April 13, 2024

Exploding stars are rare, but they emit torrents of radiation – a star close enough to Earth could threaten life on the planet

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Stars like the Sun are remarkably constant. They vary in brightness by just 0.1% over years and decades, thanks to the fusion of hydrogen into helium that powers them. This process will keep the Sun shining continuously for about another 5 billion years, but when the stars run out of nuclear fuel, their deaths could lead to pyrotechnics.

The sun will eventually die, grow, and then condense 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 happen in the Milky Way only a few times a century, and these violent explosions are usually remote enough that people here on Earth don’t notice. For a dying star to have any effect on life on our planet, it would have to become a supernova within 100 light years of Earth.

I’m an astronomer who studies cosmology and black holes.

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 close to home they can pose a threat to life on Earth.

The death of a massive star

Very few stars are massive enough 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, which means 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 discuss them. In 185 AD, a star appeared in a place where no star had been seen before. It was probably a supernova.

An animation showing 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 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 from a statistical point of view the next one should have already happened.

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 becomes a supernova, it will shine as brightly as a full moon for those observing from Earth, without causing any harm to life on our planet.

Radiation damage

If a star goes supernova close enough to Earth, 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 be 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 deep-sea sediments are the telltale 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 have cooled the climate, leading to the extinction of some ancient species.

Safety from a supernova comes with greater distance. Gamma rays and cosmic rays scatter in all directions once emitted by a supernova, so the fraction that reaches Earth decreases with greater distance. For example, imagine two identical supernovae, one of which is 10 times closer to Earth than the other. Earth would receive radiation about a hundred times stronger from the nearest event.

A supernova within 30 light years would be catastrophic, severely destroying the ozone layer, disrupting the marine food chain and likely causing mass extinction. Some astronomers guess that nearby supernovae triggered a series of mass extinctions between 360 and 375 million years ago. Fortunately, these events happen within 30 light years only every few hundred million years.

Neutron stars merge when gravity pulls them together, releasing intense radiation.

When neutron stars collide

But supernovae are not the only events that emit gamma rays. Neutron star collisions cause high-energy phenomena ranging from gamma rays to gravitational waves.

Left behind after a supernova explosion, neutron stars are balls of matter the size of a city, with the density of an atomic nucleus, that is, 300 trillion times denser than the Sun. These collisions created much of the of Earth’s gold and precious metals. The intense pressure caused by the collision of two ultradense objects forces neutrons into atomic nuclei, which creates 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 has a big impact.

If Earth were in the line of fire 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 within organisms’ cells, to 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 there’s only one collision in the Milky Way every 10,000 years. They are 100 times rarer than supernova explosions. Throughout the universe, there is a neutron star collision every few minutes.

Gamma-ray bursts may not pose an imminent threat to life on Earth, but over very long time scales, the bursts will inevitably reach 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 life has existed on Earth.

By this math, it is quite likely that a gamma-ray burst caused one of the five mass extinctions in the last 500 million years. Astronomers argue that a gamma-ray burst caused the first mass extinction 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 across the solar system and overwhelmed every gamma-ray telescope in space.

It was the brightest gamma-ray burst to occur since the beginning of human civilization. The radiation caused a sudden disturbance in Earth’s ionosphere, although 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 worrying – a similar explosion in the Milky Way would be a million times brighter.

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