Wednesday, 29 January 2014

The M82 Supernova SN2014J

Something went bang in the night...

I thought I was doing well last week. Tuesday evening I had my Rosetta blog finished, ready to publish the next morning. I was sure people would still be interested in it two days after the event. I mean, it's not as if the closest supernova since 1987 was going to go off during the night?


At 7.20pm last Tuesday evening a group of students together with their Professor, Steve Fossey, were in the UCL observatory for a practical astronomy class. It was a cloudy night, and the only bright thing left to look at in the sky was the Cigar Galaxy, officially known as M82. Turning their telescope on it, they noticed something odd: A point of light near the centre of the image so bright , it was outshining the rest of the galaxy. Quickly turning another telescope on it, they saw the same thing. A star had exploded in one of the most violent and energetic event in the Universe. A supernova.

The discovery of the supernova (bottom image) compared with an older image of M82 above it.
Fossey contacted the Palomar Transient Factory (PTF), a collection of linked telescopes in the USA that specialise in studying supernova and other unique events. They confirmed that it was a supernova and, at just twelve million light years away, the closest one to Earth since 1987. As the tenth supernova discovered so far this year, it was christened SN2014J.

They then took a spectra, splitting the light from the explosion into its separate wavelengths, or colours. Different materials absorb or release light at different wavelengths, so by looking for dark gaps or bright spikes in the spectra the PTF team could see what the exploding star was made of.

Although they are all exploding stars, the type of star which causes the supernova can vary. Many are the deaths of huge stars, eight times the mass of the Sun or more, collapsing in on themselves then blowing to pieces as they run out of the hydrogen fuel they need to hold themselves up. It was a supernova of this kind, known as core-collapse supernovae, that produced the millisecond pulsar I talked about in the first post. SN1987A, the closest recent supernova, was one of these.

The closest recent Supernova, which went off in 1897 in the Large Magellanic cloud, a dwarf galaxy orbiting our own Milky Way. This picture  was taken by the Hubble Space Telescope in 2006, showing the still-expanding cloud of debris from the massive explosion.

The spectra of the M82 supernova showed that it was something different. SN2014J is a Type 1A supernova, the explosion of a White Dwarf star.

White dwarfs are the leftover cores of stars like our Sun. Stars are held in a delicate balance, stopped from collapsing under their own weight by the energy generated by hydrogen fusion in their cores. When this fuel runs out the core starts to collapse, whilst the rest of the star expands into a red giant. The outer layers are gradually blown away, leaving behind a hot, dense core of carbon and oxygen.

White dwarfs are strange objects. They are so dense that they have around the same mass as the Sun squeezed in to an object no bigger than the Earth. They are held up only by something called electron degeneracy pressure, the fundamental rule that two things can't be in the same place at once.

This pressure can only hold up so much. If the mass of the white dwarf goes over 1.44 times that of the Sun then it collapses, exploding so violently that it releases more energy than the entirety of the galaxy that contains it. These are known as supernova progenitors.

How to get a white dwarf above that 1.44 Solar mass limit is a subject of debate. For decades the accepted model was that the white dwarfs are in binary systems with another star. When that star turns into a red giant, its outer layers get close enough to the white dwarf that the gravity of the white dwarf beings to pull the material of the red giant onto itself. If this carries on for too long the white dwarf slips over the mass limit and it's all over.
Artists impression of the traditional model for the cause, or progenitor, of a Type 1A supernova. Material from a Red Giant flows onto a White Dwarf, causing it to grow above 1.44 Solar masses and explode. 
Although that model makes sense, actually getting the physics to work has been tricky. Few computer simulations can get the white dwarf to explode. In many of them the material being dumped onto the white dwarf is simply blown away before it can build up to the limit.

This has lead to suggestions that instead of a white dwarf taking matter from a red giant, Type 1A supernovae may be caused by the collision of two white dwarfs. This model works better in simulations, as well as providing a better explanation for some observations.

Working out which of these options cause Type1A supernovae is important because of one crucial difference. In the white dwarf-red giant model, every white dwarf explodes at exactly 1.44 Solar masses. With two white dwarfs, the double degenerate model, that's not necessarily true. And that has implications for the spookiest ghost in physics: Dark Energy.

If white dwarfs always explode at 1.44 solar masses, then the explosion will always have roughly the same brightness. This means that if we see two Type 1A supernova, one of which is brighter than the other, then the dimmer supernova must be further way. Objects with that behaviour are known as standard candles, and are our main way of measuring distances in space. By observing Type 1A supernova in far away galaxies, we can measure the size of the Universe.

In the late 1990s Type1A supernovae were used to investigate how quickly the universe was expanding. The expectation was that as time went on the expansion of the Universe will slow down, as gravity begins to overcome the initial outwards explosion of the Big Bang. The Type1A measurements showed the opposite. The expansion of the Universe is accelerating, and we don't know why. The cause of the acceleration is known as Dark Energy, although the only things we know about it are that it isn't dark and it probably isn't energy.

Should Type1A supernovae turn out not to be standard candles then this whole measurement and much of modern cosmology would be thrown into doubt. What makes SN2014J so interesting then is that it is close enough to investigate what the progenitors were. 

Already the Hubble Space Telescope has swung round to take a look, joining a host of other telescopes in space and around the world to study the new supernova. Meanwhile astronomers are scouring older pictures of M82, trying to spot what stars were at the exact spot where the supernova went off.

Hubble image of the new supernova. Astronomers everywhere have been shocked by the appearance of a giant arrow over M82...

My own experience with the supernova has been quite interesting. The news was breaking as I woke up last Wednesday. I tweeted about it, and a few hours later was emailed by a Spanish science news website asking for more details! The article is in Spanish, but Google Translate does a fair job on it.

Because this supernova is so close, anyone can take a look. It's not quite bright enough to see with the naked eye, but a small telescope should be able to spot it. Over the next week or so it it should brighten so much that it will be visible through binoculars. M82 is just above the Plough, near the North Star.

I hope you get a chance to go out and see it. Space science is often a long process, with missions and observations taking years to come to fruition and gives results. Events like this, coming out of nowhere, are relatively rare, but have the potential to reshape our knowledge of the Universe.

For new blogs and various other science news (including any more nearby supernovae), follow me on Twitter

UPDATE: On 27th February, I finally managed to get the telescope out and have a look. I only have a small telescope so M82 wasn't much more than a fuzzy blob, but there was definitely a small point of light just off centre. Success!

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