How to observe the death of a star?

In the movie "The Wandering Earth", the sun is about to become a red giant and submerge the earth. Did you know that this is indeed the stage that the sun will go through in its later years? So, what will happen when the sun is about to end its long life? Hello everyone, I am Han Wenbiao, and today I will talk to you about the death of stars and how scientists observe it.

The closest star to us, the Sun, is 4.6 billion years old. Its lifespan is about 11 billion years, and it is now in its prime. In more than 5 billion years, the Sun will exhaust its core hydrogen fuel and become a red giant. Its core may also experience the helium flash described in "The Wandering Earth". The dying Sun will continue to grow in size and can expand all the way to the Earth's orbit. In hundreds of millions of years or even longer, the outer shell will be dispersed and become a beautiful planetary nebula, while the core will leave a dense white dwarf.

After talking about the sun, everyone must also want to know what happens when other stars in the universe die. Generally speaking, the death of a star means that the star no longer emits light and heat stably, and ends the main sequence stage, that is, the stage of hydrogen nuclear fusion. A star is in the main sequence stage for most of its life, and once the hydrogen fuel is exhausted, people will say that the star is beginning to reach the end of its life. However, the word death is not particularly accurate. I prefer to use metamorphosis to describe this process. Because when a star ends the main sequence stage, it actually enters a new life cycle. So when all stars die, will they first become a red giant like the sun, and then become a white dwarf?

Actually, this is not the case. Scientists have discovered that what happens when a star dies depends on its mass. If it is a red dwarf with a mass of only 0.08-0.5 solar masses, their lifespan can be as long as 100 billion years, which is longer than the current age of our universe. Therefore, we have not observed the death of such stars. Stars with a mass larger than red dwarfs but less than 8 times the mass of the sun, including the sun, will end up as mentioned above, first becoming huge red giants, and then after the outer layers are dissipated, a dense white dwarf will be left in the center.

What may be of more interest to everyone is the death process of more massive stars, which is accompanied by an extremely violent explosion - a supernova explosion. The electromagnetic radiation produced by the explosion can often illuminate the entire galaxy in which it is located, and may last for weeks, months or even years before gradually fading. During this period, the radiation energy released by a supernova can be equivalent to the total radiation energy of the sun in its lifetime. Therefore, as early as 185 AD during the Eastern Han Dynasty, the ancient Chinese observed and recorded a supernova explosion that shone in the night sky for eight months.

Stars can explode by throwing most or almost all of their matter outward at nearly one-tenth the speed of light, radiating shock waves to the surrounding interstellar matter. This shock wave causes a shell-like structure composed of expanding gas and dust, which is called a supernova remnant. At the same time, the core may form a neutron star or a black hole and continue to exist. So why do supernovas occur when massive stars die?

This is because a larger mass leads to a higher temperature and pressure in the core, which in turn leads to a runaway. When a massive star finishes burning hydrogen, the central core becomes a helium core. The helium core contracts under the influence of gravity and the temperature rises. When it reaches 100-200 million degrees, helium burns to produce carbon, oxygen, neon and releases energy. After the burning is over, the central core is mainly carbon and oxygen. When the carbon-oxygen core contracts and the temperature rises to 500 million to 1 billion degrees, carbon combustion occurs, producing oxygen, neon and magnesium, releasing energy. After the end, the central core is an oxygen-neon-magnesium core. When the core further contracts and the temperature continues to rise to 1.2 billion degrees, oxygen combustion occurs; when the temperature reaches 2 or 3 billion degrees, silicon combustion occurs, and the product is iron and releases energy. When the iron core contracts and the temperature soars to 5 billion degrees, the iron core will react with gamma photons, but this reaction does not release energy but requires energy absorption. This energy can only be provided by capturing a large number of electrons, which in turn causes a sharp drop in electron pressure. The iron core collapses violently to release huge energy, exploding the outer layer of the star. This is the brilliant supernova explosion. As I said before, this explosion does not really mean death, because it is very likely that a neutron star or a black hole will be left behind after the explosion. What is important is that a supernova explosion will produce a large amount of heavy elements and spread them into the universe, becoming an indispensable component of the richness of the universe, including our life. As the saying goes, when a star explodes, all things come into being.

Today's astronomers use advanced astronomical equipment, such as the high-altitude cosmic observatory Lasso and gravitational wave detectors, to observe supernova explosions and try to understand this brilliant cosmic firework. In December 2023, a research team led by Tsinghua University, based on early warning signals, immediately used telescopes around the world to conduct very detailed observations and studies on a supernova explosion 50 million light-years away from the Earth on May 19, 23, and obtained information such as light changes and spectra before and after the explosion. What is particularly interesting is that they discovered for the first time the process of shock waves changing from red to blue, revealing the shock wave process of supernova explosions that had not been discovered before. This study not only used professional astronomical observation equipment, but also used the amateur observation results of astronomy enthusiasts. Therefore, there are several astronomy enthusiasts among the authors of this Nature paper. It shows that astronomical research is a science that the public can widely participate in. Scientists and astronomy enthusiasts work together to combine multi-messenger astronomical observations of electromagnetic waves, neutrinos, high-energy cosmic rays and gravitational waves to reveal more secrets behind this cosmic wonder.

This article is a work supported by Science Popularization China Starry Sky Project

Author: Han Wenbiao, researcher at Shanghai Astronomical Observatory, Chinese Academy of Sciences

Reviewer: Gou Lijun, researcher at the National Astronomical Observatory of the Chinese Academy of Sciences

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.