Mainly introduce the four celestial bodies of white dwarf, red giant, neutron star and black hole. I hope that after reading it, you can add some interest to the reading of this book.

White dwarfs are a very special kind of celestial objects, which are small in size and low in brightness, but have large mass and extremely high density. For example, the Sirius companion star (it was the first white dwarf to be discovered) is not much larger than Earth, but it has about the same mass as the Sun! That is, its density is around 10 million tons / cubic meter.

Based on the radius and mass of a white dwarf, it can be calculated that its surface gravity is equal to 10 million to 1 billion times the Earth's surface. At such high pressures, any object ceases to exist, even the atoms are crushed: the electrons break out of the atomic orbit and become free electrons.

A white dwarf is a late star. According to modern theories of stellar evolution, white dwarfs form at the center of red giants.

When the outer region of the red giant expands rapidly, the helium nucleus is strongly contracted inward by the reaction force, and the compressed material continues to heat up, and eventually the core temperature will exceed 100 million degrees, so helium begins to fuse into carbon.

After millions of years, the helium nucleus burned out, and now the structural composition of the star is not so simple: the outer shell is still a mixture dominated by hydrogen; And there is a helium layer underneath it, and there is a carbon ball buried inside the helium layer. The nuclear reaction process becomes more complex, and the temperature near the center continues to rise, eventually transforming the carbon into other elements.

At the same time, unstable pulsating oscillations begin to occur on the outside of the red giant: the radius of the star sometimes increases and sometimes decreases, and the stable main sequence star becomes a huge and extremely unstable fireball, and the nuclear reaction inside the fireball becomes more and more unstable, sometimes intense, sometimes weak. At this time, the density of the inner core of the star has actually increased to about ten tons per cubic centimeter, and we can say that at this time, inside the red giant, a white dwarf star has been born.

Why are white dwarfs so dense?

We know that an atom is made up of a nucleus and electrons, and the mass of an atom is mostly concentrated in the nucleus, and the volume of the nucleus is very small. For example, the radius of a hydrogen atom is one hundredth of a centimeter, while the radius of a hydrogen nucleus is only one trillionth of a centimeter. If the nucleus is the size of a glass sphere, the electron orbit will be two kilometers away.

Under tremendous pressure, the electrons will break away from the nucleus and become free electrons. This free electron gas will occupy as much space between the nuclei as possible, so that the amount of matter contained in the unit space will also be greatly increased, and the density will be greatly increased. Figuratively speaking, at this point the nucleus is "immersed" in electrons.

This state of matter is generally referred to as a "degenerate state". The degenerate electron gas pressure and the strong gravitational balance of the white dwarf maintain the stability of the white dwarf. Incidentally, as the mass of the white dwarf increases further, the degenerate electron gas pressure may not be able to resist its gravitational contraction, and the white dwarf will collapse into a denser object: a neutron star or a black hole.

In the case of a single star system, since there is no thermonuclear reaction to provide energy, white dwarfs emit light and heat while cooling at the same rate. After 10 billion years, the old white dwarfs will gradually stop radiating and die. Its body became a huge crystal harder than a diamond, a black dwarf, and it was immortalized.

In the case of multi-star systems, the evolution of white dwarfs may be altered. （

If you're amazed by the sheer density of white dwarfs, here's something even more surprising to you! We'll introduce here a more dense type of star: the neutron star.

The density of a neutron star is 10 to the 11th power of kilograms per cubic centimeter, which is the mass of 100 million tons per cubic centimeter! Compared to the tens of tons per cubic centimeter of white dwarfs, the latter seems to be nothing to mention. In fact, the mass of a neutron star is so great that the mass of a neutron star with a radius of 10 kilometers is comparable to the mass of the Sun.

Like white dwarfs, neutron stars are late-stage stars that form at the center of older stars. It's just that the stars that can form neutron stars are more massive. According to scientists' calculations, when an older star has a mass greater than the mass of ten suns, it may eventually become a neutron star, while a star with a mass of less than ten suns often changes into only one white dwarf.

However, the difference between neutron stars and white dwarfs is by no means just the mass of the stars that produce them. Their state of physical existence is completely different.

To put it simply, the density of a white dwarf, although large, is still within the maximum density that can be achieved by the normal structure of matter: electrons are still electrons, and nuclei are still nuclei. In neutron stars, the pressure is so great that the degenerate electron pressure in a white dwarf can no longer bear it: the electrons are compressed into the nucleus, and the homoprotons are neutralized into neutrons, making the atom consist only of neutrons. And the whole neutron star is formed from such nuclei close together. A neutron star, so to speak, is a huge nucleus. The density of a neutron star is the density of the nucleus of an atom.

Neutron stars are very similar to white dwarfs in terms of how they are formed. When a star's shell expands outward, its core contracts by a reaction force. The nucleus undergoes a complex series of physical changes under enormous pressure and the resulting high temperatures, culminating in the formation of a neutron star core. And the entire star will come to life with a spectacular explosion. This is the famous "supernova explosion" in the sky.

It is easy to imagine a "black hole" as a "big black hole", but it is not. The so-called "black hole" is such a celestial body: its gravitational field is so strong that not even light can escape.

According to the general theory of relativity, the gravitational field will bend space-time. When a star is large, its gravitational field has little effect on space-time, and light from a point on the star's surface can shoot in a straight line in any direction. The smaller the radius of a star, the greater its curvature of space-time around it, and the light emitted at certain angles will return to the star's surface along the curved space.

When the radius of a star reaches a certain value (called the Schwarzschild radius in the sky), even the light emitted from the vertical surface is captured. At this point, the star becomes a black hole. To say that it is "black" means that it is like a bottomless pit in the universe, and once any matter falls into it, it "seems" that it can no longer escape. In fact, black holes are truly "invisible", as we'll get to in a moment.

So, how are black holes formed? In fact, like white dwarfs and neutron stars, black holes are likely to evolve from stars.

We have described in some detail the process of the formation of white dwarfs and neutron stars. When a star ages, its thermonuclear reaction has exhausted the fuel (hydrogen) of the center, and there is not much energy produced by the center. In this way, it no longer has enough strength to carry the enormous weight of the shell. So under the weight of the outer shell, the core begins to collapse until it finally forms a small, dense star that regains its ability to balance with the pressure.

Stars with less mass mainly evolve into white dwarfs, while stars with more mass may form neutron stars. And according to the calculations of scientists, the total mass of a neutron star cannot be greater than three times the mass of the Sun. If this value is exceeded, there will be no more force to compete with its gravity, causing another collapse.

This time, according to scientists' conjectures, matter will march inexorably towards the central point until it becomes a "point" where the volume tends to zero and the density tends to infinity. And when its radius shrinks to a certain point (Schwarzschild radius), as we described above, the huge gravitational pull makes it impossible for even light to shoot outward, thus cutting off all contact between the star and the outside world - the "black hole" is born.

Compared to other celestial bodies, black holes are too special. For example, black holes have "stealth" that cannot be directly observed, and even scientists can only speculate about its internal structure. So, how do black holes hide themselves? The answer is curved space. We all know that light travels in a straight line. This is the most basic common sense. However, according to the general theory of relativity, space bends under the action of a gravitational field. At this time, although the light still travels along the shortest distance between any two points, it is no longer a straight line, but a curve. Figuratively speaking, it seems that light was supposed to go in a straight line, but the strong gravitational pull pulled it away from its original direction.

On Earth, this bending is negligible due to the small action of the gravitational field. And around a black hole, this distortion of space is very large. In this way, even if the light emitted by a star blocked by a black hole will fall into the black hole and disappear, another part of the light will pass through the curved space around the black hole and reach the Earth. So, we can effortlessly observe the starry sky on the back side of the black hole, as if the black hole does not exist, which is the stealth of the black hole.

What's even more interesting is that some stars not only emit light energy towards Earth that reaches Earth, but also light emitted in other directions can be refracted by the strong gravitational pull of a nearby black hole to reach Earth. In this way, we can see not only the "face" of the star, but also its side and even its back!

"Black holes" are undoubtedly one of the most challenging and exciting theories of this century. Many scientists are hard at work to demystify it, and new theories are being proposed. However, these latest achievements in contemporary astrophysics cannot be explained here in a few words. If you are interested, you can refer to the special treatise.

When a star passes its long prime of life, main-sequence star, and enters old age, it will first become a red giant.

Calling it a "superstar" is to highlight its huge size. During the giant phase, the size of the star will expand to as much as a billion times.

It is called a "red" giant because as the star expands rapidly, its outer surface is getting farther and farther away from the center, so the temperature will decrease and the light emitted will become more and more reddish. However, although the temperature has decreased a little, the red giant is so large that its luminosity has become very large and extremely bright. Many of the brightest stars seen with the naked eye are red giants.

In the Hera-Royd diagram, the red giants are distributed in a fairly dense area in the upper right of the main sequence region, trending almost horizontally.

Let's take a closer look at the formation of red giants. We already know that stars burn up with thermonuclear fusion inside them. As a result of nuclear fusion, every four hydrogen nuclei are combined into a helium nucleus, and a large amount of atomic energy is released, forming radiation pressure.

For a star in the main sequence phase, nuclear fusion occurs mainly in its central (core) part. The radiation pressure is balanced by the gravitational pull of its own contraction.

Hydrogen is burned very quickly, and a helium nucleus forms in the center and grows in size. As time progresses, there is less and less hydrogen around the helium nucleus, and the energy produced by the central nucleus is no longer enough to sustain its radiation, so the equilibrium is broken and gravity prevails. Stars with helium nuclei and hydrogen shells shrink under gravitational pull, increasing their density, pressure, and temperature. The combustion of hydrogen is pushed into a shell around the helium nucleus.

Since then, the process of star evolution has been as follows: the inner core shrinks, the outer shell expands - the helium nucleus inside the burning shell shrinks inward and heats up, while the stellar shell expands outward and keeps getting colder, and the surface temperature is greatly reduced. This process lasted only a few hundred thousand years, and the star became a red giant in rapid expansion.

Once the red giant star is formed, it heads towards the next stage of the star, the white dwarf. When the outer region expands rapidly, the helium nucleus is strongly contracted inward by the reaction force, and the compressed material continues to heat up, and eventually the core temperature will exceed 100 million degrees, igniting helium fusion. The final outcome will be the formation of a white dwarf star in the center.