Danger from supernovae

Suppose a star explodes into a supernova at a distance of about 100 lightyears. Is the earth protected against the radiation from that explosion? More general, at what distance (in lightyears) can we say that we are safe against the dangers (radioation) of a supernova ? Is protection possible, for example going underground, and for how long ?

A few years ago, there was a theory that perhaps the dinosaurs were killed off by the effects of an explosion of a nearby star. This theory has been more recently supplanted by the idea of a giant asteroid impact doing the deed, but the exploding star theory did spur some interest. This question also comes up occasionally on the newsgroup sci.astro, so some of the folks there put together a very good synopsis of the results of a nearby supernova (SN), or exploding star. In turn, I will give a synopsis of their results.

Supernovae are very bright, and they put out light at all different wavelengths. Different wavelengths do different damage; for example, ultraviolet light can directly cause skin damage (even cancer), while infrared does relatively little damage. Certainly x-rays can wreak havoc as well! But before we panic, let's look at the numbers.

[Note (added June 11, 2004): For some reason, I originally said in the following paragraph that Betelegeuse is 1000 light years away. I don't know whay I wrote that! I know it is much closer than that. Anyway, I corrected the number below. Sorry for any confusion.]
First, let's note that the nearest star that can explode is probably the star Betelgeuse in the constellation of Orion. Distance estimates to it vary, but it is about 500 light years away. Keep that number in mind when we go through the explanation...

For a typical supernova to be as bright to the human eye as the Sun, it would have to occur at a distance of about a light year from the Earth. That's close! The nearest star (other than the Sun) is over 4 light years away. Now, a supernova would certainly affect the life cycles of plants and animals if it were even 1/100000 as bright as the Sun (the full Moon is perhaps 50000 times fainter than the Sun, and we know that has a profound affect on some plants and animals). Since objects get dimmer as the square of their distance, this means it could be 300 times farther away (the square root of 100000), or a distance of 300 light years, to have any real effect on the Earth. This is still much closer than the nearest SN candidate.

X-rays are a different story. Solar flares put out x-rays, and without the protection of the Earth's atmosphere they could do serious damage. For a supernova to be as bright as a good solar flare, it would have to be at a distance of about 5-60 light years or so away. This is still far closer than the nearest possible SN. Some SNe are very bright in x-rays, though, and could be as far as 500 light years away and still put out a solar flare's worth of x-rays. This could actually do serious damage to satellites in orbit or interplanetary probes.

It turns out that gamma rays are possibly dangerous to us here on Earth. A SN at 3000 light years can dump as much as 1000 solar flares' worth of gamma rays onto the Earth. However, exactly how this would affect the Earth is unclear. Solar flares happen all the time without damaging the Earth overmuch, so maybe this is not a big deal either. Remember, the Earth's atmosphere protects us very well against radiation from space.

The point here is that a supernova would have to be very close, certainly closer than the nearest possible SN candidate, before doing Hollywood-disaster-movie type damage to us.

The Mad Scientist moderator who forwarded your question also asked me how close a SN could be before looking at it would hurt your eye. We know that looking at the Sun can hurt your eye, but looking at the full Moon doesn't. A supernova would have to be at least as close as perhaps 200 light years before being as bright as the Moon. The situation is complicated further because a supernova would appear as a point, while the light from the Moon is spread out. The full Moon can make you squint; imagine how it would be if all that light were concentrated into a single point! I called some ophthalmologists to see if they knew just how bright a source could be before it damages the eye, but none seems to know. I will continue to look into this. If I find out, I will send an update to the Mad Scientists, and I'll also ask to get the info added to the web page about SN risks that was mentioned above.

If you want to see more about supernovae, my professional research page also has links to supernova related work. The supernova information is about 2/3 of the way down the page. Here are some pictures of the supernova SN1987A using the Hubble Space Telescope, also from my home page.