What is a supernova explosion? How terrifying is it?

People who don't know astronomy may not know about supernova explosions. When the evolution of some stars is nearing the end, they will end their lives with a violent explosion, and the remaining parts will become other celestial bodies. This kind of explosion is called a supernova explosion. The term was coined by Walter Baade and Fritz Zwicky in 1931.

This explosion will release huge electromagnetic radiation, including visible light, in a short period of time, which can even illuminate the entire galaxy it is in. Theoretical calculations show that the energy radiated by a supernova during this period can reach 10^44 joules, which is a very large energy value, comparable to the total energy radiated by the sun in its lifetime (about 10 billion years), and some supernovae release much more energy in a short period of time when they explode than the sun releases in its lifetime.

Supernova explosions can be divided into two major categories.

One is when a massive star is at the end of its life, its internal nuclear fuel is exhausted, and the internal radiation pressure is ultimately unable to resist its own gravity, causing rapid gravitational collapse, and an explosion caused by the release of gravitational potential energy. Usually only stars with a mass of more than 8 times that of the sun will experience a supernova explosion. In other words, the sun will not experience a supernova explosion.

The other type occurs in a binary star system, where two white dwarfs merge, or the matter that a white dwarf absorbs from a companion star accumulates to a sufficient mass, reaching the Chandrasekhar limit (1.4 times the mass of the sun), which can also trigger a supernova explosion. This explosion sounds uncommon, but in fact binary star systems are very common in the universe.

Neutron stars and black holes are formed in supernova explosions. These two types of dense celestial bodies are extremely dense, with the density of neutron stars alone reaching 1 billion tons per cubic centimeter. However, supernova explosions do not always form such dense celestial bodies, and sometimes they are blown to pieces.

When a star at the end of its life explodes as a supernova, it ejects most of its matter at speeds up to one-tenth the speed of light, creating a shock wave that leads to the formation of a shell-like structure made of expanding gas and dust. This structure is the remnant of the supernova explosion.

In 1054, ancient Chinese astronomers saw a supernova explosion that was so bright that it could be seen during the day. According to relevant historical records, scientists have confirmed that the remains of the explosion are the Crab Nebula that can be seen today, which is 6,500 light-years away from the Earth. After the supernova explosion, its core collapsed into a pulsar (a pulsar is a rapidly rotating neutron star). The pulsar was discovered in 1969 and is the first celestial body confirmed as a relic of a historical supernova explosion.

Supernova explosions are really terrifying because the energy released by the explosion is extremely huge and it is one of the highest energy astronomical events in the universe.

The energy released when a supernova explodes is too great to be underestimated. If the explosion occurs too close to the Earth, the powerful gamma-ray radiation generated during the explosion will sweep across the Earth, which is enough to change the Earth's atmosphere and cause extinction of life. This deadly radiation is called a gamma-ray burst, and it is not only produced during a supernova explosion, but also when a neutron star or black hole merges.

There have been five mass extinction events in the history of the Earth. According to scientists' research, the Ordovician-Silurian extinction event that occurred 450 million years ago was most likely caused by the explosion of a supernova near the Earth at that time. The radiation generated by the explosion destroyed a large part of the Earth's ozone layer at the time, exposing a large number of organisms to deadly radiation.

However, the probability of a supernova explosion posing a threat to humans is relatively small, so there is no need to worry.

First of all, most stars are very far apart. The nearest star to the sun, Proxima Centauri, is only 4.2 light years away. Except for supernova explosions caused by supermassive stars, general supernova explosions can only have an impact on the Earth when they are within 50 to 100 light years of the Earth.

Secondly, most stars in the universe are smaller than the sun, and these smaller stars usually have a longer lifespan, longer than the sun. The universe is only 13.8 billion years old, and on average, there are only two or three supernova explosions in the Milky Way every hundred years. It's just that there are too many galaxies in the universe, so supernova explosions occur every day in the universe, but they are all outside the Milky Way, too far away from the earth, and will not pose any threat to the earth.

Although supernova explosions are terrifying, they are also one of the most important links in the evolution of celestial bodies in the universe. Without supernova explosions, life would not even be possible in the universe, because the elements after iron in the periodic table are basically formed during supernova explosions. Two light elements can fuse into heavy elements, but the conditions are extremely harsh, requiring temperatures of hundreds of millions or even billions of degrees. Such harsh high temperature and high pressure conditions can only be achieved during supernova explosions.

In addition, in the 1960s, astronomers discovered that the absolute magnitude of the maximum brightness of a Type Ia supernova explosion has a clear functional relationship with the light curve, which can be used to measure the distance to a galaxy and serve as a standard candle for astronomical distance measurement.