Answer to Question #10108 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
The answer to this question is actually not as simple as it might seem because there are several factors to consider. The first is that supernovae release energy in a number of different ways: they give off visible light, they blow off their outer layers and a lot of radiation (ultraviolet and gamma ray) is emitted, and they emit huge numbers of neutrinos (subatomic particles that don’t interact with matter very much at all). Not only that, but supernovae produce a lot of radioactive atoms--as much radioactive nickel as our sun weighs--that emit radiation as they decay over a couple of years. This is a big list, but these are all forms of radiation that we have to think about, so let’s go into what happens with each of these.
Most of the supernova’s energy comes out in the form of neutrinos, particles that are so insubstantial and that interact so poorly with matter that a single neutrino can pass through about a light-year’s worth of lead before it is likely to come to a stop. By my calculations, the neutrinos from even a nearby supernova are not likely to cause any health problems at all because they just aren’t likely to interact with our bodies. They will just zip right through without causing any DNA damage or having any other effects. The visible light is unlikely to cause problems either, but for different reasons: unless a supernova is incredibly close to us, the light won’t be nearly as bright as our sun and it will fade within a few weeks. This leaves the atoms (radioactive and otherwise), the ultraviolet light, and the gamma rays.
Believe it or not, there is proof that radioactivity produced by supernovae has landed on Earth. In the late 1990s and the first years of this century, some German scientists found traces of radioactive iron in deep-sea sediments that could only have come from a nearby supernova. Since then there have been a few other similar discoveries, and there is no doubt that radioactive supernova debris have hit our planet in the fairly recent (from a geologic perspective) past. Having said that, there was never enough radioactivity to cause any health problems. What is more interesting is that the radioactivity produced by a supernova emits a ton of radiation over the period of a few years--almost as much energy comes out in gamma rays as is emitted by the visible light of the supernova. About 90 percent of this is absorbed by the gas and dust blown out into space so we are exposed to only a fraction of this energy, which is much less than the amount that can cause radiation sickness.
The last form of radiation is the ultraviolet and this can actually cause the most trouble. According to research by some astronomers at the University of Texas in Austin (Craig Wheeler and John Scalo), as well as Adrian Melott at the University of Kansas, the ultraviolet radiation from a supernova can destroy our ozone layer and can also cause chemical reactions in the atmosphere that can create problems by blocking solar radiation. So in just a few days much of the Earth might be deprived of sunlight and, after the smog dissipates, ultraviolet radiation levels can be quite high. This might, in fact, be the most dangerous thing about a supernova. Having said that, the Earth has survived hundreds of these events so they are obviously unable to kill all life on Earth.
The other factor to consider is how far away the supernova is. The radiation dose drops off with the square of the distance so a supernova that might be deadly at a distance of one light year would hardly have any effect at all at three times that distance. We can also calculate how often we’d expect our sun to be close enough to a supernova for it to have an effect on us: it doesn’t happen very often (maybe only a few times since the universe first formed).
So, now some intervals! Or, in other words, how often do supernovae go off that are close enough to cause any problems?
Without going into the math, astronomers have worked out how often we can expect to see a supernova at any distance from the Earth. Starting with the facts that there are two or three supernovae in our galaxy every century and that supernovae are concentrated in the galaxy’s spiral arms, we can calculate that about every 10,000 years there will be a supernova close enough to cause the ultraviolet radiation mentioned earlier. About every 10 million years the radiation emitted by the radioactive nickel and cobalt will be enough that it might cause mild radiation sickness, and there might have been one or two instances over the entire life of the Earth (about 4.6 billion years) in which radiation levels were high enough to pose a threat to living organisms on the Earth’s surface.
Now (finally!) to put all of this in perspective: there is no doubt that the Earth has been affected by supernovae in its past, and that some of our run-ins would have been uncomfortable or worse. And this is going to happen again in the future. But the odds of anything like this happening during our lifetime or our children’s lifetime is so low that we really need not worry about it.P. Andrew Karam, PhD, CHP