Originally published 14 November 1983
One Friday evening in August of 1975, while working late, I heard a radio report of a new star in the constellation Cygnus the Swan. I rushed outside, where it took but an instant to recognize the intruder.
This part of the sky is one of the most brilliant for northern observers. Here can be found the fullest stream of the Milky Way, split from top to bottom by the Great Rift. And here are three first magnitude stars — Vega, Deneb and Altair — that make up the famous “Summer Triangle.”
Deneb is the Arabic word for “tail.” The star is the tail of a swan that wings its way south along the stream of the Milky Way. On that particular August evening it looked as if a tail feather had been plucked from the swan as it flew past. Not far from Deneb, where no star had been before, there was a new star almost as bright as Deneb.
The new star was short lived. By the following evening it had dimmed noticeably. Within a week the star was visible only to observers with telescopes or binoculars.
Nova means “new,” but a nova is not really a “new star.” What we see is the sudden brightening of an old star that was too far away to be previously visible. The star that brightened in August of 1975 was thousands of light years away, all the way across one of the spiral arms of the galaxy, beyond the reach of the largest telescopes.
The flaring of a nova occurs near the end of a star’s life, when the energy balance within the star becomes unsteady and the star blows off its outer layers. The most massive stars, the giants of the galaxy, depart this world with even greater violence. Those superstars blow themselves to bits in cataclysms called supernovas. Supernovas pour into space enormous shock waves of matter and energy. It is an unlucky planet that finds itself in the way.
There are hundreds of billions of stars in the Milky Way Galaxy. There have been four supernovas on our side of the Galaxy during the past 1000 years. The last was observed on Earth in 1604. None of the four stars was close enough to pose a threat to life on Earth.
Astronomers have estimated the frequency of supernova events. A reasonable guess is that a supernova will occur within a 50 light-year neighborhood of the Earth once every few hundred million years.
A devastating blow
If one of the stars in our 50 light-year neighborhood went supernova, for a time it would shine as brightly as the full moon. We could read by its light. Just as the brilliant new object appeared in our sky we would be hit by a first wave of destruction: gamma rays, x‑rays and high energy cosmic ray particles. In addition to the direct biological effects of this radiation, the blast would disrupt the ozone layer that protects life on Earth from the sun’s ultraviolet light. It would also modify climate by increasing ionization of the upper atmosphere. Several thousand years after the initial blast, the shell of exploded stellar material would arrive, also wreathed in deadly radiation. The death of the nearby star would deliver a one-two punch which life on Earth would have to absorb.
Is there any evidence that life has stumbled beneath such blow in the past?
In March of 1982, David Raup of the Field Museum of Natural History in Chicago, and J. John Sepkoski of the University of Chicago, published a compilation of fossil data which continues to excite biologists and earth scientists. The survey showed that there has been a steady increase in the number of marine families over the past 600 million years. The general upward slop of the curve represents a net gain of the tendency of evolution toward diversity over the background of extinctions that goes on all the time.
What is most interesting about the Raup-Sepkoski survey is the handful of episodes in the history of life when the rate of extinctions suddenly soared and the number of marine families took a sharp dip. The most spectacular of these was the so-called “Time of the Great Dying” which occurred 230 million years ago. Nearly half of all animal families disappeared from the Earth. Seventy-five percent of the amphibians and 80 percent of the reptiles were wiped out.
The causes of these calamitous breaks in the fossil record are unknown, but nearby supernovas must certainly be considered among possible causes. We can be sure that supernovas have occurred in the neighborhood of the sun. It would seem likely that those events have left some record in the history of life.
But supernovas are not the only instruments for mass extinctions. Polarity reversals in the Earth’s magnetic field, the impact of massive asteroids, crescendos of volcanic activity, changing sea levels and climatic change have all been charged with punctuating the story of life.
The last straw
Perhaps no single cause was responsible for the episodes of extinction. At times of severe geological stress, life may be more vulnerable to punctuative events like supernovas or magnetic field reversals. The explosion of a nearby star or the flip of the magnetic field may simply be the straw that broke the camel’s — or dinosaur’s — back.
Novas such as Nova Cygni 1975 occur several times each century. Supernovas are rarer. We are perhaps overdue for a supernova in our part of the galaxy. Astronomers look forward to the event. They hope the dying star will be near enough to be a fine object of study, but not so close as to pose a threat.
A supernova could wreak havoc on Earth. But we should not underestimate the resiliency of life. The Raup-Sepkoski curve of diversity climbs in spite of occasional devastating setbacks. The Earth has been buffeted by cosmic violence throughout its history. Life will survive the death of a nearby star.
