SN 1987A, 20 years later
Published on Feb 23, 2007 at 1:32 pm.
2 Comments.
Filed under stars, supernova.
20 years ago today, I was in graduate school. Though I didn’t hear about it for a couple of days, Ian Shelton had discovered a supernova. Then, I heard that the first supernova of the year, SN 1987A (the second, if another were found, would be called SN 1987B, the third SN 1987C, and so forth). Well, that was significant, because back in those days, supernovae were discovered far less frequently than they are today with modern technology and automated searches. But, what made this discovery the most exciting was that this supernova was visible with the naked eye. It got up to third magnitude! That made it the brightest supernova in nearly four centuries. Also, it was the closest supernova since that time. Even so, Supernova 1987A was not within our own galaxy. It was in the Large Magellanic Cloud, a smaller galaxy that is a satellite of our own Milky Way Galaxy. That put the supernova at a distance of 168,000 light-years away. And that, as far as it is, is the closest that a supernova has been seen since the beginning of the Seventeenth Century. Unfortunately, this supernova was not visible from most of the northern hemisphere. If I hadn’t been a poor graduate student, I’d have caught a flight to somewhere where I could see this event. Who knows when the next one will be?
A supernova is an exploding star. When stars of sufficient mass die, they explode in a titanic explosion that momentarily shines nearly as bright as a whole galaxy full of stars. We had developed theories about what goes on with a star before it goes supernova, but most supernovae are so far away that we never actually get to see the star that blew up ahead of time. Being the closest supernova since the invention of the telescope, we actually had the opportunity to have already observed the star that exploded. The star that exploded had already been cataloged. It was named Sanduleak -69° 202a. But, this is when things got weird. Stars that go supernova are supposed to be red supergiants. This was a blue supergiant. Then things got stranger. The supernova showed classic spectral lines of hydrogen. That’s what you expect when a massive star blows up. The outer parts of the star are made of hydrogen. But, then the hydrogen lines faded. We’ve seen a few other supernovae since then with this characteristic, but this was the first one. Astronomers were baffled. And, then the Hubble Space Telescope was launched, and it was turned towards the location of the supernova, and it showed the picture above. Now, everyone was really confused. The multiple rings in the picture are not caused by the supernova itself. They were ejected long before, perhaps as long as 20,000 years before the supernova. 
They are glowing because they are reflecting the light of the supernova. As seen in this photo, at the center of the ring is a small blob of expanding material. That is the supernova remnant. (Click on the picture for a full sized view.) The rings have remained a mystery. There have been suggestions that the rings may have been the outer layers shed by the star somehow before it went supernova. More recently, computer simulations have shown that the inner ring may have actually been caused by the merger of two stars.
But, this marvelous supernova wasn’t through with its show. The blob in the center is the rapidly expanding supernova remnant. It quickly became apparent that the supernova remnant was going to overtake the ring. The outermost parts of the supernova remnant began to run into the ring in the early 1990’s. And when it did, the gas from the supernova remnant running into the ring material heated it up even more and it began to glow. I’m including a mosaic of images showing the ring as time passes and more of the gas from the supernova remnant hits the ring.
Now, the ring is virtually completely lit up, as seen in the even more recent image below, and the bulk of the supernova remnant hasn’t even reached it! The ring will likely become even more interesting as the next decade passes.
And, if all that weren’t enough, the Earth, 168,000 light-years away, got blasted with neutrinos from the supernova. Neutrinos are tiny weakly interacting subatomic particles that are given off in certain nuclear reactions, such as those going on with fusion in the Sun, or in the core collapse of a massive star in a supernova. Remember that I said that the supernova was shining for a while like a whole galaxy? Well, it was undergoing nearly an entire lifetime’s worth of nuclear interactions in a short period of time, too. All these neutrinos raced outwards through space. And, there were so many produced that even spread out across a sphere 168,000 light-years in radius, there were more neutrinos passing through us from the supernova than there were from the Sun for a while! That’s pretty impressive. We now think that these neutrinos play a crucial role in how supernovae happen. We have learned so much from watching just this one supernova unfold.
But, all through this, we need to be careful to remember that this was not a normal supernova. This was a blue supergiant blowing up, and most supernovae don’t have hydrogen spectral lines that disappear as the supernova develops. So, what gives? As I said earlier, a recent model for what may have happened is that this was originally a binary star system. Eventually, the more massive of the stars ran out of hydrogen to fuse in its core, so it began to swell up to be a red giant, as such stars do. But, the stars were close enough that when the massive star expanding, it started to distend and dump the mass from its outer layers onto the smaller star. As the smaller star gained mass, it had to begin fusing hydrogen faster to support itself, and it too became larger. Perhaps the stars began to overlap some. So much mass was being shed from the larger star that it was dumped into space around the system, forming a disk of gas. (There is some speculation that something like this may be happening with a star much closer to home called Eta Carinae). Eventually, the stars are moving in this common envelope of gas, and they spiral towards one another, eventually merging. The resulting star has to fuse furiously to keep up with all the mass, and it becomes a blue supergiant. But, it is a strange blue supergiant, because it is far too rich in the heavier elements such as helium and carbon common in dying stars. Hydrogen was a far smaller percentage of its mass than normal. So, it rapidly began going through the stages of nuclear fusion, reaching the end state of fusing into iron far sooner than would normally have occurred, and not allowing the outer layers of the star to expand into a red supergiant. In the 20,000 years that this took, the blue supergiant blew away the disk, forming the bright ring that was later lit up by the supernova. We still don’t know for sure what the outer rings are about. Perhaps they are from some sort of violent mass loss events when the star was a close binary, similar to those that Eta Carinae has been observed to have. Eventually, the star went supernova as the core of the star collapsed. The rest is history. NASA produce a neat graphic of this sequence of events, and I’ve posted it here. Click on it for a full sized view.
There is so much more that I can say about Supernova 1987A. We have learned so much from supernovae from just this one (albeit strange) example. And, we are still learning. Only the most massive of stars explode in this way. By far the majority of stars die much more quietly, “gently” shedding their outer layers into space before the core collapses to be a piece of ultra condensed material, mostly carbon, about the size of the Earth and a little over half the mass of the Sun. We call these stellar corpses white dwarfs. But, while stars that go supernovae are extremely rare, these explosions are among the brightest things in the universe. We can, and do, see supernovae billions of light-years away.
-Astroprof
(Images courtesy of NASA, HST, STScI)
Mark on February 25, 2007 at 1:43 pm: 1
by the way— my comment to the lunar eclipse (as good as that entry was) was actually meant for this entry! This blog entry was awesome.
Astroprof on February 25, 2007 at 4:10 pm: 2
Thanks for the kind words. I started this blog for fun, but it is nice to know that people get something out of it.