I discovered (via random folks posting to my twitter feed) this morning that we (as in "humanity") discovered a new supernova in the galaxy M82 in the last day or so. This is very cool for a lot of reasons.
So what is M82? M82 is a nearby galaxy, as galaxies go. It's not quite a spiral or an elliptical; it's usually categorized as an irregular galaxy. It's very close to the galaxy M81; the two galaxies are less than a degree apart on the sky, and are found not far from the tip of the bowl of the Big Dipper.
In the image above, M81 is the face-on spiral galaxy towards the bottom, and M82 is the thin line of a galaxy above it. If you look at this system with a radio telescope, you see that the two galaxies actually share a stream of gas; it's clear that the two of them are interacting with each other.
M82 is also what we call a starburst galaxy. This is a galaxy that's undergoing a burst of star formation. Stars are being formed in it much faster than they are in a typical everyday star-forming galaxy like our own. When stars are being formed rapidly, this will include some high-mass, short-lived stars. Higher mass stars may only live a few million or a few ten million years, which is a very short lifetime for a star. (For comparison, our star will live nearly ten billion years.) These high-mass stars also die with a bang: they explode in acore-collapse supernovae. So, you'd expect core-collapse supernovae to be more common in starburst galaxies than in regular everyday galaxies, simply because the high-mass stars that are the progenitors of core-collapse supernovae are being made at a faster rate there than they are elsewhere.
Unfortunately, often these starburst galaxies are very dusty, and the sites of most active current star formation are buried deep inside dust. As such, some of those supernovae may explode but never be caught because the light of the explosion is shrouded by dust.
M81 and M82 are very nearby galaxies, on the cosmic scale. According to NED, M82 is between 3 and 5 Mpc away; I'll use NED's "average" value of 3.8 Mpc. That means that M82 is 12 million light-years away. Whereas the supernova was just seen here on Earth in the last couple of days, it actually exploded 12 million years ago.
The new supernova was reportedly discovered by amateur astronomers in Russia (please suggest a better link or citation to said amateurs in the comments if you have one!), and has been confirmed by numerous people. As the image in the buzzfeed "discovery" link I just gave you shows, this supernova is actually on the outskirts of M82. What's more, this supernova has been shown by its spectrum to be a thermonuclear supernova rather than a core-collapse supernova.
Very massive stars (more than eight times the mass of the sun) end their lives in a core-collapse supernova, blowing themselves all to hell and leaving behind a neutron star. Stars of lower mass end their lives in a much more sedate (but still pretty awesome) planetary nebula and leave behind a white dwarf. A white dwarf is a star that's about the mass of the Sun, but only the size of the Earth. It is hot just because of left over heat from its formation; it's no longer actively generating energy inside it. It just sits there and cools off. A typical white dwarf is made up of carbon and oxygen (although in a rather exotic state that I won't go into right now). It's not as mind-bogglingly dense as a neutron star, but it's still amazingly dense. There is an upper limit to the possible mass of a white dwarf star (at 1.4 times the mass of the Sun); one over the size of that would collapse under gravity. If a real white dwarf does grow to this size (either by colliding with another white dwarf, or by pulling matter off of a companion star over time), it starts to collapse under gravity, but doesn't complete that collapse. It gets dense enough to trigger runaway nuclear fusion of the carbon that composes it. Basically all of the carbon in the white dwarf undergoes fusion, and a chain reaction of fusion continues until all the atoms have combined to make iron (or something right about the same size as iron). In other words, the white dwarf becomes... a thermonuclear bomb that is one and a half times the mass of the sun.
That's what happened in M82.
Because these supernovae are so amazingly bright, they can be seen out to extreme distances. I'm not talking M82; I'm talking many, many, many times farther away. Whereas M82's supernova exploded just 12 million years ago, we've seen supernova out to distances that means they exploded 10 billion years ago. This property, together with other properties, of this type of supernova makes them lighthouses useful for measuring the history of the Universe. Because we can see them so far away, we can see supernova from when the Universe was much younger. Indeed, it was measurements of these supernovae that allowed us to measure the history of the expansion rate of the Universe and discover that the Universe's expansion is actually accelerating.
It'll be fun to watch over the next weeks and months as this new nearby supernova gets brighter and then fades away.