Here are some randomly assorted tidbits about SN2011dh and other supernovae.
First, the simple...in 1572, astronomer Tycho Brahe noticed a new star where there had been none. This was just before the age of the telescope in astronomy, and Brahe was one of the first scientists to collect careful data about what he saw. He published a book about this (and many other things) called De nova stella (On the new star) the next year. "Nova" is Latin for "new". In time, astronomers came to call "new stars" novas, or novae. In reality, novae are not new stars at all, they are stars that have become brighter, but that would not be known for a while yet.
In the 20th century, astronomers began to finally understand how impossible big the universe is. When novae were observed in other galaxies, it was obvious they must be pretty bloody big explosions. The term supernova was coined in the 1920s to describe those.
There are two types of supernova. Type I involves stars about the size of our own, which for various reasons blow off their outer layers. Our star runs by converting hydrogen into helium, using the vast heat created by its own gravity. Once the hydrogen runs out, it will fuse the helium into other stuff like carbon, and then sort of shrink into a white lump of hot stuff called a white dwarf. That will be the end of the road for our own Sun. But if a white dwarf has a close companion star, and if they are close enough together, it can collect matter from that star. Once enough stuff is collected, it explodes; that is a Type I supernova, and it usually leaves the original star not too badly damaged. Binary stars can do this repeatedly, and many planetary nebulae are the gases blown off by Type I explosions, still lit from within by the parent star or stars.
But really big stars can keep fusing elements into heavier and heavier stuff, until the core is finally made up of iron. Unfortunately for the star, fusing iron doesn't release any extra energy, and the core collapses under the incredible force of its own gravity, and at nearly 1/4th the speed of light. Once it collapses so far that the nuclei in its atoms are touching, it can't go any further, and the collapse is stopped so rapidly that a shock wave bounces back out, blowing away the remaining part of the star. THAT is a Type II supernova. If the original star was less than 20 times the mass of our sun, it will leave behind a Neutron Star, such as the one that I "discovered" earlier this year. If it' over 20 solar masses, a black hole will result.
Supernovas used to be observed only every century or so, but with the sheer number of amateur astronomers today, there are a few hundred per year seen throughout the visible universe. They are still very rare-out of the scores of billions of stars in our galaxy, supernovae only occur once every 50 years or so. Supernovae are given names made up of the year they were observed, followed by a letter A-Z. After the first 26, they start with aa, then ab, etc. This would therefore be the 112th supernova detected in 2011, although it is certainly one of the most visible.
Even though Type I and Type II events are so totally different, to the eye they appear very similar. Only when instruments were invented that could detect the presence of certain elements was it learned that there were two types. A spectrograph can show what elements are present at a light source. A Type I explosion shows no hydrogen, since the parent star has already used up its hydrogen. The kind of star that is big enough to implode can do so before the hydrogen runs out, so it does show hydrogen spectra. The names Type I & II were well established before it was understood just how unrelated the two things really are. Words have a way of sticking around unbidden at times.
With an estimated peak brightness of around 1 billion solar luminosities, if SN2011dh had occurred 30,000 light years away it would appear to us to be as bright as our sun.30,000 light years is a terrible long distance...our entire galaxy is perhaps 100,000 light years across. If such a supernova happened within 3,000 light years, it could be pretty bad for us, since the radiation from the blast would remove all of the ozone from our atmosphere. That is bad for our DNA, since ozone blocks much of the dangerous stuff from our own sun. There is some evidence that the Ordovician Extinction nearly half a billion years ago was caused by a nearby supernova.
There are no stars within that distance today that are candidates for such an event, so the job of wiping out all the life on our planet is left in our capable hands.
All of the elements heavier than iron are formed in the explosions of this type of supernova. If you are wearing any gold jewelry, that gold was created in the few moments after the collapse of the core in a supermassive star. By heavier, I mean having a greater atomic weight-go Google "periodic table" and have a look.
SN2011dh hit its peak brightness within a week or so, and will fade over the next year until we can't see it in amateur scopes any longer. After a couple more weeks, it is expected to fade to a plateau for several months, then continue fading. that plateau will still be easy to photograph with scopes like mine, however.
As a last unrelated thought-if they are correct about the progenitor of SN2011dh being around 25 solar masses, then it formed a black hole in the first moment of the event. However, contrary to the popular notion, it will not "suck up" everything in its area. For a simple thought experiment, imagine that our star were to become a black hole (it can't, but just play along.) Assuming it had the same mass as it did when it was just our sun, the Earth would not be affected at all-it would still be orbiting something with the mass of one sun. Our year would remain unchanged; the moon would still orbit us at the same rate, etc. Of course, it would get very, VERY cold.
No comments:
Post a Comment