Why m82’s new supernova excites astronomers

Galaxy M82, home of the new supernova. Credit: NASA, ESA, The Hubble Heritage Team, (STScI / AURA)

Last Wednesday, a supernova kept me up at night. The supernova itself was over and done with 12 million years ago, when a white dwarf star in M82 nuclear-exploded itself into oblivion, lighting up brighter than its home galaxy, and condemning to extinction any unfortunate life-forms that might have been within a 50 light years of the blast. As the light from the shock wave made its journey across the cosmos, slowly brightening toward its peak luminosity, the news spread through the network of sky-watchers around the world. Observers passed around sky coordinates to use to get their own images; amateur astronomers searched their recent photos; theorists speculated about the supernova’s ignition mechanism, and night-sky enthusiasts generally enthused. I watched Twitter long into the night, giddy with the excitement of it all – firing off questions, tweeting incoming discoveries, and doing back-of-the-envelope calculations about what kinds of discoveries the supernova might bring.

Astronomers are used to things that change slowly, over millions or billions of years, so when something goes BANG in the sky, we tend to kind of lose it. It’s certainly partially just the novelty, but it’s also because events like this give us a chance – and a fleeting one at that – to watch some of the most energetic and revealing processes in the Universe as they happen. In this case, there were plenty of reasons to get excited. The supernova is nearby – there’s been nothing significantly closer since the 80s – and the closest of its kind that we’ve caught in progress since modern observations began. It was also extremely bright. In fact, the supernova was so bright that automated supernova searches missed it, accidentally flagging it instead as a foreground object. It wasn’t until a lecturer at University College London was going through sky images his students took for a class that someone pointed out a bright speck that didn’t seem to have been there before. An alert went out to the community and images of the supernova were found in data from other telescopes, taken as long as a week before the discovery.

Perhaps the most exciting thing about the supernova, from a cosmologist’s point of view, is that it’s a Type Ia supernova. Supernova types are based on spectral properties of the blast as seen from Earth, and what kinds of elements are seen, but physically they correspond to different explosion mechanisms. Very massive stars end their lives as Type II (core-collapse) supernovae, in which the inner core of the star runs out of fuel after fusing everything it can into iron, and the star collapses upon itself before blowing apart its outer layers, leaving either a black hole or neutron star in its place. Low-mass stars, like our Sun, have a chance of ending their lives relatively peacefully, as slowly fading white dwarfs. But if a white dwarf has a companion star, it can strip off the outer layers of its companion, growing in mass until it reaches the Chandrasekhar limit – the maximum mass before a white dwarf will collapse on itself. That collapse creates a spectacular nuclear explosion, destroying the entire star in the process.

To a stellar astronomer, witnessing this explosion can provide invaluable data about the structure of the star and the mechanism by which white dwarfs manage to exceed the Chandrasekhar limit. It’s not a solved problem – some have suggested that instead of growing too massive by stripping mass from a companion star, a white dwarf explosion can be triggered by colliding and merging with another white dwarf. A bright, nearby supernova should help us determine if Type Ia supernovae come from the single-degenerate (binary star) scenario, or the double-degenerate (white dwarf merger) possibility – or if perhaps both mechanisms can occur.

To a cosmologist, though, the stellar astronomy is only an intermediate gain. Type Ia supernovae are used by cosmologists as “standard candles” – explosions whose intrinsic brightness we know already, and therefore whose apparent brightness can tell us their distance. We know that white dwarf stars always explode when they exceed the Chandrasekhar limit, and we know that explosion happens in pretty much the same way for every white dwarf star, with just a few extra variables that need to be calibrated from nearby supernovae whose distances we know by other means.

Standard candles serve as distance markers throughout the cosmos. They let us determine the distances to galaxies that are being pulled away from us by the expansion of the Universe. We can measure a redshift in the light of these galaxies due to the cosmological expansion, so we know how fast they’re moving away from us, and if we know their distance from the supernovae, we can measure the expansion as a function of distance or time. Expansion data from Type Ia supernovae first led astronomers to discover the accelerated expansion of the Universe. We know from these measurements that dark energy is the dominant component of the cosmos, but we still don’t know what it is. Better expansion data, from improved supernova calibration, will bring us closer to understanding this deeply mysterious phenomenon, and help us predict how dark energy will change the structure of the Universe in the future.

Last week’s supernova, now dubbed SN 2014J, is still getting brighter. At its peak in the next week or so, it might even be bright enough to see with good binoculars, if you happen to live in the northern hemisphere and have access to a very dark sky. While we won’t be able to detect neutrinos from this supernova (unfortunately only Type II supernovae create significant numbers of those), observations happening now in as many wavelengths as existing telescopes can see will give us a truly unprecedented view of an immense cosmic explosion. And that could lead us to something even bigger – the structure and fate of the universe itself. Exciting times, indeed.

Katie Mack

Dr Katie Mack is an astrophysicist based at North Carolina State University. Her webpage is www.astrokatie.com.

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