Wednesday, January 22, 2014

Closest Supernova Since 1987 Explodes in M82 Galaxy!

Well this completely interrupted the other post I was working on, but for good reason! A supernova has gone off in the nearby (11.5 million lightyears is what astronomers consider "nearby") galaxy Messier 82, shown below in a Hubble Space Telescope visible/near-infrared composite image.

Meanwhile, my Twitter feed has exploded with people who are excited about the supernova and talking amongst themselves about what's going on, who found it, what type it is, why that's unexpected, and so on. Yes, that's right. I follow a lot of astronomers on Twitter.

The supernova was officially discovered by University College London with the University of London Observatory. The picture below is the discovery image, which shows images of the galaxy taken on December 10th, 2013 and January 22nd, 2014.
Image source: http://www.ucl.ac.uk/maps-faculty/maps-news-publication/maps1405
The supernova itself was observed, and spectra of the supernova itself have determined that it is a Type Ia supernova (source: Astronomer's Telegram). This means that the spectra the observers saw looked something like the following image. (Note that these are examples and NOT the spectra of our friend in M82).
Image source: http://www.aip.de/highlight_archive/christensen/index.html
Astronomers typically classify supernova by the absorption lines observed in the spectra of the supernova. A Type I supernova has no hydrogen lines in its spectra, while a Type II supernova has hydrogen absorption lines. Type I supernova are further sub-divided into subtypes Ia, Ib, and Ic. Type Ia supernova feature large silicon absorption lines (which you can see in the example spectrum above), Type Ib feature large helium absorption lines, and Type Ic feature none of the above.

Hilariously enough, the spectral classifications of supernova have NOTHING to do with their origins. Types Ib, Ic, and II are all core-collapse supernovae, meaning they originated from the death of a massive star in the type of explosion that is responsible for creating all of the heavy elements (heavier than nickel) in our universe. A Type Ia supernova, which our friend in M82 here happens to be, is the result of a white dwarf exploding somehow. Interestingly enough, we're still not sure exactly how that happens.

As a brief aside, a white dwarf is a type of stellar remnant that comes from stars that are not massive enough to explode as supernovae. Stars support themselves with the energy released from nuclear fusion reactions that take place in the stellar cores. One consequence of this is that you steadily build up heavier elements that are harder to fuse together (because positively charged objects like nuclei will repel one another).  In a star like the Sun, you'll build up a ball of un-reactive carbon and oxygen that will basically just sit there, because the core won't become hot enough to continue fusion. The only reason it won't collapse on itself is because of a quantum mechanical oddity known as degeneracy pressure. In simple terms, degeneracy pressure is quantum mechanics telling the electrons in the carbon atoms that they have to stay a certain distance apart from one another. Meanwhile the rest of the star will become unstable and will eventually lose its outer layers as a planetary nebula (the best example of which, I think, is the Ring Nebula). The mostly carbon & oxygen core is left behind as a white dwarf.

There are currently two competing models for how a Type Ia supernova occurs. One model suggests that a white dwarf slowly steals mass from a stellar companion (usually a giant star that has a hard time holding on to its outer layers) until it reaches a total mass of about 1.4 times that of the Sun. At this point, the quantum mechanical forces responsible for holding a white dwarf up fail, and the white dwarf collapses upon itself, squeezing everything inside it together such that it flash-fuses into heavier elements like silicon, iron, nickel, and so forth. This flash-fusion blows the once-white dwarf apart and releases a LOT of energy (the same amount as the Sun will release over its entire Main Sequence lifetime). The second model has two white dwarfs merging with one another to pretty much the same effect, though it seems that this can cause supernovae at both higher and lower total masses. The progenitor mass is important because it affects how luminous the supernova will be. A higher total mass means more material to fuse, so you get brighter Ia supernovae than you would expect, and vice versa for lower total mass. We do, in fact, observe Ia supernovae that are both brighter and fainter than the median by significant amounts. The single white dwarf model has some trouble explaining this.

Now, I've taken a seminar on this exact topic, and it's almost comical to see people argue back and forth in published, peer-reviewed scientific journals over which model of Type Ia supernova best supports their data or their simulations and so on. We astronomers literally have no clue whatsoever which model is correct! The main reason for this is that the progenitor objects for Type Ia supernova are a lot harder to find than, say, a red or blue supergiant exploding as a core-collapse supernova. So until we get REALLY lucky, it's just a guessing game. I personally feel like the actual answer is that both are equally plausible and what we see is the result of both different mechanisms (this is a pet hypothesis with no actual evidence, but it fits what I think are certain trends in the data concerning the spread in Type Ia luminosities).

Few other points worth noting before I close. M82 is in the constellation Ursa Major (you know, the Big Dipper) which is pretty prominent in the sky right around now. This image from Universe Today (clearly made by someone using Stellarium) shows you roughly where to point your telescopes relative to the Big Dipper if you want to take a look for yourself. Please note that this only shows the relative position of M82, Polaris (in the Little Dipper) and the end of the Big Dipper. Remember, stars aren't stationary in the sky, so the Big Dipper won't always be in that exact position. Rotate the image as necessary.

The supernova is currently 11th magnitude, which can likely be seen with a 6 inch telescope (if you're not in a major city), but is still getting brighter. It is predicted to reach peak brightness at about 8th magnitude (yes, the magnitude scale is ass-backwards) in 12 days (February 3, right after the Superbowl!) at which point you should be able to easily see it with a decent pair of binoculars. I know I'll be looking!

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