There are two to three hundred billion stars in the Milky Way galaxy, and of course there is no shortage of massive stars, which can be called a huge “repository” of potential supernovae. And once a supernova erupts here, will it have an impact on life on Earth?
It’s not unfounded worry
Since the 1950s, some scientists have proposed that supernova explosions may be one of the main culprits of species extinction in the history of life on Earth. Theoretically, the gamma rays released by the explosion of a nearby supernova can destroy the ozone layer for decades, exposing the earth’s surface to ultraviolet rays that are harmful to living things.
At the end of the 20th century, astronomers discovered that they could track the traces of ancient supernovae near the sun by searching for iron-60. Iron-60 is an isotope of iron. What we are exposed to is the stable iron-56. Its nucleus contains 26 protons and 30 neutrons. Iron-60 has 4 more neutrons. Lighter elements like carbon, nitrogen, and oxygen are all produced by nuclear fusion inside stars. The heaviest elements that stars can produce are only iron, such as gold, platinum, uranium, and other elements heavier than iron. It can only be formed in the extremely high temperature environment of supernova explosion or neutron star collision. Iron-60 comes from a supernova, which is a radioactive element with a half-life of about 2.6 million years. Therefore, if iron-60 is found on Earth, then they must have come from a supernova explosion not far away in the past few million years.
By the beginning of the 21st century, German scientists had indeed detected trace amounts of iron-60 atoms in a 2.5 million-year-old sediment drilled on the seafloor. In 2016, scientists also found iron-60 in the space dust around the earth, including samples brought back from the moon by the Apollo project, and it took more than 2.5 million years for them to reach the earth. This shows that there was indeed a supernova explosion near the earth about 2.5 million years ago! It is about 150-300 light-years away from us.
Where does the danger come from?
Astronomers carefully studied this supernova explosion and found that in addition to gamma rays, cosmic rays generated from supernovae are more deadly threats to life!
Cosmic rays are high-energy protons or atomic nuclei that break into the earth from the universe. When they enter the atmosphere, they collide with atoms such as nitrogen and oxygen to form neutrons and a kind of “particle rain” called muons. Most muons decay into electrons within a few microseconds, but many still reach the surface, with an average of about 10,000 per square meter per minute. They also pass through the human body, and a small part of them will interact with the human body. On average, they account for about one-sixth of the total radiation exposure to the human body.
Usually, a small amount of muons will not cause serious harm. However, if the cosmic rays are increased by 100 times, the radioactive materials produced by the radiation will accumulate in the bodies of large animals, reducing their reproductive capacity, until they are sterilized and extinct. If cosmic rays continue to increase by 100 times, insects and single-celled organisms will also disappear from the earth. When the supernova exploded 2.5 million years ago, a hundred times more muons reached the depths of the ocean and lasted for thousands of years. Some large ancient creatures, such as the “megalodon”, went extinct during this time, and it is hard to say that the two are not related.
Are humans safe?
Studies have shown that if a supernova erupts within 20 light-years of our distance, life on Earth will face extinction. The “safe distance” for humans may be 50-100 light-years away.
Fortunately, first of all, the sun closest to us will not erupt into a supernova. Secondly, within 50 light-years, there are no massive stars that will become supernovae. But we can’t be 100% assured, because a small-mass white dwarf star may also explode into another type of supernova-Type Ia supernova after accreting material. According to the latest observations made by the European Space Agency’s Gaia (GAIA) satellite, it is estimated that 80% of white dwarfs have been found within 75 light-years of the Earth, and no supernova “candidates” have been found. The remaining possibility remains to be ruled out by future observations.
Beyond a safe distance, the most promising “soon” known to erupt into a supernova is Betelgeuse in Orion, more than 600 light-years away. Its mass is 10-20 times that of the sun, and its radius is about 800 times that of the sun. It belongs to the largest red supergiant among stars. Its life span is no more than ten million years, which is too short in the celestial bodies of the universe.
Betelgeuse Orion may burst into a supernova “at any time”. When is it exactly? The answer may be tomorrow, maybe tens of thousands of years later, maybe it has exploded but the light has not yet reached the earth. After all, compared with its lifespan, tens of thousands of years is only a moment’s time. Who can tell whether it will advance or postpone a “moment”? Its eruption should not bring a major disaster, but there will be one more star in the sky that is as bright as the moon.