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Ancient nova is now thought to be a stellar collision

Locating Nova Sagittarii 2015 No. 2, the new bright light in the sky. Alex Cherney, Author provided

Ancient nova is now thought to be a stellar collision

Locating Nova Sagittarii 2015 No. 2, the new bright light in the sky. Alex Cherney, Author provided

Astronomers have shed new light on what was thought to be one of the earliest recorded observations of a nova, or exploding star.

It comes as observations continue to track a faint new nova currently visible in the east before sunrise within the constellation Sagittarius. Designated Nova Sagittarii 2015 No. 2, it was discovered on March 15 by Australian amateur astronomer, John Seach.

It has been brightening since that time so that it is now visible to the naked eye from moderately dark skies.

Spectroscopy confirms that this is a classic nova -– what we are seeing is the aftermath of nuclear explosions occurring on the surface of a white dwarf star.

But a new look at a nova from 1670, shows that some nova can occur by different means.

Early observations

Nova Vul 1670, was seen by European astronomers over 340 years ago, when it appeared within the faint constellation of Vulpecula. The new research, published in the journal Nature, shows this stellar outburst was not an ordinary nova but may well be the result of two stars merging together.

This chart of the position of a 1670 nova (marked in red) was recorded by the famous astronomer Hevelius and published by the Royal Society in England in their journal Philosophical Transactions. Royal Society, CC BY

In classical novae, such as Nova Sagittarii 2015 No 2, the nova is seen to brighten, then fade away. After increasing in brightness over the past week or so, the nova now seems to have peaked at about a magnitude of 4.3, making it the brightest nova in Sagittarius since at least 1898.

There was another novae in Sagittarius discovered last month – which is why this nova is labelled No 2 – but it did not become bright enough to be seen without a small telescope.

Current expectations for Nova Sagittarii 2015 No 2 are that it is now fading away. Check here for latest magnitude estimates via the American Association of Variable Star Observers.

A variable sight

In contrast, the outburst of Nova Vul 1670 lasted nearly three years. It was easily visible to the naked eye, reaching a peak magnitude of 2.6 (that’s almost six times brighter than Nova Sagittarii 2015 No 2). During its first two years its brightness was recorded to vary, then it disappeared and reappeared twice before finally fading away.

In the newly published results, the region around Nova Vul 1670 has been observed at radio and submillimetre wavelengths, using the European Southern Observatory’s Atacama Pathfinder Experiment telescope (APEX), the Smithsonian Astrophysical Observatory’s Submillimeter Array (SMA) and the Max Planck Institute’s Effelsberg radio telescope.

This picture shows the remains of the new star seen in 1670, created from a combination of visible-light images from the Gemini telescope (blue), a submillimetre map showing the dust from the SMA (green) and a map of the molecular emission from APEX and the SMA (red). ESO/T. Kamiński, CC BY

The great thing about observations at these wavelengths, is that they can be used to determine what molecules are present within that part of space. It was found that the area is surprisingly rich in nitrogen and that the isotopic ratios of carbon, nitrogen and oxygen were very peculiar and not consistent with those expected from a nova.

But they could reflect the kind of chemical composition found inside a star that was converting hydrogen to helium via a fusion reaction process known as the CNO cycle.

In addition, an estimate of the total mass of the gas has determined that it is much too high for a classical-nova explosion.

Nova Vul 1670 appears to be part of a new class of object known as a luminous red nova. This is a relatively new classification that was first proposed in 2007 from observations of an outburst within the galaxy M85.

Such objects are around 10 to 100 times brighter than classical novae but are much fainter than supernovae.

It’s also found that the light curve of red novae (also known as red transients) have multiple peaks, in a similar way to how Nova Vul 1670 appeared to vary in brightness when it was observed over 340 years ago.

When stars collide

The mechanism behind a luminous red nova is thought to be the merging of two stars. This can occur when stars within a binary system end up coming too close together -– possibly because their orbit decays or because one of the stars reaches old-age and as it swells to become a red giant star, the other star finds itself stuck inside the red giant’s outer envelope.

A similar fate awaits the Earth, when our sun reaches its red giant phase in around 5 billion years time.

When the stars eventually merge, material is dredged up from inside the stars and ejected into space. What’s left behind, matches Nova Vul 1670 fairly closely - a faint remnant embedded in cool gas that is rich in molecules and dust.

And what of that nova that we can currently see? Sagittarius is visible from Australia from around 2am until sunrise, when it will be high in the east. But it won’t be around for long, so try and see it while you can.

Sagittarius is found high in the east before sunrise from across Australia and just above the constellation is the planet Saturn. Museum Victoria/Stellarium