The Local Bubble is an irregularly shaped region of hot (million-degree) but tenuous gas (plasma) in which our solar system and many other stars reside. A chain of supernova explosions may have formed it, perhaps the same ones that deposited the iron-60.
It may sound unreasonable to have so many supernovae all going off in the same area at nearly the same time, but it isn’t. The massive stars that make type II supernovae are often born in associations, and therefore clump together. The Orion Nebula (M42), a favorite of amateur astronomers, is a large example of this.
The stars that make the powerful supernovae have fairly short lives, so a group that is born together with the same starting mass will tend to explode more or less together. Astronomers estimate that the stars that dump the iron-60 each contained about 10 times the mass of the Sun, and should live only a relatively short 30 million years. This line of thinking demonstrates that the idea of a chain of explosions is reasonable, but by no means proven.
For the first time, instead of general expectations, we have a definite event to discuss. It was not close enough (30 light-years) to generate a mass extinction but close enough to affect Earth. Compare this with historical supernovae thousands of light-years away — the ones with written records, allowing us to find their remnants in the sky from the descriptions of their locations. For the events that dumped iron-60, we don’t have such information. Although there is a lot of uncertainty about how many supernovae occurred in this series, the last one clearly happened about 2.5 million years ago, at a distance of 150 to 300 light-years. It gives us something to work with. My group has been working out what kind of effects we should expect.
Interestingly, Charles Sheffield wrote a pair of science fiction novels, Aftermath (1998) and Starfire (2000), in which he portrayed a nearby supernova with surprisingly accurate descriptions of many of the effects that our group has calculated. Later, when mineral evidence was found for the dinosaur-killing asteroid or comet, researchers also had been looking for evidence of a nearby supernova. So all of this is not new; the issue has been considered at least since 1950. Still, what we have found recently surprised us because the important effects turned out to be different from the ones usually discussed.
Hazard lights
First, we looked at the effects of blue light generated by the supernova. It sounds silly, but insomnia would be a hazard if the event were visible on Earth’s night side. It turns out that the blue wavelengths of light are not at all healthy for sleeping creatures. (Get rid of any blue LED alarm clocks!) Both the intensity and color of such an object in the night sky would be detrimental to sleeping animals, but only for a few weeks at most.
A more commonly discussed hazard is ozone depletion in Earth’s atmosphere, resulting in a big increase in ultraviolet light at ground level. This is a side effect of radiation breaking up nitrogen gas (N2) in our atmosphere. The chemical bond is so strong that life on Earth has generally lived with a nitrogen shortage. (It can’t be used unless atomic nitrogen [N] is freed from the molecule.) Most radiation hazards break up the nitrogen in the stratosphere, after which the freed nitrogen makes compounds with oxygen, thereby destroying ozone (O3), which is converted to ordinary oxygen (O2).
Ozone in the stratosphere blocks the part of the ultraviolet spectrum called UVB, whose wavelengths are between 380 and 420 nanometers. UVB can cause severe burning of the skin. It gets absorbed by protein and, most importantly, DNA — the localized burst of energy breaks chemical bonds and can lead to cancer and mutation. For many decades, the disaster scenarios of the effects of nearby supernovae have hinged on this effect. It turned out not to matter in this case. To explain why, we have to talk about cosmic rays.