Stars like the Sun are remarkably constant. They vary in brightness by just 0.1% over years and decades, thanks to the hydrogen-helium fusion that powers them. This process will keep the Sun shining steadily for another five billion years or so, but when stars exhaust their nuclear fuel, their death can lead to pyrotechnics.
The sun will eventually die by growing large and then condensing into a type of star called a white dwarf. But stars with more than eight times the mass of the Sun die violently in an explosion called a supernova.
Supernovas only occur a few times a century in the Milky Way, and these violent explosions usually occur so far away that people here on Earth don’t notice. For a dying star to have any effect on life on our planet, it must go supernova within 100 light-years of Earth.
I am an astronomer who studies cosmology and black holes.
In my writing on cosmic endings I have described the threat posed by stellar catastrophes such as supernovae and related phenomena such as gamma-ray bursts. Most of these disasters occur in remote places, but when they happen closer to home, they can threaten life on Earth.
The death of a massive star
Few stars are so big that they die in a supernova. But if you do, it briefly rivals the brightness of billions of stars. With one supernova every 50 years, and with 100 billion galaxies in the universe, a supernova explodes somewhere in the universe every hundredth of a second.
The dying star emits high-energy radiation in the form of gamma rays. Gamma rays are a form of electromagnetic radiation with wavelengths much shorter than light waves, meaning they are invisible to the human eye. The dying star also emits a flood of high-energy particles in the form of cosmic rays: subatomic particles that travel at nearly the speed of light.
Supernovae in the Milky Way are rare, but a few have been close enough to Earth that historical records discuss them. In 185 AD, a star appeared in a place where no star had ever been seen. It was probably a supernova.
Observers around the world saw a bright star suddenly appear in 1006 AD. Astronomers later compared it to a supernova 7,200 light-years away. Then in 1054 AD, Chinese astronomers recorded a star visible in the daytime sky, which astronomers subsequently identified as a supernova 6,500 light-years away.
Johannes Kepler observed the last supernova in the Milky Way in 1604, so in a statistical sense the next one is already too late.
At 600 light-years away, the red supergiant Betelgeuse in the constellation Orion is the nearest massive star approaching the end of its life. When it becomes a supernova, it will shine as brightly as the full moon for those watching from Earth, without causing any damage to life on our planet.
Radiation damage
If a star close enough to Earth goes supernova, the gamma rays can damage some of the planetary protection that makes life on Earth possible. There is a delay due to the finite speed of light. If a supernova goes off 100 light years away, it will take 100 years before we see it.
Astronomers have found evidence of a supernova 300 light-years away that exploded 2.5 million years ago. Radioactive atoms trapped in seafloor sediments are the telltale signs of this event. Radiation from gamma rays erodes the ozone layer, which protects life on Earth from the sun’s harmful radiation. This event would have cooled the climate, leading to the extinction of some ancient species.
Safety against a supernova involves a greater distance. Gamma rays and cosmic rays scatter in all directions once emitted by a supernova, so the fraction that reaches Earth decreases with distance. For example, imagine two identical supernovae, one ten times closer to Earth than the other. Earth would receive radiation that is about a hundred times stronger as the event approaches.
A supernova within 30 light years would be catastrophic, severely depleting the ozone layer, disrupting the marine food chain and likely causing mass extinction. Some astronomers suspect that nearby supernovae caused a series of mass extinctions 360 to 375 million years ago. Fortunately, within a radius of 30 light years, these events only occur every few hundred million years.
When neutron stars collide
But supernovas aren’t the only events that emit gamma rays. Collisions between neutron stars cause high-energy phenomena ranging from gamma rays to gravitational waves.
Neutron stars, left behind after a supernova explosion, are spheres of matter the size of a city with the density of an atomic nucleus, so 300 trillion times denser than the Sun. These collisions created much of the gold and precious metals on Earth. The intense pressure caused by the collision of two ultra-dense objects forces neutrons into atomic nuclei, creating heavier elements such as gold and platinum.
A collision with a neutron star generates an intense burst of gamma rays. This gamma radiation is concentrated in a narrow beam of radiation that packs a big punch.
If Earth were within 10,000 light-years, or 10% of the galaxy’s diameter, in the line of fire of a gamma-ray burst, the burst would severely damage the ozone layer. It would also damage the DNA in the cells of organisms, at a level that would kill many simple life forms such as bacteria.
That sounds ominous, but neutron stars don’t typically form in pairs, meaning only one collision occurs in the Milky Way about every 10,000 years. They are a hundred times rarer than supernova explosions. Throughout the universe, a collision of neutron stars occurs every few minutes.
Gamma-ray bursts may not pose an immediate threat to life on Earth, but in the very long term, bursts will inevitably impact Earth. The chance of a gamma-ray burst causing a mass extinction is 50% in the past 500 million years and 90% in the 4 billion years since life has been on Earth.
Based on these calculations, it is very likely that a gamma-ray burst caused one of the five mass extinctions in the past 500 million years. Astronomers have argued that a burst of gamma rays caused the first mass extinction 440 million years ago, when 60% of all marine life disappeared.
A recent memory
The most extreme astrophysical events have a large range. Astronomers were reminded of this in October 2022, when a pulse of radiation swept through the solar system, overloading all the gamma-ray telescopes in space.
It was the brightest gamma-ray burst ever to occur since the dawn of human civilization. The radiation caused a sudden disruption of Earth’s ionosphere, even though the source was an explosion nearly 2 billion light-years away. Life on Earth was unaffected, but the fact that it changed the ionosphere is sobering: a similar burst in the Milky Way would be a million times brighter.
This article is republished from The Conversation, an independent nonprofit organization providing facts and trusted analysis to help you understand our complex world. It was written by: Chris Impey, University of Arizona
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Chris Impey receives funding from the National Science Foundation and the Howard Hughes Medical Institute.