Tuesday, June 14, 2011

A new type of supernova?

Image Credit: Caltech / Robert Quimby / Nature
Last week, a group of astronomers led by Caltech astronomer Robert Quimby announced that they had learned a few crucial pieces of information about these enigmatic sources.  This new evidence suggests that we are seeing a new type of stellar explosion, though we still don't know exactly what we are seeing.

Two years ago, I attended a conference on supernovae (exploding stars), and I blogged about weird objects that we could not explain.  In apparently blank parts of the sky, a couple of "new stars" had appeared and slowly faded away, just like supernovae.  Only these new objects changed their brightness on much longer time scales than normal supernovae, they did not appear to be located inside another galaxy, and their spectra showed weird features that could not be identified with certitude.  Many different explanations were proposed, from white dwarf stars in our own Milky Way Galaxy to carbon stars being shredded by black holes halfway across the Universe.

These new types of supernovae were discovered as part of the Palomar Transient Factory program.  This program continuously searches several patches of the night sky, looking at these same patches over and over and over again, night after night, week after week.  The program uses an automated telescope and camera that looks for things changing in brightness and tries to figure out what they are.  Automated routines try and figure out what the objects might be, such as a variable star or a supernova, and then flags the most interesting ones for further investigation.  Several similar searches are underway and are discovering ever-increasing numbers of "transient" events, or objects changing their brightness on short time scales.  These searches have turned up new types of supernova explosions before.

So, while these weird new types of supernovae are rare, the automated searches never tire and found many instances.  When Quimby's team finally had several examples to go off of, they looked for similarities between each of the supernovae.  We'll look at a few of them.

First, and most importantly, Quimby and his collaborators were able to figure out where these objects were.  (As I said above, as of two years ago, we didn't know if they were right next door to our Solar System or halfway across the Universe, billions of light-years away).  Each of these "new stars" was associated with a different, extremely faint galaxy.  These faint galaxies have less than 1/100th the number of stars that exist in our Milky Way, which makes them hard to see.  It also means that astronomers have to explain why these explosions only happen in very tiny galaxies.  You'd expect that this type of explosion should be much more common in galaxies that have hundreds of times more stars, but they don't seem to be.

Second, the team was able to measure distances to the weird supernovae.  These distances ranged from a few billion light-years (relatively nearby, in astronomical terms) to ten billion light-years (a good fraction of the size of the visible universe).  Once we know how far away something is, and since we know how bright it appears to be, we can calculate how bright it actually is.  Doing so, the team found that these objects are much brighter than most normal supernovae.

Third, we know that the Universe is expanding, and that expansion changes what we can see in the spectrum of an object due to the Doppler shift.  The spectrum of an object contains information about what it is made out of, its temperature, and even how fast it is moving.  Most astronomical objects have fairly recognizable spectra, and so astronomers can calculate the amount of the Doppler shift and account for it in their analysis.  But these weird supernovae had spectra that astronomers could not recognize.  Quimby's team finally cracked the code by finding a couple of recognizable features in the spectra, which allowed them to measure the Doppler shift and correct for it.  When they did so, they found that all of the weird supernovae had very similar spectra (showing that they are the same type of object), and that these supernovae have very different chemical makeups than normal supernovae.

These three new bits of information are crucial.  When we discover new things in astronomy, we always want to know how far away they are, how bright they are, and what they are made out of.  And we now have that information.  But we still don't know exactly what we are seeing! 

Think of it this way: suppose you see smoke on the horizon, and after some measurement you figure out that the smoke is from a wood fire 5 miles away with flames leaping 50 feet in the air.  That's all well and good, but you probably would like to know what is burning.  Is it a tree?  Is it a barn?  Is it a bonfire?  Is it your house?  Or is it something else entirely?  The answer to this question could be very important to you! 

Likewise, here we see something that in some ways looks like an exploding star, but yet it is brighter and has a different chemical makeup.  Also, this explosion seems to only happen in dwarf galaxies and not in big galaxies like our Milky Way.  What could it be?

One idea that Quimby's team proposed is a hypothetical kind of supernova called a "pair-instability supernova".  As stars go through their life cycles, they convert hydrogen into helium, and helium into heavier elements like oxygen, calcium, silicon, iron, and every other element known in nature.  The more stars a galaxy has, the more of these heavy elements that are made.  So, big galaxies like the Milky Way have a lot of these heavy elements, while dwarf galaxies (like the host galaxies of these weird supernovae) have a lot less. 

It turns out that if you try and make really big stars, say stars that are a hundred or more times the mass of the sun, it gets really hard if the star is polluted by a lot of heavy elements.  The most massive stars in the Milky Way seem to tip the scales at about 100 times the mass of the sun.  But in dwarf galaxies, with few metals around, you can make even more massive stars.

Not only that, but metal pollution changes the life cycle of a star.  Stars with lots of heavy elements tend to shed a lot of weight as they get older.  A heavily-polluted star that starts out with 100 times the mass of the sun might end its life with only 1/10th that amount.  But a star with very little heavy element pollution can keep all of that mass.

Computer models of extraordinarily massive stars that keep all of their material as they approach their death show that these stars can get ridiculously hot in their centers.  They can get so hot that they begin to produce antimatter.  Producing this antimatter makes the star unstable, and it blows itself apart in a supernova, and likely one that is very bright with different chemical composition than the more-common, typical supernovae we see coming from metal-polluted stars in big galaxies.

So, are these weird supernovae really the hypothetical pair instability supernovae?  Maybe.  Maybe not.  Quimby's team mentions another possibility (involving a magnatar, a star with a super-strong magnetic field).  There may also be other, even more exotic explanations.  And I've seen other, different types of supernovae explained using pair-instability supernovae -- since it is a hypothetical kind of explosion, they make good scapegoats for things we just don't understand well.  But this is how science often works.  These new, weird supernovae and pair instability supernovae are two pieces of the giant jigsaw puzzle of the Universe.  Maybe they fit together, maybe not, but we might as well try it and see!

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