Stars like the Sun are surprisingly constant. The nuclear fusion of hydrogen and helium causes the brightness to vary by only 0.1% over years and decades. Through this process, the Sun will continue to shine steadily for about 5 billion years. But if a star uses up all its nuclear fuel, its death can result in a fireworks display.
The Sun eventually grows larger and dies when it condenses into a type of star called a white dwarf. But stars eight times more massive than the Sun die violently in explosions called supernovas.
Supernovas only occur across the Milky Way a few times every 100 years, and these violent explosions are usually far enough away that people here on Earth don't notice them. For a dying star to affect life on our planet, a supernova would have to occur within 100 light-years of Earth.
I am an astronomer who studies cosmology and black holes.
In my post about the end of the universe, I described the threats posed by stellar cataclysms, such as supernovae, and related phenomena, such as gamma-ray bursts. Most of these cataclysms are far away, but if they happen close to home, they could pose a threat to life on Earth.
death of a giant star
Few stars are massive enough to die in a supernova. But if you do that, it will briefly rival the brightness of billions of stars. A supernova occurs once every 50 years, there are 100 billion galaxies in the universe, and a supernova explodes somewhere in the universe every hundredth of a second.
Dying stars emit high-energy radiation in the form of gamma rays. Gamma rays are a form of electromagnetic radiation with a much shorter wavelength than light waves and are invisible to the human eye. Dying stars also emit a torrent of high-energy particles in the form of cosmic rays. In other words, it is a subatomic particle that moves close to the speed of light.
Supernovae in the Milky Way are rare, but there are instances where they have been close enough to Earth to be discussed in the historical record. In 185 AD, a star appeared where no star had previously been seen. It was probably a supernova.
Observers around the world saw a bright star suddenly appear 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 in a statistical sense the next supernova is being delayed.
The red supergiant Betelgeuse, located 600 light-years away in the constellation of Orion, is the nearest giant star nearing the end of its life. When a supernova occurs, it will shine as brightly as the full moon to observers on Earth and will cause no harm to life on Earth.
radiation damage
If a star gets close enough to Earth and goes supernova, the gamma-ray radiation could damage some of the planet's safeguards that allow life to thrive on Earth. Because the speed of light is finite, there is a time delay. If a supernova explodes 100 light years away, it will take us 100 years to see it.
Astronomers have discovered evidence of a supernova 300 light-years away that exploded 2.5 million years ago. Radioactive atoms trapped in seafloor sediments are a tell-tale sign of this event. Radiation from gamma rays has eroded the ozone layer that protects life on Earth from the sun's harmful radiation. This event may have cooled the climate, causing the extinction of some ancient species.
Safety from supernovas comes from greater distances. When gamma rays and cosmic rays are emitted from a supernova, they spread out in all directions, so the rate at which they reach Earth decreases with increasing distance. For example, imagine two identical supernovae, one 10 times closer to Earth than the other. Earth will receive about 100 times more intense radiation from closer events.
A supernova within 30 light years would be a catastrophe, severely destroying the ozone layer, disrupting marine food chains, and possibly causing mass extinction. Some astronomers speculate that a nearby supernova triggered a series of mass extinctions 360 to 375 million years ago. Luckily, these events occur within 30 light years and only once every few billion years.
When neutron stars collide
But supernovae are not the only events that emit gamma rays. Neutron star collisions produce high-energy phenomena ranging from gamma rays to gravitational waves.
Neutron stars left behind after supernova explosions are city-sized chunks of atomic-density material, 300 trillion times more dense than the Sun. These collisions created a lot of gold and precious metals on Earth. The intense pressure created by the collision of two superdense objects forces neutrons into the atomic nucleus, creating heavier elements such as gold or platinum.
Neutron star collisions produce intense gamma-ray bursts. These gamma rays are focused into a narrow jet of radiation that produces a large impact.
If Earth were to be in the field of a gamma-ray burst within 10,000 light-years, or 10% of the diameter of the galaxy, the burst would seriously damage the ozone layer. It can also damage the DNA inside an organism's cells to a level that can kill many simple life forms, such as bacteria.
It may sound ominous, but neutron stars don't typically form in pairs. Therefore, a collision only occurs in a galaxy approximately once every 10,000 years. It is 100 times rarer than a supernova explosion. Neutron star collisions occur every few minutes throughout the universe.
A gamma-ray burst may not pose an immediate threat to life on Earth, but after a very long time, the explosion will inevitably hit the Earth. The probability of a mass extinction caused by a gamma-ray burst is 50% over the last 500 million years and 90% over the next 4 billion years since life existed on Earth.
According to these calculations, a gamma-ray burst likely caused one of five mass extinctions in the past 500 million years. Astronomers have claimed that gamma-ray bursts caused the first mass extinction 440 million years ago, when 60% of all marine life disappeared.
Recent Notifications
The most extreme astrophysical phenomena have a long reach. Astronomers were reminded of this in October 2022 when pulses of radiation swept through the solar system and overloaded every gamma-ray telescope in the universe.
This was the brightest gamma-ray burst since the dawn of human civilization. The radiation caused a sudden disturbance in Earth's ionosphere, even though the explosion occurred nearly two billion light-years away. Although life on Earth was not affected, the fact that it changed the ionosphere is alarming. A similar explosion in the Milky Way would be a million times brighter.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Image credit: NASA, ESA, Joel Kastner (RIT)