There is a plethora of potential events outside human control that can lead to mass extinction on Earth. It’s happened before. Fortunately, such events are few and far between. Nevertheless, it is worth paying attention to the potential for at least a few reasons. First, a little awareness is appropriate where danger lurks. Also, big problems help put small problems into perspective. More importantly, humanity has a way of solving problems, even those that seem insurmountable. One of those potential problems is a Gamma Ray Burst, or “GRB.”
GRBs are a relatively new discovery. They are extremely energetic explosions that can be as short as ten milliseconds. Astronomers have observed thousands of GRBs over the past five decades. These occurred in distant galaxies, but were powerful enough to detect. If one occurs in our galaxy, even from thousands of light years away, it could cause great harm if 
Earth is within the cone of the burst. In fact, a GRB is a strong candidate for the Ordovician-Silurian extinctions about 450 million years ago (not Earth’s “worst” extinction event, but about 85% of marine species died, including many of the Trilobite families). Depending on the circumstances, a GRB can either radiate lifeforms directly, or it can destroy Earth’s ozone with catastrophic consequences.
A “long” GRB (lasting from several seconds to several minutes) is the suspected culprit, thought to be caused by the collapse of a “Wolf-Rayet” star, which would be an old, fast-spinning, behemoth star more than 25 times the mass of the sun. Basically, as its core collapses into a black hole, jets of strong radiation shoot outward from its poles at nearly light speed. If the O-S extinctions were caused by a long GRB, the collapsed star would have been somewhere within our galaxy, and its spin axis would have been pointed at Earth. It was too long ago to determine what star (now a black hole) that might have been. Stars move a lot in 450 million years. Our sun makes a full orbit of the galaxy in just 226 million years.
WR 104. Composite image by Christmann. Background credit NASA Hubble Telescope. Inset credit U.C. Berkeley Space Sciences Laboratory, W.M. Keck Observatory.
At present, there is only one Wolf-Rayet star we know of that is close enough (about 7,500 light years) and oriented in such a way (pointed generally toward us) that it might pose an eventual threat — WR 104 in the constellation Sagittarius. It might collapse sometime within the next 500,000 years. Right now it appears to be askew enough to miss us. Hopefully that doesn’t change over the next half-million years. So far, the odds appear to be on our side.
However, be it WR 104 or another local Wolf-Rayet star, the fact remains that long GRBs are a relatively common occurrence in the universe, and some hypothesize them as one answer to the Fermi Paradox, which in essence asks: If there is extraterrestrial intelligence in the universe, why do we not see any sign of it? If galaxies are periodically swept clean of complex organisms by GRBs, then the lifespan of civilizations might be relatively short. See Braun, B., A Possible Answer to Fermi’s Paradox: Gamma-Bursts, Postcards from the Universe, March 29, 2013; Annis. J. An astrophysical explanation for the “great silence”. Journal of the British Interplanetary Society. 52, 19-20 (1999).
Interestingly, although many species perished, life on Earth survived after the GRB hit (if that’s what it was) 450 million years ago. If Earth has another 500 million years before the next blast, how far along might humanity and its technology be? Considering the track record, it is not beyond imagining that we could handle a GRB given hundreds of millions of years to prepare.
J. C. Conway 