Tucson (USA), March 30 (The Conversation) Stars like the Sun are remarkably constant. Their brightness varies only 0.1% over years and decades, thanks to the fusion of hydrogen into helium that fuels them. This process will keep the Sun shining steadily for about 5 billion more years, but when stars run out of nuclear fuel, their death can trigger pyrotechnics.
The Sun will eventually die by growing and then condensing 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.
Supernovas occur across the Milky Way only a few times per century, and these violent explosions are usually remote enough that people here on Earth don't notice them. For a dying star to have any effect on life on our planet, it would have to go supernova less than 100 light years from Earth.
I am an astronomer who studies cosmology and black holes.
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In my writings on cosmic endings, I have described the threat posed by stellar cataclysms such as supernovae and related phenomena such as gamma-ray bursts. Most of these cataclysms are remote, but when they occur closer to home they can pose a threat to life on Earth.
The death of a massive star.
Very few stars have enough mass to die in a supernova. But when it does, it briefly rivals the brightness of billions of stars. With a 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 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 releases a torrent of high-energy particles in the form of cosmic rays: subatomic particles that move at close to the speed of light.
Supernovae in the Milky Way are rare, but some have been close enough to Earth for historical records to analyze. In the year 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 a bright star suddenly appear in 1006 AD. Astronomers later linked it to 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 should have already occurred.
At 600 light-years away, the red supergiant Betelgeuse in the constellation Orion is the closest massive star approaching the end of its life. When it goes supernova, it will shine as bright as the full Moon to those observing it from Earth, causing no harm to life on our planet.
Radiation damage
If a star goes supernova close enough to Earth, the gamma-ray radiation could damage some of the planetary protection that allows life to thrive on Earth. There is a delay due to the finite speed of light. If a supernova explodes 100 light years away, it will take us 100 years to 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 eroded the ozone layer, which protects life on Earth from the Sun's harmful radiation. This event would have cooled the climate, causing the extinction of some ancient species.
Safety against a supernova is given by a greater distance. Gamma rays and cosmic rays propagate in all directions once emitted by a supernova, so the fraction that reaches Earth decreases with greater distance. For example, let's imagine two identical supernovae, one of them 10 times closer to Earth than the other. Earth would receive approximately one hundred times stronger radiation from the nearest event.
A supernova within 30 light years would be catastrophic, severely depleting the ozone layer, disrupting the marine food chain, and likely triggering a mass extinction. Some astronomers hypothesize that nearby supernovae triggered a series of mass extinctions between 360 and 375 million years ago. Fortunately, these events occur within 30 light years only every few hundred million years.
When neutron stars collide
But supernovae are not the only phenomena that emit gamma rays. Neutron star collisions cause high-energy phenomena ranging from gamma rays to gravitational waves.
Neutron stars, left behind after a supernova explosion, are city-sized balls of matter with the density of an atomic nucleus, or 300 billion times denser than the Sun. These collisions created huge part of the gold and precious metals of the Earth. The intense pressure caused by the collision of two ultradense objects forces neutrons toward atomic nuclei, creating heavier elements such as gold and platinum.
The collision of a neutron star generates an intense burst of gamma rays. These gamma rays are concentrated into a narrow jet of radiation that has a large impact.
If Earth were in the line of fire of a gamma ray burst 10,000 light years away, or 10% of the galaxy's diameter, the burst would severely damage the ozone layer. It would also damage the DNA within organisms' cells, to a level that would kill many simple life forms such as bacteria.
This sounds ominous, but neutron stars don't usually form in pairs, so there is 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 imminent threat to life on Earth, but on very long time scales, bursts will inevitably hit Earth. The chances of a gamma ray burst triggering a mass extinction are 50% in the last 500 million years and 90% in the 4 billion years since there was life on Earth.
According to these calculations, it is quite likely that a gamma ray burst caused one of the five mass extinctions of the last 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 creatures disappeared.
A recent reminder
The most extreme astrophysical phenomena have a long 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.
It was the brightest gamma ray burst to occur since human civilization began. The radiation caused a sudden disturbance in 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 altered the ionosphere is sobering: a similar explosion in the Milky Way would be a million times brighter. (The conversation)
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