Anatomy of a Supernova

Stars of all masses spend the majority of their lives fusing hydrogen nuclei into helium nuclei: we call this stage the main sequence. When all of the hydrogen in the central regions of a star is converted into helium, the star will begin to "burn" helium into carbon. However, the helium in the stellar core will eventually run out as well; so in order to survive, a star must be hot enough to fuse progressively heavier elements, as the lighter ones become exhausted one by one. Stars heavier than about 5 times the mass of the Sun can do this with no problem: they burn hydrogen, and then helium, and then carbon, oxygen, silicon, and so on... until they attempt to fuse iron. Iron is special in that it is the lightest element in the periodic table that doesn't release energy when you attempt to fuse it together. In fact, instead of giving you energy, you end up with less energy than you started with! This means that instead of generating additional pressure to hold up the now extended outer layers of the aging star, the iron fusion actually takes thermal energy from the stellar core. Thus, there is nothing left to combat the ever-present force of gravity from these outer layers. The result: collapse! The lack of radiation pressure generated by the iron-fusing core causes the outer layers to fall towards the centre of the star. This implosion happens very, very quickly: it takes about 15 seconds to complete. During the collapse, the nuclei in the outer parts of the star are pushed very close together, so close that elements heavier than iron are formed.

What happens next depends on the mass of the star. Stars with masses between about 5 and 8 times the mass of our Sun form neutron stars during the implosion: the nuclei in the central regions are pushed close enough together to form a very dense neutron core. The outer layers bounce off this core, and a catastrophic explosion ensues: this is the visible part of the supernova. Stars with masses greater than about 10 times the mass of the Sun meet a very different fate. The collapse of the outer regions of the star is so forceful that not even a neutron star can support itself against the pressure of the infalling material. In fact, no physical force is strong enough to counter the collapse: the supernova creates a black hole, or a region of spacetime that is so small and so dense that not even light can escape from its clutches. In this case, the details of how the ensuing explosion actually occurs have still to be worked out. Observationally, supernovae are found by patiently observing the sky and looking for bright objects where there were none before. At its peak luminosity, the supernova resulting from a single star may be bright enough to outshine an entire galaxy.

The conclusion seems to be that a supernova would need to be within tens or hundreds of light-years from us to cause significant damage to the Earth and life on our planet.

As for the damage that a supernova would cause, the x-ray and gamma ray light emitted by the supernova would probably be our biggest concern. Without the Earth's atmosphere to protect us, x-rays and gamma rays can do significant damage to the molecules that make up living organisms. And supernovae do put out a huge number of x-rays and gamma rays; even if a supernova is thousands of light years away, it will still dump gamma rays on us at a faster rate than the sun does during its most active periods (i.e. when it is undergoing solar flares).

Luckily, though, our atmosphere easily protects us against solar flares and would probably do a good job against much larger gamma ray fluxes as well. You'd have to get to the point where the gamma ray flux was so high that it was destroying a significant percentage of the molecules in the protective layer of our atmosphere before you could really say that the supernova was damaging our environment.

Originally posted by azdude1804
ive never really seen much of a show from the universe, except i think 2 comets, one being hail/bob (or something like that)...