Stars like the Sun are fairly stable. Their brightness changes by just 0.1% over years and decades, thanks to the conversion of the hydrogen that powers them into helium. This process will allow the Sun to shine steadily for about 5 billion more years, but when stars exhaust their nuclear fuel, their death can lead to pyrotechnics.
The Sun will eventually die by growing larger and later turning into a type of star called a white dwarf. But stars eight times more massive than the Sun die violently in an explosion called a supernova.
Supernovae occur in the Milky Way only a few times a century, and these violent explosions are usually too far away for people on Earth to notice. For a dying star to affect life on our planet, it must go supernova 100 light-years from Earth.
I’m an astronomer who studies cosmology and black holes.
In my article on cosmic endings, I described the threat posed by stellar catastrophes such as supernovae and related events such as gamma-ray bursts. Many of these disasters occur over long distances, but when they occur closer to home they can pose a threat to life on Earth.
The death of a great star
Very few stars are large enough to die in a supernova. However, when we achieve this, we can rival the brightness of billions of stars for a short time. A supernova occurs every 50 years, and while there are 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 as gamma rays. Gamma rays are a form of electromagnetic radiation whose wavelengths are much shorter than light waves, meaning they are invisible to the human eye. The dying star also releases a flood of high-energy particles in the form of cosmic rays: subatomic particles traveling at near the speed of light.
Supernovae in the Milky Way are rare, but a few have occurred close enough to Earth for historical records to dispute. In 185 AD, a star appeared in a place where no star had been seen before. It was probably a supernova.
Observers around the world saw the sudden appearance of a bright star in 1006 AD. Astronomers later matched 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 later identified as a supernova 6,500 light-years away.
Johannes Kepler observed the last supernova in the Milky Way in 1604, so statistically speaking the next supernova is overdue.
At 600 light-years away, red supergiant Betelgeuse in the constellation Orion is the nearest massive star nearing the end of its life. When it turns supernova, it will be as bright as a full moon for those watching from Earth and will not cause any harm to life on our planet.
radiation damage
If a star goes supernova close enough to Earth, gamma-ray radiation can damage some of the planetary protection that has allowed life to develop on Earth. There is a time delay due to the finite speed of light. If a supernova exploded 100 light years away, it would take 100 years for us to see it.
Astronomers have found evidence of a supernova that exploded 2.5 million years ago, 300 light-years away. Radioactive atoms trapped in seafloor sediments are clear signs of this event. Radiation from gamma rays has eroded the ozone layer, which protects life on Earth from the Sun’s harmful radiation. This event would cool the climate, leading to the extinction of some ancient species.
Protection from a supernova comes with greater distance. Gamma rays and cosmic rays spread in all directions after being emitted from a supernova, so the fraction reaching Earth decreases with distance. For example, imagine two identical supernovae, one 10 times closer to Earth than the other. Earth would receive radiation a hundred times stronger than a more recent event.
A supernova 30 light years away would be catastrophic, severely depleting the ozone layer, disrupting the marine food chain, and possibly causing mass extinction. Some astronomers estimate that nearby supernovae triggered a series of mass extinctions between 360 and 375 million years ago. Luckily, these events only occur every few hundred million years within 30 light years.
When neutron stars collide
But supernovae are not the only events that emit gamma rays. Neutron star collisions cause high energy events, from gamma rays to gravitational waves.
Neutron stars, left behind after a supernova explosion, are city-sized balls of matter the density of atomic nuclei, 300 trillion times denser than the Sun. These impacts created most of the gold and precious metals on Earth. The intense pressure caused by the collision of two ultradense objects forces neutrons into the atomic nucleus, resulting in the formation of heavier elements such as gold and platinum.
A neutron star collision creates an intense gamma-ray burst. These gamma rays are concentrated into a narrow radiation jet that creates a huge impact.
If Earth were in the line of fire of a gamma-ray burst from a distance of 10,000 light-years, or 10% of the diameter of the galaxy, the burst would severely damage the ozone layer. It will also damage the DNA inside the cells of organisms to a level that can kill many simple life forms such as bacteria.
This may sound ominous, but neutron stars don’t usually form in pairs, so there’s only one collision in the Milky Way about every 10,000 years. They are 100 times rarer than supernova explosions. A neutron star collision occurs every few minutes throughout the universe.
Gamma-ray bursts may not pose an immediate threat to life on Earth, but bursts will inevitably hit Earth over very long periods of time. The probability of a gamma-ray burst triggering a mass extinction is 50% in the 500 million years since life began on Earth and 90% in 4 billion years.
According to this math, it’s quite possible that one in five mass extinctions in the last 500 million years was caused by a gamma-ray burst. Astronomers have suggested that a gamma-ray burst caused the first mass extinction, 440 million years ago, when 60% of all marine life disappeared.
A recent reminder
The most extreme astrophysical events have a long impact range. Astronomers were reminded of this in October 2022, when a pulse of radiation swept through the solar system and overloaded all gamma-ray telescopes in space.
This was the brightest gamma-ray burst since the beginning of human civilization. Although its source was an explosion approximately 2 billion light-years away, the radiation caused an immediate disturbance in Earth’s ionosphere. Life on Earth was not affected, but the fact that it changed the ionosphere is thought-provoking; A similar explosion in the Milky Way would be millions of times brighter.
This article is republished from The Conversation, an independent, nonprofit news organization providing facts and authoritative analysis to help you understand 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.