Expert: Philip Stahl - 10/20/2007

Question
How close would a supernova have to be to Earth to kill people? How close
would it have to be to adversely affect electronics? How close would it have to be
to affect radio transmissions?

Answer
Hello,

Both Type I and Type II supernovas can occur, and the Type II have strong hydrogen spectral lines while Type I do not. This means the Type I are older, more evolved stars (which naturally would have used up most of their hydrogen).

Despite that, the energy (visual) given off in each case is about 10^42 Joules, and the total kinetic energy is about 10^44 Joules.

The key thing to bear in mind for such events, is that the inverse square law for radiation applies: that is, the intensity of radiation (whether x-rays, gamma rays, etc) will fall off as the square of the distance.

If 10^8 radiation intensity units are received by a planet at 100 light years distance from some supernova X, then 10^4 such units will be received at 200 light years distance from the same X. Thus, distance affords protection.

Given this, I don't see that a supernova would be a major threat in terms of loss of life, or electronic damage unless it was closer than 400 light years. What is more worrisome is the *neutrino flux* which will have almost as much energy as the total kinetic for the event (e.g. 10^44 J) and can pass right through matter.

Given such high neutrino flux, I suspect that at least 1000 light years would have to separate us from such a supernova!