So apparently there are big burning things in the sky called stars. They are unstable, and I don't want to scare you, but they occasionally explode. When they explode, you don't want to be near them. But how far are you supposed to get away? It doesn't really matter, since if we're too close to any star there's nothing at all we can do about it, but let's find out if we're all doomed.
When a star gets sufficiently massive, its entire life is a balancing act between heat and gravity. As gravity pulls in the star's outer layers, the pressure of them causes the center of the core to heat up through fusion. As the center of the core heats up, it pushes the outer layers away, releasing pressure. Eventually, as fuel runs out in the inner core of the star, the outward push of the heat can suddenly fail, causing the outer layers to fall in fast. That causes a massive explosion of heat and energy inside the stars and blows off the outer layers once and for all in a supernova.
No one wants to be standing close to that, for a lot of reasons. Not only can the outright heat and debris annihilate anything in its path, the gamma radiation - radiation with high enough energy to rip electrons away from atoms - will do a lot of damage as well. A too-near supernova could kill off most of life on Earth simply by allowing the radiation to strip away our ozone layer. Anyone who has taken a gander at the sky has noticed that these ticking time bombs are all around us. How close can we get to any of them before they blow us away?
The livable distance would depend on the size of the nova, but Earth has seen a couple of nearby supernovas in its time. Millions of years ago, Australopithecans would have seen an extra sun in the sky for a little while. The brightness in the sky originated in the Sco-cen cloud, then between 130 and 450 light years away. It may have changed Earth's ecosystem and ushered us from the Pliocene to the Pleistocene era. Since then, supernovas have been recorded in the year 185, 1572, and 1604, their distances ranging from a few hundred to thousands of light years away, but none would have been as near as Sco-cen. So we can weather something between 130 and 450 light years away. Other than that, estimates cover a wide range. In 2010, there was a short kerfuffle when we were worried about a star 3,000 light years away. Back in 2002, we were concerned that a star about a 150 light years away would gather fuel and become large enough to eventually go nova. However, it seems that NASA estimates, with a wide margin of error, that we'll keep struggling along as long as we have about 25 light years between us and a supernova. Since there are only about 75 stars within around 20 light years of us, it seems we don't need to be biting our fingernails just yet.
Top Image: NASA/JPL