SN 2012 aw
2 galaxies 38 million light years away in Leo
Eocene - first cats appeared
Here I have photographed two galaxies in Leo, much further away than M31 (Andromeda). The stars in this photo are all in our own Milky Way, with one exception. The one marked with short lines is in that distant galaxy.
Normally we amateurs cannot possibly resolve individual stars in other galaxies. This is one that has exploded as a supernova, almost outshining its galaxy.
I read that someone had spotted that this one had occurred and I went out to try to photograph it.
A supernova can occur in more than one way but one possibility is as follows. This could have been a star larger than the Sun. It reached the end of its hydrogen fuel. That meant that the radiation coming out was no longer enough to balance gravity. So it started to collapse. That caused the temperature and pressure to increase until new kinds of nuclear reaction set in: helium converting to heavier elements such as nitrogen, oxygen, carbon, and so on up to iron. This process generates much more radiation and after only a matter of hours the star explodes. Hence the bright object we see. Importantly for us, that sends the heavy atoms out into space. Eventually they may become incorporated into new stars and planets around them.
We would not want to be near a supernova when it happens but without such events we could not be here: the universe would only consist of hydrogen and helium.