▲ Sirius A and B, an ordinary (similar to the sun) star and a white dwarf in the binary system
News from October 30, Beijing time. According to foreign media reports, when we study the celestial bodies in the universe, they are usually divided into two categories:
1. Self-luminous objects, such as stars, can produce light by themselves;
2. Non-self-luminous celestial bodies need external energy to make themselves visible.
The latter category includes planets, satellites, dust and clouds, etc. The light they emit is either reflecting other light sources or absorbing external energy before emitting light.
But is a self-luminous object necessarily a star? Surprisingly, not only are many self-luminous celestial bodies not necessarily stars, but many self-luminous celestial bodies with “star” in their names are not actually stars . Brown dwarfs, white dwarfs, and even neutron stars are not stars, while red dwarfs, yellow dwarfs (such as our sun), and all giant stars are indeed stars. The reason for this difference is this.
▲ Stars of various sizes, colors and masses (including many bright blue stars) have masses dozens or even hundreds of times that of the sun.
In our daily conversations, most of us habitually think that what we can see is a star, that it is a huge material ball, which can emit light by itself and radiate energy to the universe. In a sense, this is fine: all stars are indeed like this. They are indeed a large mass of matter, reaching hydrostatic equilibrium under the action of gravity. A series of physical processes take place inside them, transferring energy outward to the surface. Then from their boundary, that is, the star’s photosphere, part of the energy falls within the range of visible light, and then radiates into the universe.
All of this applies to stars, but it also applies to other celestial bodies, some of which are not stars at all. In the eyes of astronomers, if you want to become a new star, you need to cross a stricter threshold: the inner core ignites the nuclear fusion reaction. Please note that it is not a random nuclear fusion reaction, but a nuclear fusion reaction that fuses hydrogen into helium, or the product of the reaction continues to fuse into heavier elements. Without this, astronomers will not regard a celestial body as a star.
▲ The evolution of stars with the same mass as the sun
This may sound arbitrary, but please don’t rush to conclusions. There are important reasons behind this: if we start with the gas nebula, the reason will be obvious. In the universe, all the stars we currently know originate from gas nebulae. Gaseous nebulae spread throughout the universe, mainly composed of hydrogen and helium, and other trace amounts of heavier elements. And if gaseous nebulae become cold enough or large enough, or the interior becomes unstable enough, they will begin to collapse.
When the gravitational collapse begins, the density of some areas will inevitably be higher than the average density. Compared with other areas, the high-density area exerts greater attraction to the material, so it will become denser and denser over time. What happened next was that different regions competed to attract as much material as possible. But there is a problem with this situation: when the gaseous nebula collapses, the particles inside will collide and heat, thereby preventing the gaseous nebula from collapsing further.
▲ The Eagle Nebula is known for its continuous star formation, which contains a large number of Bock spheres or dark nebulae
The only way out is that these collapsing gaseous nebulae can radiate energy out in some way: they must cool themselves. The most effective way is to use those trace amounts of heavier elements, which can radiate energy more than hydrogen or helium atoms. As some areas of matter in the nebula become hotter, the heated gas not only begins to radiate energy, but also traps the energy inside, causing the internal temperature to rise sharply.
This gas may glow, but it is not a star, at least not yet. However, we can treat it as a protostar cloud for the time being, because it may become a mature star in the future. But for the protostellar cloud to become a mature star, its internal temperature needs to rise again, and only when matter is continuously drawn into high-density areas, the temperature will continue to rise, trapping more heat.
When the core temperature exceeds 1 million Kelvin, the initial fusion reaction takes place.
▲ There is a ring of protoplanetary disk around the protostar IM Lup, which not only has a ring shape, but also has the characteristic of a spiral toward the center
The first thing that happened was that deuterium (a hydrogen isotope of a proton and a neutron) fuse with a free proton to form a helium-3 nucleus: with two protons and a neutron. After passing this level, the nebula officially becomes a “protostar”: a large mass of matter that continues to accumulate mass from the surrounding molecular cloud, with its core supported by pressure. This pressure comes from the deuterium fusion reaction that is taking place, which exactly counteracts gravity.
In most cases, in this huge cloud of gas, there will be many cores desperately growing, attracting more mass to themselves, and constantly moving away from other protostars. In this competition, there are winners and losers, because some protostars can gain enough mass to heat to temperatures above about 4 million Kelvin. At this time, they can ignite the chain reaction. It is this chain reaction that provides energy for our sun: the proton-proton chain reaction. If you pass this level, congratulations, you are the winner of the universe: hope to become a real star. If you fail, then you will remain in this “swing” state where you can only fuse deuterium, and then become a brown dwarf: a failed star.
▲ Gliese 229 is a red dwarf star surrounded by a brown dwarf star Gliese 299b orbiting
Brown dwarfs have a mass between 13 and 80 times Jupiter: about 7.5% of the mass of the sun. Although they are called brown dwarfs, they are not real stars because they have not reached a critical threshold: they cannot undergo the fusion reactions necessary to become a mature star. If a brown dwarf merges with another brown dwarf or coexists with another brown dwarf to obtain sufficient mass, and then successfully crosses this mass threshold, it can be promoted to become a red dwarf: merge hydrogen into helium and become A real star.
These real stars vary in mass, color, and brightness. Stars with a mass between 7.5% and 40% of the sun are red dwarfs: they burn hydrogen into helium, but nothing more; they can never reach higher temperatures to do other things. Stars with a mass between 40% and 800% of the sun will eventually evolve into red giant stars, and then fuse helium into carbon until they run out of fuel. The more massive stars will evolve into supergiant stars, and they will explode into supernovae at the end of their lives.
▲ Modern star classification system-Morgan-Keenan spectral classification system
All stars that burn hydrogen, helium, carbon, or other heavier elements (with the heaviest element not exceeding iron)—whether they are the size of dwarfs, giants, or supergiants—are stars. As long as they can fuse lighter elements into heavier elements through the energy release process of nuclear fusion, we can say that they are stars. Some stars are stable, some are pulsating and flares; some are constant, some are variable; some are red, some are blue; some are very weak, and some are millions of times the sun.
But it doesn’t matter; they are still stars. As long as nuclear fusion is taking place in the core of these celestial bodies (except for deuterium combustion), they are stars.
However, every star has limited fuel. According to Einstein’s famous equation E = mc², they can only convert limited mass into energy. When the fusion stops, and the core shrinks, the temperature further rises, and no new fusion occurs, the star’s life is over. After this day, the only question is what will happen next.
▲ The life of a massive star
As far as we know, depending on the mass and condition of the star, there are five choices at this time:
1. The red dwarf will be completely composed of helium, and the entire (former) star will shrink into a white dwarf, and eventually cool down and extinguish into a black dwarf
2. The outer gas shell of a star similar to the sun will be blown away, and then become a planetary nebula, while the core of the star shrinks into a carbon-oxygen white dwarf, and finally slowly cools and extinguishes into a black dwarf;
3. Heavier stars are destined to explode into supernovae, and low-mass supernovae will produce neutron stars in their cores with a mass of 2.5 to 2.75 times that of the sun;
4. High-quality supernovae will still explode, but their cores are too large to produce neutron stars, but will produce black holes;
5. Or, in rare cases, the outer gas shell of a supergiant star that would have exploded into a supernova is stolen. In this case, “fancy” white dwarfs such as neon white dwarfs or magnesium white dwarfs will be produced inside the star that has lost its outer shell.
However, these general destinies—white dwarfs, neutron stars, and black holes—only represent the possibilities we know.
▲ In the core of the most massive neutron star, a single nucleus can be decomposed into quark-gluon plasma
Of course, there will be more peculiar possibilities. A neutron star can merge with a giant star to form a Thorne-Zutkov object. A very supernova or tidal force collapse event will tear up the entire supergiant star, and nothing will be left in the end. Perhaps the compressed matter will have further degenerate forms-like strange stars, quark stars, precursor stars and so on. We just haven’t discovered and identified them yet. In addition, all white dwarfs will slowly cool and extinguish, first emitting red light, then infrared light, and finally turning black after a long, long time.
Although these stellar remains have stars in their names, they are no longer stars at all. Once the fusion reaction stops inside their cores, they are just stellar remains: remnants of former stars. White dwarfs are not stars, and the ultimate fate of white dwarfs is black dwarfs. Neutron stars are not stars; black holes are not stars either. Other strange stars, such as strange stars, quark stars, or precursor stars, are not stars even if they exist. If the heavier elements continue to accumulate inside the Thorne-Zutkov celestial body, the star identity can be retained; but as long as the fusion reaction stops, it will no longer be a star.
▲ Thorne-Zutkov celestial body is a hypothetical star, which is a red giant or red supergiant star with a neutron star in its core
When you put all this information together, we can clearly distinguish which are stars and which are not. The collapsed core is supported by radiation and continues to absorb cloud gas from the surrounding molecular cloud, which is a protostar, but not a real star. Only deuterium is fused inside the core and nothing else. It is a brown dwarf (a star that has failed evolution), but it is not a real star either. Only those who successfully fused hydrogen into helium or helium (or heavier elements) into other heavier elements at a temperature of 4 million Kelvin or above in the core can be granted the status of “star”.
However, once the fusion reaction in the core stops, you are no longer a star. Any kind of stellar remains-white dwarfs, neutron stars, black dwarfs, etc., are not stars, and can only be said to have been brilliant. These wrecks may continue to shine for trillions of years, and they may shine longer than the lifespan of the real star that gave birth to them, but in the final analysis, even if the name has “star”, they are no longer real stars. . Although they can still glow without fusion, they are no longer stars after all.
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