The media loves to grab onto any new technology or experiment that has even the tiniest whiff of controversy about it and whip up some fanciful scare stories about how it will (not if, will) end the world and we all have to sign a petition and protest. Fair enough, with big experiments there is always going to be a small risk involved. The potential benefits to science and progress always far outweigh the potential risks, however, as if they didn’t the experiment just wouldn’t be feasible.
A recent example is the LHC. Everyone got into a huge panic shortly before the first beam last September when the idea came to light that a micro black hole might be formed. People immediately hear the words ‘black hole’ and ‘new experiment’ and ‘expensive’ and immediately assume a group of geurilla scientists are trying to take over the world. These people should not be allowed to leave the house.

The purpose of this post is not to address these trivial issues though. It is infact to address a much more serious and much more likely issue. That a nearby supernova will wipe out all life on Earth.

A supernova is what happens when a massive star begins to collapse under its own gravity, then, not being able to take the immense pressures and forces involved, suddenly begins rapid nuclear fusion and catastrophically explodes, expelling a vast amount of energy and leaving behind a neutron star or black hole.
One of the most famous supernovae is SN1987A, which is the first supernova to occur in 1987. It was highly luminous and even visible to the naked eye for a while. But even then, 1987A was a distant supernova.
A supernova closer to home, say, within a kiloparsec, has a high chance of ending a lot of life.

Some calculations:

Assumptions:
1. The stars in the Milky Way are spherically distributed.
2. The mass of the Milky Way is entirely composed of stellar objects.
3. All stars in the Milky Way are roughly as massive as the Sun.

Taking these assumptions into consideration, we will most likely come out with an underestimate of the stellar number density of the Milky Way, purely based on assumption number 1.

Mass of the Milky Way: ~ 6×10^11 Solar Masses (1.42×10^42 kg)
Radius of the Milky Way: ~ 50,000 Light Years (4.7×10^20 m)
Average Density of the Milky Way: ~ 3.2×10^-21 kg/m^3

Now assumption number 4, the stars are uniformly distributed throughout the Milky Way.

Density within a radius of 1kpc: ~4×10^38 kg
Stars within a radius of 1kpc (using assumptions 2 and 3): ~200,000,000

So if we make an underestimate that 1% of the stars in the Milky Way are large enough to produce enough energy in a supernova to kill us all, that makes close to 2 million high mass stars that are close enough to wipe out all life on Earth.

In fact, it’s been proposed that the reason for one of the mass extinctions in the Earth’s history was caused by a nearby supernova.

The reason we aren’t all kicking up a fuss about it? Well, for one, there’s absolutely nothing we can do about it. There’s no way to predict when a supernova will happen (aside from observe the stars for evidence of their evolutionary stage) and there’s definately no way to stop one. The second reason is that a supernova is a relatively rare event. The Milky Way has around 100 billion stars. The average rate of supernovae in our galaxy is around 5 per century. So the probability of a single star in the Milky Way going supernova is around 3×10^-14. This means that we can expect a nearby (within 1kpc) star to go supernova around once every 1,000,000 years.
A mass extinction caused by a nearby supernova has, literally, a one in a million chance of happening.