by Stars like the Sun are extremely constant. They change in brightness by only 0.1% over years and decades, thanks to the fusion of hydrogen into helium that powers them. This process will keep the Sun shining steadily for about another 5 billion years, but when stars exhaust their nuclear fuel, their death can be caused by pyrotechnics.
The Sun will eventually die by growing large and then condensing into a type of star called a white dwarf. But stars more than eight times more massive than the Sun die violently in an explosion called a supernova.
Supernovae across the Milky Way happen only a few times a century, and these violent explosions are usually remote enough to go unnoticed by people here on Earth. For a dying star to have any effect on life on our planet, it would have to go supernova within 100 light years of Earth.
I am an astronomer who studies cosmology and black holes.
In my writing about cosmic endings, I have described the threat of stellar cataclysms such as supernovae and related phenomena such as gamma-ray bursts. Most of these cataclysms are remote, but when they happen closer to home they can threaten life on earth.
Death of a giant star
Few stars are massive enough to die in a supernova. But when one does, it briefly rivals the brightness of billions of stars. At one supernova every 50 years, and with 100 billion galaxies in the universe, somewhere in the universe a supernova explodes every hundredth of a second.
The dying star emits high-energy radiation as 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 releases a torrent of high-energy particles in the form of cosmic rays: subatomic particles moving at close to the speed of light.
Supernovae are rare in the Milky Way, but some are close enough to Earth that historical records record them. In 185 AD, a star appeared where no star had been seen before. It was probably a supernova.
Observers around the world saw a bright star suddenly appear in 1006 AD Astronomers later matched it with a supernova 7,200 light years away. Then, in 1054 AD, Chinese astronomers recorded a star visible in the daytime sky that astronomers later 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 overdue.
At 600 light years away, the red supergiant Betelgeuse in the constellation Orion is the closest massive star to the end of its life. When it goes supernova, it will shine as bright as the full Moon to those watching from Earth, without causing any damage to life on our planet.
Radiation damage
If a supernova star passes close enough to Earth, the gamma-ray radiation could damage some of the planetary defenses that allow life to thrive on Earth. There is a time delay due to the limited speed of light. If a supernova passes 100 light years away, it takes us 100 years to see it.
Astronomers have found evidence of a supernova 300 light years away that exploded 2.5 million years ago. Signs of these events are radioactive atoms trapped in the seabed sediment. Gamma-ray radiation erodes the ozone layer, which protects life on Earth from the sun’s harmful radiation. This event would have caused a cooling of the climate, causing some ancient species to become extinct.
Safety from a supernova comes with a larger distance. Gamma rays and cosmic rays are scattered in all directions when they are emitted from a supernova, thus reducing the fraction that reaches Earth with greater distance. For example, imagine two identical supernovae, one 10 times closer to Earth than the other. The Earth would receive radiation approximately a hundred times stronger from the closer event.
A supernova within 30 light years would be catastrophic, severely depleting the ozone layer, disrupting the marine food chain and likely causing a mass extinction. Some astronomers estimate that a nearby supernova triggered a series of mass extinctions 360 to 375 million years ago. Fortunately, these events occur within 30 light years but every few hundred million years.
When neutron stars collide
But supernovae are not the only events that emit gamma rays. Neutron star collisions create high-energy phenomena ranging from gamma rays to gravitational waves.
Left after a supernova explosion, neutron stars are balls of material with the density of an atomic nucleus, and therefore 300 trillion times denser than the Sun. These collisions created many of the gold and precious metals on Earth. The intense pressure caused by two ultradense objects causes neutrons to collide in atomic nuclei, creating heavier elements such as gold and platinum.
A neutron star collision generates an intense burst of gamma rays. These gamma rays are concentrated into a narrow jet of radiation that packs a big punch.
If Earth were in the firing line of a gamma-ray burst within 10,000 light-years, or 10% of the galaxy’s diameter, the burst would severely damage the ozone layer. It would also damage the DNA inside 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 normally form in pairs, so the Milky Way only has one collision about every 10,000 years. They are 100 times rarer than supernova explosions. All over the universe, a neutron star collision happens every few minutes.
Gamma-ray bursts may not pose an immediate threat to life on Earth, but over a very long period of time, bursts will certainly hit Earth. There is a 50% chance that a gamma-ray burst has caused 50% mass extinction in the last 500 million years and 90% in the 4 billion years since life on Earth.
According to that math, there is every chance that one of the five mass extinctions in the last 500 million years was caused by a gamma-ray burst. Astronomers have argued that the first mass extinction was caused by a gamma-ray burst 440 million years ago, when 60% of all sea creatures disappeared.
A recent reminder
The most extreme astrophysical events have a long reach. Astronomers were reminded of this in October 2022, when a pulse of radiation swept through the solar system and overloaded all the gamma-ray telescopes in space.
It was the brightest gamma-ray burst since the dawn of human civilization. The radiation caused a sudden disturbance in the Earth’s ionosphere, even though the source exploded almost 2 billion light years away. Life on Earth was unaffected, but the fact that it changed the ionosphere is tantalizing – a similar explosion in the Milky Way would be a million times brighter.
This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of our complex world. 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.
