustered together. Astronomers believe the nature of a galaxy’s surroundings or its environment can affect its growth pathways, similar to how people in large cities can access more resources than those in rural areas.</p>
<p>When many galaxies are grouped together they may interact. And this interaction can stimulate bursts of star formation within a particular galaxy. </p>
<p>That said, this growth spurt may be short-lived, as gas and stars can be stripped away through the gravitational interaction between multiple galaxies, thereby limiting future star formation and growth in a single galaxy. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/406407/original/file-20210615-21-kqv2zq.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/406407/original/file-20210615-21-kqv2zq.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/406407/original/file-20210615-21-kqv2zq.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/406407/original/file-20210615-21-kqv2zq.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/406407/original/file-20210615-21-kqv2zq.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/406407/original/file-20210615-21-kqv2zq.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/406407/original/file-20210615-21-kqv2zq.jpeg?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">
<figcaption>
<span class="caption">This image was captured using the Hubble Space Telescope. It shows a group of spiral galaxies, which astronomers can clearly determine due to the high resolution of the image.</span>
<span class="attribution"><span class="source">Rebecca Allen</span></span>
</figcaption>
</figure>
<p>But even if a galaxy can’t form stars, it can still grow by merging with or consuming smaller galaxies. For example, the Milky Way will one day consume the smaller Magellanic clouds, which are dwarf galaxies. It will also merge with the slightly larger Andromeda galaxy one day, to form one giant galaxy.<br>
Yet, while many studies have been conducted unpack galaxy evolution, we still can’t say all our questions have been answered. </p>
<p>It took billions of years for the galaxy clusters we observe today to form. But if astronomers can leverage the latest technologies and peer further into the distance than ever before, we will hopefully gain clues about how a galaxy’s environment can impact its growth.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/406599/original/file-20210616-3839-td4xlq.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/406599/original/file-20210616-3839-td4xlq.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=616&fit=crop&dpr=1 600w, https://images.theconversation.com/files/406599/original/file-20210616-3839-td4xlq.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=616&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/406599/original/file-20210616-3839-td4xlq.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=616&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/406599/original/file-20210616-3839-td4xlq.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=774&fit=crop&dpr=1 754w, https://images.theconversation.com/files/406599/original/file-20210616-3839-td4xlq.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=774&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/406599/original/file-20210616-3839-td4xlq.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=774&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Pictured are two groups of distant galaxies that existed when the universe was one-quarter of its current age. These galaxy groups will eventually come together and form a structure similar to the Virgo cluster. I have studied them both to learn more about how the galaxies within them are growing.</span>
<span class="attribution"><span class="source">Rebecca Allen</span></span>
</figcaption>
</figure>
<h2>The bending of spacetime reveals secrets</h2>
<p>With decades of observations and millions of galaxies captured in surveys, experts have many theories regarding how galaxies form, and how the universe evolves. This field is called cosmology.</p>
<p>Thanks to Albert Einstein, we know the gravitational force of massive objects in space causes space to bend. This has been observed through a phenomena known as “lensing”, where vast amounts of matter are concentrated in one area within objects such as black holes, galaxies or galaxy clusters. </p>
<p>Their gravity distorts spacetime, acting as a giant lens to reveal warped images of more distant objects behind them. Using lensing, astronomers have developed ways to find and study distant galaxies that would otherwise be hidden from view. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/406597/original/file-20210615-13-1p74hbo.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/406597/original/file-20210615-13-1p74hbo.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/406597/original/file-20210615-13-1p74hbo.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/406597/original/file-20210615-13-1p74hbo.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/406597/original/file-20210615-13-1p74hbo.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/406597/original/file-20210615-13-1p74hbo.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/406597/original/file-20210615-13-1p74hbo.png?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">A set of galaxy-galaxy lenses. The massive foreground galaxy’s gravity distorts spacetime, acting as a lens that reveals a warped image of a distant background galaxy.</span>
<span class="attribution"><span class="source">Rebecca Allen</span></span>
</figcaption>
</figure>
<p>These observations continue to drive our understanding of galaxy evolution. They’re challenging our theories of when and how galaxies form and grow. </p>
<p>One 2018 discovery made by a group of researchers, including myself, revealed a set of massive and already evolved galaxies from when the universe was only about one-sixth of its current age. They would have had to form and grow at an extremely rapidly to fit our current models of galaxy growth.</p>
<p>In an upcoming investigation, Swinburne Professor Karl Glazebrook will lead me and my team to become some of the first astronomers granted access to Nasa’s James Webb Space Telescope to study these early galaxies.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/407588/original/file-20210622-21-6yk933.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/407588/original/file-20210622-21-6yk933.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/407588/original/file-20210622-21-6yk933.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=364&fit=crop&dpr=1 600w, https://images.theconversation.com/files/407588/original/file-20210622-21-6yk933.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=364&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/407588/original/file-20210622-21-6yk933.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=364&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/407588/original/file-20210622-21-6yk933.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=458&fit=crop&dpr=1 754w, https://images.theconversation.com/files/407588/original/file-20210622-21-6yk933.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=458&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/407588/original/file-20210622-21-6yk933.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=458&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">One of the massive quiescent galaxies which our team will investigate. While extremely large, its older stars and distance make it appear as a tiny red nugget among the much brighter and closer galaxies.</span>
<span class="attribution"><span class="source">Rebecca Allen</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure><img src="https://counter.theconversation.com/content/162759/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>More than 60 images capturing huge expanses of sky are sent to us from Chile. Within them we can see thousands of bright spots. What do we find when we look closer?Rebecca Allen, Coordinator Swinburne Astronomy Online | Program Lead of Microgravity Experimentation, Space Technology and Industry Institute, Swinburne University of TechnologySara Webb, PhD candidate in Astrophysics, Swinburne University of TechnologyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1531182021-01-12T16:23:54Z2021-01-12T16:23:54ZGalaxies eject gas when they merge, preventing new stars forming – new research<figure><img src="https://images.theconversation.com/files/378284/original/file-20210112-21-1vo1ilm.jpg?ixlib=rb-1.1.0&rect=264%2C189%2C2519%2C1368&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Elliptical galaxies are filled with extremely old stars.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/m31-andromeda-galaxy-satellites-m32-m110-30317167">Igor Chekalin/Shutterstock.com</a></span></figcaption></figure><p>Most stars in the universe today are found in massive galaxies called ellipticals, named for their stretched-out-circle shape. Unlike our own galaxy, which is a spiral with arms extending out from the centre, the edges of elliptical galaxies are smooth.</p>
<p>At first glance, these galaxies might seem like simple systems. However, they are among the most mysterious objects in the cosmos. Elliptical galaxies host <a href="https://academic.oup.com/mnras/article/404/4/1775/1081595">extremely old stars</a> and are not forming new stars.</p>
<p>Exactly how these dead galaxies form is a question both observational and theoretical astronomers have tried to answer <a href="https://www.nature.com/articles/s41550-018-0558-1">for a long time</a>. <a href="https://www.nature.com/articles/s41550-020-01268-x">Our new study</a>, published in Nature Astronomy, could help solve this puzzle.</p>
<p><a href="https://theconversation.com/study-of-distant-galaxies-challenges-our-understanding-of-how-stars-form-90147">Stars form</a> when huge clouds of gas within galaxies collapse under gravity. Eventually, enough mass clumps together and a star is formed. But galaxies need gas for this to happen. Our team detected a galaxy, called ID2299, that is ejecting nearly half of its star-forming gas. The galaxy is throwing out the equivalent of 10,000 Suns per year in gas, removing 46% of the total cold gas the galaxy contains. </p>
<p>Because the galaxy is also forming stars very rapidly, hundreds of times faster <a href="https://iopscience.iop.org/article/10.1088/0004-637X/806/1/96/pdf">than our Milky Way</a>, the gas that remains will be quickly consumed. At the current rate, ID2299 will shut down in just a few tens of million years. This is much faster than the typical duration of star formation episodes in galaxies, which is a few billion years. </p>
<h2>Tidal tails</h2>
<p>This exceptional massive ejection is being caused by a tidal tail, produced by the galaxy’s merger with another galaxy. Tidal tails are elongated streams of stars and gas extending into the interstellar space, as a result of tidal forces caused by the interaction – like the moon’s tidal pull on Earth. </p>
<p>Tidal tails are commonly seen in nearby merging galaxies, but it is difficult to identify them in the distant universe because of their low luminosity. Luckily, even though ID2299 was observed at a time when the universe was only 4.5 billion years old (our universe is now about 14 billion years old), we were able to see this ejection when it first started happening, when these tails are usually at their brightest.</p>
<figure class="align-center ">
<img alt="Artist's impression of a galaxy with a tidal tail." src="https://images.theconversation.com/files/378249/original/file-20210112-17-s76pks.jpg?ixlib=rb-1.1.0&rect=580%2C479%2C2357%2C1754&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/378249/original/file-20210112-17-s76pks.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=382&fit=crop&dpr=1 600w, https://images.theconversation.com/files/378249/original/file-20210112-17-s76pks.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=382&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/378249/original/file-20210112-17-s76pks.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=382&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/378249/original/file-20210112-17-s76pks.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=480&fit=crop&dpr=1 754w, https://images.theconversation.com/files/378249/original/file-20210112-17-s76pks.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=480&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/378249/original/file-20210112-17-s76pks.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=480&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Some of the galaxy’s gas is being ejected as a result of a merger.</span>
<span class="attribution"><a class="source" href="https://www.eurekalert.org/multimedia/pub/253013.php?from=488893">ESO/M. Kornmesser</a>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>We discovered this exceptional galaxy by inspecting a survey of galaxies made with the <a href="https://www.almaobservatory.org/en/home/">Atacama Large Millimeter/submillimeter Array (ALMA)</a>, designed to study the properties of the cold gas in more than 100 galaxies in the distant universe.
Data from ALMA provided the spectrum of the cold, star-forming gas. The ejection was observed as a broad emission line, near the very prominent emission line associated with the galaxy. From this spectrum, we were able to measure the mass and velocity of the ejected gas. </p>
<p>This is the first time we have observed a typical massive star-forming galaxy in the distant universe about to die because of a massive cold gas ejection. Our study provides an important observational confirmation of the fact galaxies can stop forming stars as a result of gas expulsions. </p>
<figure class="align-center ">
<img alt="A simulated collision between two galaxies." src="https://images.theconversation.com/files/378286/original/file-20210112-23-f77esg.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/378286/original/file-20210112-23-f77esg.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/378286/original/file-20210112-23-f77esg.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/378286/original/file-20210112-23-f77esg.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/378286/original/file-20210112-23-f77esg.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/378286/original/file-20210112-23-f77esg.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/378286/original/file-20210112-23-f77esg.jpeg?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">
<figcaption>
<span class="caption">Part of the gas is ejected in large tails and as the galaxies get closer they merge to form a single system.</span>
<span class="attribution"><a class="source" href="https://www.eurekalert.org/multimedia/pub/253015.php?from=488893">Jeremy Fensch, et al</a></span>
</figcaption>
</figure>
<p>Simulations and previous observational results suggested gas ejections were associated with galactic winds produced either by the accretion of gas onto a <a href="https://www.nature.com/articles/s41550-017-0165">supermassive black holes</a> or intense star formation episodes. In our study, we showed that the ejection detected in ID2299 cannot be explained by a galactic wind. The results might therefore lead us to revise our understanding of how galaxies stop forming their stars.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/study-of-distant-galaxies-challenges-our-understanding-of-how-stars-form-90147">Study of distant galaxies challenges our understanding of how stars form</a>
</strong>
</em>
</p>
<hr>
<p>Our study shows that mergers have a crucial role in the evolution of galaxies because they are capable of expelling large quantities of gas from galaxies, shutting down star formation and affecting galaxy growth. </p>
<p>Future studies with deeper and higher resolution data will allow us to better understand the dynamics of the ejected gas in ID2299. Observing more ejections in other distant galaxies will also be important to understand how common these phenomena are.</p><img src="https://counter.theconversation.com/content/153118/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Annagrazia Puglisi received funding from Region Île-de-France and Incoming CEA fellowship from the CEA-Enhanced Eurotalents program, co-funded by FP7 Marie-Skłodowska-Curie COFUND program (Grant Agreement 600382). Annagrazia Puglisi also receives financial support from STFC through grants ST/T000244/1 and ST/P000541/1.</span></em></p>These results might lead us to revise our understanding of how galaxies stop forming their stars.Annagrazia Puglisi, Post Doc Research Associate in the Centre for Extragalactic Astronomy, Durham UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1446472020-08-26T12:21:40Z2020-08-26T12:21:40Z5 ways families can enjoy astronomy during the pandemic<figure><img src="https://images.theconversation.com/files/354734/original/file-20200825-22-dm4gus.jpg?ixlib=rb-1.1.0&rect=53%2C0%2C6000%2C3934&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">With the proper equipment, you can enjoy the beauty of the night sky. </span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/happy-family-are-watching-meteor-shower-nigh-sky-royalty-free-image/826077456?adppopup=true">Allexxandar via iStock/GettyImages</a></span></figcaption></figure><p>This is a challenging time for families. Schools across the U.S. are struggling to provide a meaningful online experience. The coronavirus pandemic has cut off or restricted many entertainment options. As an <a href="https://scholar.google.com/citations?user=OrRLRQ4AAAAJ&hl=en&oi=ao">astronomer</a>, I believe a great way for families to fill the void and have a meaningful science experience in the time of COVID-19 is to turn their attention to the stars they can see right outside their homes.</p>
<p>The night sky is, and always has been, safe and free.</p>
<p>Here are five ways you can get started.</p>
<h2>Naked eye</h2>
<figure class="align-left ">
<img alt="Three people point up to the night sky to look at beautiful constellations." src="https://images.theconversation.com/files/354735/original/file-20200825-14-646g1a.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/354735/original/file-20200825-14-646g1a.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=425&fit=crop&dpr=1 600w, https://images.theconversation.com/files/354735/original/file-20200825-14-646g1a.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=425&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/354735/original/file-20200825-14-646g1a.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=425&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/354735/original/file-20200825-14-646g1a.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=534&fit=crop&dpr=1 754w, https://images.theconversation.com/files/354735/original/file-20200825-14-646g1a.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=534&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/354735/original/file-20200825-14-646g1a.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=534&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">If you travel to the right places, you can witness goregous constellations at night.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/colorful-milky-way-with-silhouette-of-a-family-royalty-free-image/539946058">den-belitsky via iStock/GettyImages</a></span>
</figcaption>
</figure>
<p>You can see a lot with just your eyes. But the night sky is a strange landscape to most people. Just as you would when traveling somewhere unfamiliar, you’ll need a map. <a href="https://skyandtelescope.org/">Sky and Telescope</a> – an astronomy news publication – has a guide to get started, with <a href="https://skyandtelescope.org/wp-content/uploads/GettingStartedNorth.pdf">printable sky maps</a> for any month. The objects of the night sky migrate through a complete cycle over the course of a year.</p>
<p>As the Earth moves around the sun, different stars and constellations come into view, so you can enjoy new sights all through the year. You may find it more convenient to have a planisphere, or sky wheel, a rotating plastic disk that <a href="http://skymaps.com/store/cat04.html">shows the night sky for any date and time</a>. They can be bought for US$10-$20 online.</p>
<p>Spotting planets is trickier since they move among the stars, but there are interactive maps online that show them in the <a href="https://skyandtelescope.org/interactive-sky-chart/">night sky for any time at any location</a>. Your view of the night sky depends on your latitude, so it varies with your specific location. Even easier, there are <a href="https://www.goodhousekeeping.com/life/g26089673/best-stargazing-apps/">smartphone apps</a> that take all the work – and maybe also the fun – out of navigating the night sky.</p>
<p>Hold your phone up and the apps identify stars and overlay the constellation shapes. Some respond to voice commands and add detailed information on celestial objects or show the <a href="https://www.nasa.gov/mission_pages/station/main/index.html">International Space Station</a> as it whizzes overhead. </p>
<h2>Binoculars</h2>
<p>You probably have a pair of binoculars somewhere in your house. If you don’t or you want an upgrade, new binoculars cost anywhere from $35 to over $500, and the lower end of the range is just <a href="https://www.space.com/26021-best-binoculars.html">fine for stargazing</a>. </p>
<p>Perhaps you use them at concerts or for bird-watching. Well, they’re also <a href="https://earthsky.org/human-world/top-tips-for-using-ordinary-binoculars-for-stargazing">perfect for stargazing</a>. There are two numbers on binoculars. They represent the magnification and the lens diameter, so 7 x 50 will magnify an image by a factor of 7 using 50-millimeter lenses. At a dark location, your naked eye will see about 3000 stars. With binoculars this number goes up to 100,000.</p>
<p>The <a href="https://stardate.org/sites/default/files/pdfs/teachers/ObservingTheMoon.pdf">moon is spectacular through binoculars</a>. It’s fun for kids to track a cycle of the moon phases over a month, and then do an <a href="https://www.youtube.com/watch?v=wz01pTvuMa0&list=PLyO4---4Ijl_NS0JabRyGQJmF1iFES433&index=3&t=0s">activity that shows why the moon has phases</a>.</p>
<h2>Small telescopes</h2>
<figure class="align-right ">
<img alt="A young boy and his father view the night sky next to a telescope." src="https://images.theconversation.com/files/354643/original/file-20200825-16-15820bg.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C383%2C255&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/354643/original/file-20200825-16-15820bg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/354643/original/file-20200825-16-15820bg.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/354643/original/file-20200825-16-15820bg.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/354643/original/file-20200825-16-15820bg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=502&fit=crop&dpr=1 754w, https://images.theconversation.com/files/354643/original/file-20200825-16-15820bg.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=502&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/354643/original/file-20200825-16-15820bg.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=502&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Using telescopes one can view hundreds of stars from miles away.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/father-and-son-watching-stars-royalty-free-image/80464402">DAJ/GettyImages</a></span>
</figcaption>
</figure>
<p>If you want to make a bigger commitment to exploring the night sky, consider getting a small telescope. Peering through a telescope opens up a world of star clusters, galaxies and nebulae. You can see Saturn’s rings and the moons of Jupiter.</p>
<p>It’s a good idea to <a href="https://skyandtelescope.org/wp-content/uploads/AboutScopes.pdf">read a guide</a> before you take the plunge. Small telescopes range from a hundred to several thousand dollars, but you can get a <a href="https://www.space.com/31229-best-beginner-telescopes.html">good starter one</a> for as little as $200. These basic telescopes usually have a viewfinder attached to help locate objects, but you’ll need sky maps to get the most out of them.</p>
<p>If you can shell out at least $400, you can acquire what are called “GoTo” telescopes that have motors and are computer controlled, where the <a href="https://theplanets.org/best-telescopes-buying-guide/">telescope does the work of finding the deep sky objects</a>. You just have to type the name in or choose from a list. Now you’ll be ready to learn <a href="https://cosmicpursuits.com/1943/how-to-see-averted-vision-and-dark-adaptation/">tricks of the trade</a>, like using a red LED flashlight to preserve your night vision, and looking slightly to the side, which lets you see deeper because the cells near the edge of your retina are more sensitive to low light levels.</p>
<h2>Online resources</h2>
<p>The internet is a great resource for backyard astronomy. You might want to start with a <a href="https://www.youtube.com/watch?v=L-Wtlev6suc&t=266s">Crash Course</a> in naked-eye astronomy. In addition to reviews of apps and binoculars and telescopes, there are <a href="https://skyandtelescope.org/astronomy-news/new-videos-sky-telescopes-skywatching-series-12192014/">tutorials on how to use your new telescope</a>.</p>
<p>You’ll also want to check out Sky and Telescope’s weekly <a href="https://skyandtelescope.org/observing/sky-at-a-glance/">“Sky at a Glance.”</a> BBC’s <a href="https://www.skyatnightmagazine.com/">Sky at Night magazine</a> has a <a href="https://www.youtube.com/channel/UCNXzteckwM7bKNNLwWu5OLw">longer monthly summary</a> of what you can observe.</p>
<p>And if you just want to be inspired by the <a href="https://www.youtube.com/watch?v=xTvvQ65jWVs&t=57s">visual splendor</a> of a dark night sky, there are a number of <a href="https://www.youtube.com/watch?v=Z3xkHmC-KQE">time-lapse videos</a> you can enjoy.</p>
<h2>Dark skies</h2>
<p>Hopefully you’re excited about backyard astronomy, but what you can actually see will depend on where you live.</p>
<p>For tens of thousands of years, the night sky was a familiar friend to our ancestors, and they used it to navigate, tell time and project their myths into the stars and constellations. But the glories of the night sky have been steadily eroded by industrial activity and artificial lights.</p>
<p>[<em>The Conversation’s science, health and technology editors pick their favorite stories.</em> <a href="https://theconversation.com/us/newsletters/science-editors-picks-71/?utm_source=TCUS&utm_medium=inline-link&utm_campaign=newsletter-text&utm_content=science-favorite">Weekly on Wednesdays</a>.]</p>
<p>You can see recent measures of night sky brightness in a <a href="https://cires.colorado.edu/Artificial-light">zoomable map</a> of the U.S., where it’s clearly harder to find a dark sky in the eastern half of the country.</p>
<p>To measure how sky brightness affects what you can see, amateur astronomers use something called the <a href="https://nightskypix.com/bortle-scale/">Bortle scale</a>, where 9 is an inner city and 1 is a pristine wilderness. <a href="https://www.darksky.org/light-pollution/">Light pollution</a> is the effect of artificial lighting on the night sky, and you can also see how it affects the familiar <a href="https://academo.org/demos/bortle-scale/">Big Dipper and Orion</a> constellations. </p>
<p><a href="https://www.npca.org/articles/1806-celebrate-dark-skies-at-these-27-national-parks">National parks</a> are great places to enjoy the sky because they try to protect it from artificial lights, and in normal times many national parks offer <a href="https://www.nps.gov/subjects/nightskies/stargaze.htm">astronomy programs</a>. Many are also beginning to <a href="https://www.nps.gov/subjects/nightskies/events.htm">resume those programs</a> with COVID-19 restrictions in place.</p>
<p>If you want to contribute to the effort to raise awareness of light pollution by monitoring the sky brightness where you live, the U.S. national observatories run a project called “<a href="https://www.globeatnight.org/about.php">Globe at Night</a>.”</p>
<p>Anyone can collect data and help a research project by doing <a href="https://scistarter.org/citizen-science">citizen science</a>, which is when non-scientists gather data and contribute to a collective research effort. You can become a citizen scientist and submit your own observations from a computer or smartphone.</p><img src="https://counter.theconversation.com/content/144647/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Chris Impey does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>COVID-19 may have messed up school and shut down a lot of entertainment venues. But you can still brighten things up by doing a little stargazing at night, an astronomer says.Chris Impey, University Distinguished Professor of Astronomy, University of ArizonaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/901472018-01-19T10:08:26Z2018-01-19T10:08:26ZStudy of distant galaxies challenges our understanding of how stars form<figure><img src="https://images.theconversation.com/files/202446/original/file-20180118-158519-186b12q.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">ESO/UltraVISTA team. Acknowledgement: TERAPIX/CNRS/INSU/CASU</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>The most massive galaxies in our neighbourhood formed their stars billions of years ago, early in the history of the universe. At the present day, they produce very few new stars. Astronomers have long believed that is because they contain very little gas – a key ingredient necessary to produce stars. But our new study, published in <a href="http://em.rdcu.be/wf/click?upn=KP7O1RED-2BlD0F9LDqGVeSAjZUdc-2FqgpuTsX153V7IiU-3D_nxaMCHA8HKRN7Obv30-2BkpQSlaoIvL88II6d-2FeFNo6wAVblVhHRPm9FZ88ZZc-2BU0NtNEO6fwpQzC5sY8GGY-2FgUfoxQOAKiAg9T5k7kjv1Q3W-2B3ZnK4ggQt5KVx0u7OaIRBU5e9EzQsG5Um3gqO50LS-2FgP-2BBVZ3jbVzz7H-2F1GdFcRA54qYNA6-2Bm2Zqxi-2FEA36ouf2hq-2BaRnrlu4xuDpjhDm-2BsT5yRU-2B-2FQeiIiw8LV6EGBCNUaWinJ3lSyNcwlSPOYNtmsihT4AyRzvRCGDQUcBOg-3D-3D">Nature Astronomy</a>, is now challenging this long held view.</p>
<p>Through probing the extreme environments of faraway massive galaxies, we can learn not only about their evolution and the history of the universe, but most importantly about the fundamental processes regulating <a href="https://theconversation.com/uk/search?utf8=%E2%9C%93&q=star+formation">the formation of stars</a>. Given that stars produce most of the different types of atoms in our bodies and the world around us, understanding how they were formed is essential if we are to know where we came from.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/202447/original/file-20180118-158531-3vuror.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/202447/original/file-20180118-158531-3vuror.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/202447/original/file-20180118-158531-3vuror.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/202447/original/file-20180118-158531-3vuror.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/202447/original/file-20180118-158531-3vuror.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/202447/original/file-20180118-158531-3vuror.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/202447/original/file-20180118-158531-3vuror.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">Disc galaxy Messier 101.</span>
<span class="attribution"><span class="source">NASA, ESA, CXC, SSC, and STScI</span></span>
</figcaption>
</figure>
<p>Galaxies exist in two main types: disc and elliptical. Disc galaxies, including the Milky Way, are flat and contain large reservoirs of gas that they use to continually form stars. Elliptical galaxies are massive, round and stopped forming stars long ago. Most theories assume that at some point elliptical galaxies <a href="https://arxiv.org/abs/astro-ph/0502199">lost their gas reservoirs</a>, which caused the rate of star formation to drop. </p>
<h2>Distant light</h2>
<p>Our team investigated whether there are other ways in which distant, elliptical galaxies could have lost their ability to form stars. Distance to galaxies are measured by how bright its stars are, in light years (defined as how long it takes the light to reach us in one year). As it takes so long for the light from these faraway galaxies <a href="http://hubblesite.org/reference_desk/faq/answer.php.id=45&cat=galaxies">to reach us</a>, we can work out that they appear to us as they were 10 billion years ago. </p>
<p>Ideally we would want to directly observe the gas in these galaxies, but this is extremely challenging and would require several hours of observations per galaxy – and we need to look at thousands of galaxies. Instead, we opted to study dust. Dust (cold rather than hot) only represents 1% of the interstellar matter in a galaxy, but it is found wherever cold gas is. A galaxy that contains a lot of dust therefore also contains a lot of gas.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/202449/original/file-20180118-158531-1i9tbmh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/202449/original/file-20180118-158531-1i9tbmh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=596&fit=crop&dpr=1 600w, https://images.theconversation.com/files/202449/original/file-20180118-158531-1i9tbmh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=596&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/202449/original/file-20180118-158531-1i9tbmh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=596&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/202449/original/file-20180118-158531-1i9tbmh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=749&fit=crop&dpr=1 754w, https://images.theconversation.com/files/202449/original/file-20180118-158531-1i9tbmh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=749&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/202449/original/file-20180118-158531-1i9tbmh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=749&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Elliptical galaxy called ESO 306-17 in the southern sky.</span>
<span class="attribution"><span class="source">NASA, ESA and Michael West (ESO)</span></span>
</figcaption>
</figure>
<p>We used data from the <a href="http://cosmos.astro.caltech.edu/">Cosmological Evolution Survey (COSMOS)</a>, which covers a large patch of the sky observed by most major telescopes, on Earth and in space. We used images from infrared to radio wavelengths of light, which allows us to measure both the rate of star formation and the cold dust mass in galaxies. </p>
<p>Since the galaxies we are interested in are so far away, it is impossible to detect each galaxy individually in the existing infrared or radio data. Instead, we combined the light from 1,000 galaxies and determined how much gas they contain on average and how quickly they are forming stars. </p>
<p>As a result, we made an exciting discovery. Despite having low star formation rates, the elliptical galaxies contain surprisingly large amounts of gas: 100 times more than was expected. This is surprising in two ways. It challenges our standard view of elliptical galaxies as “boring” gas-poor objects. But it also forces us to rethink the basic view of star formation processes – we have always assumed that the presence of cold gas must lead to star formation. Here, we find that elliptical galaxies form stars far less efficiently than disk galaxies at the same epoch.</p>
<p>So why is that? Nine years ago, I <a href="https://arxiv.org/abs/0905.4669">predicted</a> this possibility from numerical simulations I had run as a PhD student. I found that in disc galaxies, the gravitational pull of the stars helps the gas to collapse to form new stars. In contrast, the gas in elliptical galaxies feels a weaker pull from the stars and does not collapse so easily. It is fascinating that the global morphology of a galaxy can control what happens at the smallest scales. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/202444/original/file-20180118-158550-cw4eco.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/202444/original/file-20180118-158550-cw4eco.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=854&fit=crop&dpr=1 600w, https://images.theconversation.com/files/202444/original/file-20180118-158550-cw4eco.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=854&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/202444/original/file-20180118-158550-cw4eco.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=854&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/202444/original/file-20180118-158550-cw4eco.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1073&fit=crop&dpr=1 754w, https://images.theconversation.com/files/202444/original/file-20180118-158550-cw4eco.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1073&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/202444/original/file-20180118-158550-cw4eco.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1073&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Star forming nebula, where gas collapses to form new stars.</span>
<span class="attribution"><span class="source">ESA/NASA/JPL-Caltech</span></span>
</figcaption>
</figure>
<p>The next steps of our research will use new simulations and hopefully direct observations of the cold gas itself with the <a href="http://www.almaobservatory.org/en/home/">Atacama Large Millimeter/submillimeter Array (ALMA)</a>, an observatory in Chile, to improve our understanding of the complex interplay between star formation and galaxy morphology. This will shed light on universal processes ultimately happening in every galaxy, including our very own Milky Way.</p><img src="https://counter.theconversation.com/content/90147/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Marie Martig 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>Massive, far distant galaxies contain 100 times more gas than we thought possible.Marie Martig, Senior Lecturer in Astrophysics, Liverpool John Moores UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/618542016-07-11T19:38:49Z2016-07-11T19:38:49ZWhy do some galaxies stop making new stars?<figure><img src="https://images.theconversation.com/files/128876/original/image-20160630-30655-18nxf78.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Spiral galaxy NGC 3953 is a veritable star making machine, but why do some galaxies stop forming new stars?</span> <span class="attribution"><a class="source" href="http://www.nsatlas.org/getAtlas.html?search=name&name=ngc+3953&radius=10.0&submit_form=Submit">NASA-Sloan Atlas</a></span></figcaption></figure><p><a href="http://theconversation.com/explainer-a-beginners-guide-to-the-galaxy-49">Galaxies</a> are star-making machines, churning out new stars fuelled by cold gas collapsing under the force of gravity. Some galaxies can produce hundreds of new stars in a single year, and individual galaxies can contain many billions of stars.</p>
<p>Our own galaxy, <a href="https://theconversation.com/is-our-milky-way-galaxy-a-zombie-already-dead-and-we-dont-know-it-52732">the Milky Way</a>, is dotted with star-forming regions. One of these, <a href="http://hubblesite.org/gallery/tours/tour-orion/">the Orion Nebula</a>, is so bright you can see it with <a href="http://www.skyandtelescope.com/observing/celestial-objects-to-watch/observing-the-great-orion-nebula/">the unaided eye</a>. Look at the middle “star” of Orion’s sword, and you are actually seeing stars being born.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/128869/original/image-20160630-30635-1f3dh3t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/128869/original/image-20160630-30635-1f3dh3t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/128869/original/image-20160630-30635-1f3dh3t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/128869/original/image-20160630-30635-1f3dh3t.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/128869/original/image-20160630-30635-1f3dh3t.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/128869/original/image-20160630-30635-1f3dh3t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/128869/original/image-20160630-30635-1f3dh3t.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/128869/original/image-20160630-30635-1f3dh3t.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 middle star in Orion’s sword is actually the Orion Nebula, where new stars are being born.</span>
<span class="attribution"><span class="source">NASA, C.R. O'Dell and S.K. Wong (Rice University)</span></span>
</figcaption>
</figure>
<p>But something can break these star-making machines; many <a href="http://astronomy.swin.edu.au/cosmos/E/elliptical+galaxy">elliptical galaxies</a> have stopped forming new stars. What stops them is one of the biggest questions in astronomy. </p>
<h2>Breaking the machines</h2>
<p>A distinctive feature of elliptical galaxies is their ellipsoidal shapes, much like an Aussie rules or rugby ball. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/128870/original/image-20160630-30642-1x5u9xi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/128870/original/image-20160630-30642-1x5u9xi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/128870/original/image-20160630-30642-1x5u9xi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=436&fit=crop&dpr=1 600w, https://images.theconversation.com/files/128870/original/image-20160630-30642-1x5u9xi.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=436&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/128870/original/image-20160630-30642-1x5u9xi.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=436&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/128870/original/image-20160630-30642-1x5u9xi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=548&fit=crop&dpr=1 754w, https://images.theconversation.com/files/128870/original/image-20160630-30642-1x5u9xi.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=548&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/128870/original/image-20160630-30642-1x5u9xi.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=548&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Many elliptical galaxies have effectively stopped making new stars.</span>
<span class="attribution"><span class="source">NASA Sloan Atlas</span></span>
</figcaption>
</figure>
<p>The Milky Way, and many other large star-forming galaxies, are <a href="http://astronomy.swin.edu.au/cosmos/S/Spiral+Galaxy">spiral galaxies</a>. In spiral galaxies, stars and the gaseous fuel to make new stars circle around the galaxy in a vast flat disk.</p>
<p>Does the formation of new stars critically depend on galaxy shape? It seems plausible given most spiral galaxies are forming stars and most elliptical galaxies aren’t. </p>
<p>But how then do elliptical galaxies grow? Back in 1972, the brothers <a href="https://ned.ipac.caltech.edu/level5/Toomre/Toomre_contents.html">Alar and Juri Toomre</a> showed that new elliptical galaxies could be created by merging spiral galaxies together. Indeed, billions of years from now, <a href="http://www.nasa.gov/mission_pages/hubble/science/milky-way-collide.html">our own Milky Way will collide with the Andromeda galaxy</a> to create a new elliptical galaxy. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/fMNlt2FnHDg?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">The collision of the Milky Way with the Andromeda galaxy will form a new, elliptical galaxy.</span></figcaption>
</figure>
<p>Perhaps it is the process of galaxy mergers that breaks star-making machines. But not all plausible mechanisms for stopping star formation clearly depend on galaxy shape. </p>
<p>For example, galaxies ploughing through hot plasma can have star-forming gas stripped from them, but this process shouldn’t transform spiral galaxies into elliptical galaxies. </p>
<p>There are <a href="http://phys.org/news/2011-05-dead-galaxies.html">some elliptical galaxies that are forming stars</a>, but are there any spiral galaxies without any star formation? Is star formation intimately linked to galaxy shape or not? We decided to find out.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/128872/original/image-20160630-30646-18qvcfb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/128872/original/image-20160630-30646-18qvcfb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/128872/original/image-20160630-30646-18qvcfb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=466&fit=crop&dpr=1 600w, https://images.theconversation.com/files/128872/original/image-20160630-30646-18qvcfb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=466&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/128872/original/image-20160630-30646-18qvcfb.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=466&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/128872/original/image-20160630-30646-18qvcfb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=585&fit=crop&dpr=1 754w, https://images.theconversation.com/files/128872/original/image-20160630-30646-18qvcfb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=585&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/128872/original/image-20160630-30646-18qvcfb.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=585&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Star-forming galaxy NGC 3310 is blue because it contains short-lived hot blue stars.</span>
<span class="attribution"><span class="source">Sloan Digital Sky Survey</span></span>
</figcaption>
</figure>
<h2>Searching for star formation</h2>
<p>How do you find galaxies that are forming stars versus those that are not? Easy. You look for <a href="http://theconversation.com/explainer-what-are-stars-15235">stars</a> that die young. </p>
<p>Our yellowish sun is about halfway through its 10-billion-year life. But very luminous hot blue stars have lifetimes of just 30 million years. </p>
<p>In cosmological terms, 30 million years is a blink of the eye. Find a galaxy with these blue stars, and you are seeing a galaxy forming stars (or that formed stars very recently). Conversely, a red galaxy may not be forming any new stars. </p>
<p>There are other ways of looking for star-forming galaxies too. Hot stars warm the dust within galaxies, and that warm dust glows in <a href="http://coolcosmos.ipac.caltech.edu/page/what_is_infrared">infrared light</a>. Hot stars also cause surrounding gas to glow, producing a distinctive <a href="https://theconversation.com/explainer-seeing-the-universe-through-spectroscopic-eyes-37759">spectrum of light</a>. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/128918/original/image-20160630-30661-tkmxmh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/128918/original/image-20160630-30661-tkmxmh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/128918/original/image-20160630-30661-tkmxmh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=62&fit=crop&dpr=1 600w, https://images.theconversation.com/files/128918/original/image-20160630-30661-tkmxmh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=62&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/128918/original/image-20160630-30661-tkmxmh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=62&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/128918/original/image-20160630-30661-tkmxmh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=77&fit=crop&dpr=1 754w, https://images.theconversation.com/files/128918/original/image-20160630-30661-tkmxmh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=77&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/128918/original/image-20160630-30661-tkmxmh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=77&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Glowing hydrogen gas produces a distinctive spectrum of light.</span>
<span class="attribution"><span class="source">Jan Homann/Wikipedia</span></span>
</figcaption>
</figure>
<h2>Red and dead?</h2>
<p>We weren’t the first to look for spiral galaxies that aren’t forming stars. In 1976, Canadian astronomer <a href="http://gruber.yale.edu/cosmology/sidney-van-den-bergh">Sidney van den Bergh</a> found “anaemic” galaxies that have far less star formation than typical spiral galaxies.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/128875/original/image-20160630-30625-1m1aj66.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/128875/original/image-20160630-30625-1m1aj66.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/128875/original/image-20160630-30625-1m1aj66.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=528&fit=crop&dpr=1 600w, https://images.theconversation.com/files/128875/original/image-20160630-30625-1m1aj66.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=528&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/128875/original/image-20160630-30625-1m1aj66.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=528&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/128875/original/image-20160630-30625-1m1aj66.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=664&fit=crop&dpr=1 754w, https://images.theconversation.com/files/128875/original/image-20160630-30625-1m1aj66.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=664&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/128875/original/image-20160630-30625-1m1aj66.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=664&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Sidney van den Bergh identified NGC 718 as an anaemic spiral galaxy, with just a trickle of star formation.</span>
<span class="attribution"><span class="source">Sloan Digital Sky Survey</span></span>
</figcaption>
</figure>
<p>And British astronomer <a href="http://icg.port.ac.uk/%7Emastersk/">Karen Masters</a> has identified thousands of red spiral galaxies using the citizen science <a href="https://theconversation.com/beyond-todays-crowdsourced-science-to-tomorrows-citizen-science-cyborgs-53904">GalaxyZoo Project</a>.</p>
<p>But the spectra of red spiral galaxies identified by van den Bergh and Masters show the distinctive glow of hydrogen gas surrounding hot blue stars. These galaxies must still be forming new stars. </p>
<p>We decided to take a different approach to finding spiral galaxies without star formation, utilising images from NASA’s <a href="http://www.jpl.nasa.gov/wise/">Wide-field Infrared Survey Explorer</a>. </p>
<p>We searched for spiral galaxies without the infrared glow of warm dust heated by short-lived hot blue stars. The galaxies we found turned out to be red in ultraviolet and visible light, as expected if they aren’t forming new stars.</p>
<p>To be totally sure these spiral galaxies are truly dead, we decided to obtain their <a href="http://theconversation.com/explainer-seeing-the-universe-through-spectroscopic-eyes-37759">spectra</a>, using the <a href="http://rsaa.anu.edu.au/observatories/telescopes/anu-23m-telescope">Siding Spring 2.3-metre telescope</a>, near Coonabarabran in New South Wales. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/128880/original/image-20160630-30642-wdlllj.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/128880/original/image-20160630-30642-wdlllj.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/128880/original/image-20160630-30642-wdlllj.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/128880/original/image-20160630-30642-wdlllj.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/128880/original/image-20160630-30642-wdlllj.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/128880/original/image-20160630-30642-wdlllj.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/128880/original/image-20160630-30642-wdlllj.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/128880/original/image-20160630-30642-wdlllj.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">We used the Siding Spring 2.3-metre telescope to search for glowing hydrogen gas.</span>
<span class="attribution"><span class="source">Ssopete/Wikipedia</span></span>
</figcaption>
</figure>
<p>None of the six spectra had the distinctive signature of glowing gas heated by short-lived stars. We had finally found spiral galaxies that aren’t forming stars. </p>
<p>Our letter announcing this discovery was recently accepted for publication in <a href="http://adsabs.harvard.edu/abs/2016arXiv160603781F">Monthly Notices of the Royal Astronomical Society</a>. </p>
<h2>So what stops star formation?</h2>
<p>Clearly, star formation can be turned off without transforming spiral galaxies into elliptical galaxies. But just what is stopping star formation? There are several possibilities.</p>
<p>One option is <a href="https://astrobites.org/2013/03/13/ram-pressure-stripping-a-classic-paper/">ram pressure stripping</a>, where gas is stripped from a galaxy plunging through hot plasma. But this process should only work in <a href="https://theconversation.com/giant-galaxies-die-from-the-inside-when-they-stop-making-stars-40310">clusters of galaxies</a>, and many of our galaxies aren’t in galaxy clusters. </p>
<p>Perhaps gas cannot cool to produce new stars because of <a href="https://ned.ipac.caltech.edu/level5/Sept13/Silk/Silk8.html">heating by active galactic nuclei</a>, which are powered by the in-fall of matter towards enormous black holes. This may be true in some instances, but we didn’t see evidence for active galactic nuclei in most of our galaxies.</p>
<p>We now have a new mystery on our hands. What stops star formation in these unusual spiral galaxies?</p>
<p>Funnily enough, galaxy shapes may provide a clue. The British astronomer Karen Masters finds that spiral galaxies with little star formation often feature prominent “<a href="https://blog.galaxyzoo.org/2015/03/20/whats-all-the-fuss-about-bars-in-galaxies/">bars</a>” straddling their centres. This also seems to be true for spiral galaxies without star formation. Perhaps galaxy shape plays a critical role breaking star-making machines after all. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/128871/original/image-20160630-30664-17vdafq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/128871/original/image-20160630-30664-17vdafq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/128871/original/image-20160630-30664-17vdafq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=355&fit=crop&dpr=1 600w, https://images.theconversation.com/files/128871/original/image-20160630-30664-17vdafq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=355&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/128871/original/image-20160630-30664-17vdafq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=355&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/128871/original/image-20160630-30664-17vdafq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=446&fit=crop&dpr=1 754w, https://images.theconversation.com/files/128871/original/image-20160630-30664-17vdafq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=446&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/128871/original/image-20160630-30664-17vdafq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=446&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">NGC 4440, like many spiral galaxies with little or no star formation, features a distinctive bar straddling its centre.</span>
<span class="attribution"><span class="source">Sloan Digital Sky Survey</span></span>
</figcaption>
</figure><img src="https://counter.theconversation.com/content/61854/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Michael J. I. Brown receives research funding from the Australian Research Council and Monash University, and has developed space-related titles for Monash University's MWorld educational app.
</span></em></p><p class="fine-print"><em><span>Amelia Fraser-McKelive is the recipient of an Australian Postgraduate Award and receives funding for her research from Monash University.</span></em></p><p class="fine-print"><em><span>Kevin Pimbblet receives research funding from the University of Hull. He is affiliated with Monash University as an adjunct academic. </span></em></p>Galaxies are supposed to be the place where new stars are formed. So what causes some to stop this stellar production line?Michael J. I. Brown, Associate professor, Monash UniversityAmelia Fraser-McKelvie, PhD student in observational astronomy, Monash UniversityKevin Pimbblet, Senior Lecturer in Physics, University of HullLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/527322016-01-28T10:46:32Z2016-01-28T10:46:32ZIs our Milky Way galaxy a zombie, already dead and we don’t know it?<figure><img src="https://images.theconversation.com/files/109408/original/image-20160127-26823-vlapaf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Can a galaxy (like NGC 3810 in this case) have a classical spiral structure and also be already dead?</span> <span class="attribution"><a class="source" href="https://www.spacetelescope.org/images/potw1006a/">ESA/Hubble and NASA</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>Like a zombie, the Milky Way galaxy may already be dead but it still keeps going. Our galactic neighbor Andromeda almost certainly expired a few billion years ago, but only recently started showing outward signs of its demise.</p>
<p>Galaxies seem to be able to “perish” – that is, stop turning gas into new stars – via two very different pathways, driven by very different processes. Galaxies like the Milky Way and Andromeda do so very, very slowly over billions of years. </p>
<p>How and why galaxies “quench” their star formation and change their morphology, or shape, is one of the big questions in extragalactic astrophysics. We may now be on the brink of being able to piece together how it happens. And part of the thanks goes to citizen scientists who combed through millions of galactic images to classify what’s out there.</p>
<h2>Galaxies grow by making new stars</h2>
<p>Galaxies are dynamic systems that continually accrete gas and convert some of it into stars.</p>
<p>Like people, galaxies need food. In the case of galaxies, that “food” is a supply of fresh hydrogen gas from the cosmic web, the filaments and halos of dark matter that make up the largest structures in the universe. As this gas <a href="http://www.illustris-project.org/media/">cools and falls into dark matter halos</a>, it turns into a disk that then can cool even further and eventually fragment into stars.</p>
<p>As stars age and die, they can return some of that gas back into the galaxy either via winds from stars or by <a href="https://en.wikipedia.org/wiki/Supernova">going supernova</a>. As massive stars die in such explosions, they heat the gas around them and prevent it from cooling down quite so fast. They provide what astronomers call “feedback”: star formation in galaxies is thus a self-regulated process. The heat from dying stars means cosmic gas doesn’t cool into new stars as readily, which ultimately puts a brake on how many new stars can form. </p>
<p>Most of these star-forming galaxies are disk- or spiral-shaped, like our Milky Way.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/107855/original/image-20160111-6961-1cp7xoq.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/107855/original/image-20160111-6961-1cp7xoq.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/107855/original/image-20160111-6961-1cp7xoq.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=304&fit=crop&dpr=1 600w, https://images.theconversation.com/files/107855/original/image-20160111-6961-1cp7xoq.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=304&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/107855/original/image-20160111-6961-1cp7xoq.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=304&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/107855/original/image-20160111-6961-1cp7xoq.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=381&fit=crop&dpr=1 754w, https://images.theconversation.com/files/107855/original/image-20160111-6961-1cp7xoq.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=381&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/107855/original/image-20160111-6961-1cp7xoq.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=381&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Left: a spiral galaxy ablaze in the blue light of young stars from ongoing star formation; right: an elliptical galaxy bathed in the red light of old stars.</span>
<span class="attribution"><a class="source" href="http://www.sdss3.org/">Sloan Digital Sky Survey</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc/4.0/">CC BY-NC</a></span>
</figcaption>
</figure>
<p>But there’s another kind of galaxy that has a very different shape, or morphology, in astronomer-parlance. These massive elliptical galaxies tend to look spheroidal or football-shaped. They’re not nearly so active – they’ve lost their supply of gas and therefore have ceased forming new stars. Their stars move on far more unordered orbits, giving them their bulkier, rounder shape.</p>
<p>These elliptical galaxies differ in two major ways: they no longer form stars and they have a different shape. Something pretty dramatic must have happened to them to produce such profound changes. What?</p>
<h2>Blue=young and red=old?</h2>
<p>The basic division of galaxies into star-forming spiral galaxies blazing in the blue light of massive, young and short-lived stars, on the one hand, and quiescent ellipticals bathed in the warm glow of ancient low-mass stars, on the other, goes back to early galaxy surveys of the 20th century.</p>
<p>But, once modern surveys like the Sloan Digital Sky Survey (<a href="http://www.sdss.org/">SDSS</a>) began to record hundreds of thousands of galaxies, objects started emerging that didn’t quite fit into those two broad categories. </p>
<p>A significant number of red, quiescent galaxies aren’t elliptical in shape at all, but retain roughly a disk shape. Somehow, these galaxies stopped forming stars without dramatically changing their structure.</p>
<p>At the same time, blue elliptical galaxies started to surface. Their structure is similar to that of “red and dead” ellipticals, but they shine in the bright blue light of young stars, indicating that star formation is still ongoing in them.</p>
<p>How do these two oddballs – the red spirals and the blue ellipticals – fit into our picture of galaxy evolution?</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/107856/original/image-20160111-6992-18qgpso.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/107856/original/image-20160111-6992-18qgpso.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/107856/original/image-20160111-6992-18qgpso.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=423&fit=crop&dpr=1 600w, https://images.theconversation.com/files/107856/original/image-20160111-6992-18qgpso.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=423&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/107856/original/image-20160111-6992-18qgpso.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=423&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/107856/original/image-20160111-6992-18qgpso.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=532&fit=crop&dpr=1 754w, https://images.theconversation.com/files/107856/original/image-20160111-6992-18qgpso.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=532&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/107856/original/image-20160111-6992-18qgpso.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=532&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Galaxy Zoo allows citizen scientists to classify galaxies.</span>
<span class="attribution"><span class="source">Screenshot by Kevin Schawinski</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>Send in the citizen scientists</h2>
<p>As a graduate student in Oxford, I was looking for some of these oddball galaxies. I was particularly interested in the blue ellipticals and any clues they contained about the formation of elliptical galaxies in general.</p>
<p>At one point, I spent a whole week going through almost 50,000 galaxies from SDSS by eye, as none of the available algorithms for classifying galaxy shape was as good as I needed it to be. I found quite a few blue ellipticals, but the value of classifying all of the roughly one million galaxies in SDSS with human eyes quickly became apparent. Of course, going through a million galaxies by myself wasn’t possible.</p>
<p>A short time later, a group of collaborators and I launched <a href="galaxyzoo.org">galaxyzoo.org</a> and invited members of the public – citizen scientists – to participate in astrophysics research. When you logged on to Galaxy Zoo, you’d be shown an image of a galaxy and a set of buttons corresponding to possible classifications, and a tutorial to help you recognize the different classes. </p>
<p>By the time we stopped recording classifications from a quarter-million people, each of the one million galaxies on Galaxy Zoo had been classified over 70 times, giving me <a href="http://doi.org/10.1111/j.1365-2966.2008.13689.x">reliable, human classifications of galaxy shape</a>, including a measure of uncertainty. Did 65 out of 70 citizen scientists agree that this galaxy is an elliptical? Good! If there’s no agreement at all, that’s information too.</p>
<p>Tapping into the “wisdom of the crowd” effect coupled with the unparalleled human ability for pattern recognition helped sort through a million galaxies and unearthed many of the less common blue ellipticals and red spirals for us to study.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/109405/original/image-20160127-26823-1ia00a4.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/109405/original/image-20160127-26823-1ia00a4.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/109405/original/image-20160127-26823-1ia00a4.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/109405/original/image-20160127-26823-1ia00a4.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/109405/original/image-20160127-26823-1ia00a4.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/109405/original/image-20160127-26823-1ia00a4.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/109405/original/image-20160127-26823-1ia00a4.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/109405/original/image-20160127-26823-1ia00a4.png?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">The galaxy color-mass diagram. Blue, star-forming galaxies are at the bottom, in the blue cloud. Red, quiescent galaxies are at the top, in the red sequence. The ‘green valley’ is the transition zone in between.</span>
<span class="attribution"><span class="source">Schawinski+14</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>Unwittingly living in the green valley?</h2>
<p>The crossroads of galaxy evolution is a place called the “<a href="https://en.wikipedia.org/wiki/Galaxy_color%E2%80%93magnitude_diagram">green valley</a>.” This may sound scenic, but refers to the population between the blue star-forming galaxies (the “blue cloud”) and the red, passively evolving galaxies (the “red sequence”). Galaxies with “green” or intermediate colors should be those galaxies in which star formation is in the process of turning off, but which still have some ongoing star formation – indicating the process only shut down a short while ago, perhaps a few hundred million years.</p>
<p>As a curious aside, the origin of the term “green valley” may actually go back to a talk given at the University of Arizona on galaxy evolution where, when the speaker described the galaxy color-mass diagram, a member of the audience called out: “the green valley, where galaxies go to die!” Green Valley, Arizona, is a retirement community just outside of the university’s hometown, Tucson.</p>
<p>For our project, the really exciting moment came when we looked at the <a href="http://doi.org/10.1093/mnras/stu327">rate at which various galaxies were dying</a>. We found the slowly dying ones are the spirals and the rapidly dying ones are the ellipticals. There must be two fundamentally different evolutionary pathways that lead to quenching in galaxies. When we explored these two scenarios – dying slowly, and dying quickly – it became obvious that these two pathways have to be tied to the gas supply that fuels star formation in the first place.</p>
<p>Imagine a spiral galaxy like our own Milky Way merrily converting gas to stars as new gas keeps flowing in. Then something happens that turns off that supply of fresh outside gas: perhaps the galaxy fell into a massive cluster of galaxies where the hot intra-cluster gas cuts off fresh gas from the outside, or perhaps the dark matter halo of the galaxy grew so much that gas falling into it gets shock heated to such a high temperature that it cannot cool down within the age of the universe. In any case, the spiral galaxy is now left with just the gas it has in its reservoir. </p>
<p>Since these reservoirs can be enormous, and the conversion of gas to stars is a very slow process, our spiral galaxy could go on for quite a while looking “alive” with new stars, while the actual rate of star formation declines over several billion years. The glacial slowness of using up the remaining gas reservoir means that by the time we realize that a galaxy is in terminal decline, the “trigger moment” occurred billions of years ago.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/109406/original/image-20160127-26817-1h9dhs2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/109406/original/image-20160127-26817-1h9dhs2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/109406/original/image-20160127-26817-1h9dhs2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=192&fit=crop&dpr=1 600w, https://images.theconversation.com/files/109406/original/image-20160127-26817-1h9dhs2.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=192&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/109406/original/image-20160127-26817-1h9dhs2.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=192&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/109406/original/image-20160127-26817-1h9dhs2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=241&fit=crop&dpr=1 754w, https://images.theconversation.com/files/109406/original/image-20160127-26817-1h9dhs2.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=241&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/109406/original/image-20160127-26817-1h9dhs2.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=241&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A Hubble image of part of the Andromeda galaxy, which like our Milky Way may be a galactic zombie.</span>
<span class="attribution"><a class="source" href="https://www.nasa.gov/content/goddard/hubble-s-high-definition-panoramic-view-of-the-andromeda-galaxy">NASA, ESA, J. Dalcanton, B.F. Williams and L.C. Johnson (University of Washington), the PHAT team, and R. Gendler</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>The Andromeda galaxy, our nearest massive spiral galaxy, is in the green valley and likely began its decline eons ago: it is a zombie galaxy, according to our latest research. It’s dead, but keeps on moving, still producing stars, but at a diminished rate compared to what it should if it were still a normal star-forming galaxy. Working out whether the Milky Way is in the green valley – in the process of shutting down – is much more challenging, as we are in the Milky Way and cannot easily measure <a href="http://doi.org/10.1088/0004-637X/736/2/84">its integrated properties</a> the way we can for distant galaxies.</p>
<p>Even with the more uncertain data, it looks like the Milky Way is just at the edge, ready to tumble into the green valley. It’s entirely possible that the Milky Way galaxy is a zombie, having died a billion years ago.</p><img src="https://counter.theconversation.com/content/52732/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Kevin Schawinski receives funding from the Swiss National Fund. He is the co-founder of Galaxy Zoo. </span></em></p>Extragalactic astrophysicists want to know how and why galaxies stop forming stars, change their shape and fade away. With help from citizen scientists, they’re figuring it out.Kevin Schawinski, Assistant Professor of Galaxy & Black Hole Astrophysics, Swiss Federal Institute of Technology ZurichLicensed as Creative Commons – attribution, no derivatives.