Have you ever had a time that too much hype can take the fun out of something? Like the Superbowl, or these presidential primaries that are talked about for weeks as "crucial turning points," and the results, while important, are far from crucial or deciding?
Yesterday NASA had a press conference that they had hyped for a week. All they would say was that it would be an important discovery of something astronomers had been looking for for more than 50 years. Speculation was rampant, running from black holes to planets to dark matter to aliens. Even from my inside perspective, I had little information. All I knew was that it involved both X-ray and radio telescopes. The X-ray telescope involvement meant it had to be something with tremendous energy -- planets and aliens don't produce nearly enough X-rays for us to detect on Earth. What in the world could this amazing, earth-shattering discovery be?
When it was announced yesterday, I felt a bit let down. The discovery was of the youngest known remnant of a supernova (exploding star) in our Milky Way Galaxy. This is both interesting and important, but I don't know that it is the culmination of a 50-year hunt, and I don't see why they kept all the secrecy about the press announcement, since the paper announcing the discovery was posted to our preprint server (a place where astronomers can put papers for each other to read before the journal with the official paper comes out) on April 15, one entire month ago. Granted, we did the same thing with our press release a couple of weeks ago, but we never tried to hype up some secret new discovery. If anyone had asked, we would have told them. But, NASA seems to get its jollies from hype, even though the science alone is usually sufficient to pique the public's interest.
So, what is the science here, and why is it interesting (even without NASA's hype)? It stars with exploding stars. Supernovae, or the explosion of stars, are rare events. Various people have estimated how often a supernova should happen in the Milky Way, and the best guesses end up once every 50 to 100 years. But keep in mind that this is a long-term average; stars don't know when other stars explode, so sometimes you could have several supernovae go off in a short time frame, and other times you could get really long lags between supernovae.
The last supernova in the Milky Way Galaxy seen from Earth was in 1604. This explosion, called "Kepler's Supernova", was observed by famous astronomer Johannes Kepler, but was before the invention of the telescope! Since then, the skies have been dark, although we are all ready and waiting for a new supernova.
So, are we in one of the long lags that can sometimes happen? Maybe, but maybe not. 400 years is a long time, even given the rarity of supernovae. More likely is the fact that the Milky Way is full of dust that is great at blocking optical light. If a supernova were to happen behind one of the many dust clouds, it would be invisible in optical light on Earth, even though the supernova would outshine the entire galaxy! And since most of the stars in the Milky Way are located toward the center of our galaxy, which is hidden behind an amazingly thick layer of this dust, we would expect that many, if not most, supernovae in our galaxy would be invisible to human eyes.
We have some evidence that this dust blocking has happened in the past. One of the brightest sources of X-rays in the sky is called Cassiopeia A, or Cas A for short. In X-rays and radio waves, Cas A looks like a supernova remnant -- a lot of material is shooting out from a common center at a very high speed, just like shrapnel from any explosion would. When we look at pictures of Cas A taken several year apart, we can see the expanding cloud of shrapnel growing in size. If we run time backward and see when all of the material was in the same spot (in other words, the start of the explosion), we estimate the explosion was 300 years ago. But no explosion was seen from Earth at that time, in spite of there being many astronomers with telescopes around the world.
So, astronomers have been looking for other young supernova remnants. These would be bubbles of hot gas expanding at very high speeds. Since high-energy X-rays and certain radio waves can pierce through the dust in our galaxy, most searches fave used X-ray or radio telescopes. And by measuring the rate at which the bubble is expanding, we can again estimate the age of the supernova.
Which brings us to yesterday's press conference. One supernova remnant with the less-than-exciting name of G 1.9+0.3 was studied with both X-ray and radio telescopes (the picture at the top of this page is the combined X-ray and radio pictures of this object; click on the picture to go to the press page for more pictures and information), and the estimated age of the explosion is only about 100-150 years. In other words, the explosion probably happened around the time of the U.S. Civil War, if not even more recent. There have been many astronomers with big telescopes around, but nobody saw the explosion.
This is not surprising. The supernova remnant is located only about 1000 light-years from the center of our galaxy (whereas we live 25,000 light-years away), and is located in a region with lots of dust to block visible light. The explosion could easily have gone unnoticed by astronomers at the time.
But the search for young supernovae is not over. There may be other very young explosions waiting to be discovered by the same method. Our galaxy continues for tens of thousands of light-years on the other side of the Galactic Center, and all that part of the galaxy has been invisible to our eyes until the invention of radio and X-ray telescopes. And we need to find these young remnants if we want to be able to estimate how often stars explode, which in turn is important for understanding what kind of stars do the exploding!
Last, could we miss a supernova today? I think the answer is probably not, for two reasons. There are a lot of astronomers, especially amateur astronomers, who are constantly scanning the skies, looking for the faintest pinpoint of light that wasn't there the night before. Even with all of the dust in our Galaxy, these amateurs might be able to see a faint new star. And, in a decade or so, astronomers will have the LSST, a large telescope scanning the entire sky every few nights to look for new and changing stars.
Second, we also have full-time neutrino observatories. Neutrinos are elusive subatomic particles that are produced in large numbers by supernovae, and they are not stopped in the least by dust in our Galaxy. In 1987, a supernova in the Large Magellanic Cloud (one of our closest neighbors in space) exploded, and we detected several neutrinos from that explosion. An explosion in our galaxy will produce many more neutrinos that we will detect, and we will know that, somewhere, a star in our galaxy has just exploded. And every professional telescope in the world would start scanning the skies for that explosion for the opportunity to bring modern astronomical instruments to bear on what will be the most exciting astronomical event in decades!