: About some related astronomical knowledge.

Mainly introduce the four dwarfs of the white dwarf star, red giant star, neutron star, and black hole. I hope that after reading it, I will add some interest to the reading of this book.
White dwarf is a very special celestial body. It is small in size and low in brightness, but it is of high quality and extremely high density. For example, Sirius companion star (it was the first white dwarf to be discovered), the volume is not much larger than the earth, but the quality is similar to the sun! In other words, its density is around 10 million tons / cubic meter.
According to the radius and mass of the white dwarf, it can be calculated that its surface gravity is equal to 10 million to 100 million times the surface of the earth. Under such high pressure, any object no longer exists, and even the atoms are crushed: the electrons are separated from the atomic orbit and become free electrons.
A white dwarf is a late star. According to modern stellar evolution theory, white dwarfs are formed in the center of red giant stars.
When the outer area of ​​the red giant star expands rapidly, the reaction force of the helium nucleus is strongly inwardly contracted, and the compressed substance heats up. The final core temperature will exceed 100 million degrees, and the sputum begins to aggregate into carbon.
After millions of years, the nucleus has burned out. Now the structure of the star is not so simple: the outer shell is still a hydrogen-based mixture; there is a layer of enamel underneath it, and a layer inside the raft is buried. Carbon ball. The nuclear reaction process becomes more complicated, and the temperature near the center continues to rise, eventually turning carbon into other elements.
At the same time, the unstable pulsation oscillation begins outside the red giant star: the radius of the star becomes larger and sometimes shrinks, and the stable main star-ordered star becomes a very unstable giant fireball. The nuclear reaction inside the fireball is more and more inclined. Stable, suddenly strong, and faint. At this time, the inner core of the star has actually increased its density to about ten tons per cubic centimeter. We can say that at this time, a white dwarf star has been born inside the red giant star.
Why is the density of white dwarfs so large?
We know that atoms are made up of nucleus and electrons. The mass of atoms is mostly concentrated on the nucleus, and the size of the nucleus is small. For example, the radius of a hydrogen atom is one hundred millionth of a centimeter, and the radius of a hydrogen nucleus is only one tenth of a centimeter. If the size of the core is like a glass ball, the electron orbit will be two kilometers away.
Under tremendous pressure, electrons will break away from the nucleus and become free electrons. This free electron gas will occupy as much space as possible between the nucleus, so that the substances contained in the unit space will also be greatly increased, and the density is greatly improved. In the image, the nucleus is "immersed in" electrons.
This state of matter is generally called the "degenerate state." The degenerate electron gas pressure and the strong gravity balance of the white dwarf star maintain the stability of the white dwarf. Incidentally, when the quality of the white dwarf is further increased, the degenerate electron gas pressure may not resist the gravitational contraction of itself, and the white dwarf star will collapse into a higher density celestial body: a neutron star or a black hole.
For a single-star system, because there is no thermonuclear reaction to provide energy, the white dwarf cools at the same rate while emitting light. After a long period of 10 billion years, the old white dwarf will gradually stop radiating and die. Its body becomes a huge crystal that is harder than diamonds - black dwarfs forever.
For multi-star systems, the evolution of white dwarfs may be altered. (
If you are amazed at the huge density of white dwarfs, there is something to surprise you! We will introduce a denser star here: the neutron star.
The density of the neutron star is 10 to the power of 11 kilograms per cubic centimeter, that is, the mass per cubic centimeter is actually 100 million tons! Compared to the tens of tons/cubic centimeter of the white dwarf, the latter seems to be worth mentioning. In fact, the quality of a neutron star is so great that the mass of a neutron star with a radius of ten kilometers is comparable to the quality of the sun.
Like white dwarfs, neutron stars are stars in the late stages of evolution, and they are also formed in the center of old stars. Only the stars that can form a neutron star are of even greater quality. According to scientists' calculations, when the mass of an old star is greater than the mass of ten suns, it may eventually become a neutron star, and a star with a mass less than ten suns can often only change to a white dwarf.
However, the difference between neutron stars and white dwarfs is by no means different from the quality of the stars that produced them. Their physical state of existence is completely different.
Simply put, although the density of white dwarfs is large, it is also within the maximum density that can be achieved by normal material structures: electrons or electrons, nuclei or nuclei. In the neutron star, the pressure is so great that the degenerate electron pressure in the white dwarf can no longer afford: the electron is compressed into the nucleus, and the homogenizer is neutralized as a neutron, making the atom only composed of neutrons. The entire neutron star is formed by the close proximity of such nucleuses. It can be said that the neutron star is a huge atomic nucleus. The density of a neutron star is the density of the nucleus.
In terms of the formation process, neutron stars are very similar to white dwarfs. When the star's outer shell expands outward, its core is contracted by the reaction force. The nucleus undergoes a series of complex physical changes under enormous pressure and the resulting high temperatures, eventually forming a neutron star core. The entire star will end its life with an extremely spectacular explosion. This is the famous "supernova explosion" in astronomy.
"Black hole" is easy to imagine 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 even light cannot escape.
According to general relativity, the gravitational field will bend the space and time. When a star is very large, its gravitational field has little effect on time and space, and light from a point on the surface of the star can be emitted in a straight line in any direction. The smaller the radius of the star, the greater its effect on the surrounding space and time, and the light emitted at certain angles will return to the surface of the star along the curved space.
When the radius of a star is as small as a certain value (astronomically called "Schwarzschild radius"), even the light emitted by 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. Once any substance falls into it, it seems that it can no longer escape. In fact, the black hole is really "invisible", we will talk about it later.
So how is the black hole formed? In fact, like white dwarfs and neutron stars, black holes are likely to evolve from stars.
We have introduced in more detail the process of formation of white dwarfs and neutron stars. When a star ages, its thermonuclear reaction has exhausted the center's fuel (hydrogen), and the energy generated by the center is running out. In this way, it no longer has enough power to take up the huge weight of the outer casing. Therefore, under the weight of the outer casing, the core begins to collapse until the final formation of a small, dense star, regaining the ability to balance the pressure.
Stars with smaller masses mainly evolve into white dwarfs, and stars with larger masses may form neutron stars. According to scientists' calculations, 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 power to compete with its own gravity, causing another big contraction.
This time, according to scientists' speculation, matter will unstoppably march toward the center point until it becomes a "point" where volume tends to zero and density tends to infinity. And once its radius has shrunk to a certain degree (Schwarzschild radius), as we have described above, the huge gravitational force makes it impossible for even light to be emitted outward, thus cutting off all links between the star and the outside world - " The black hole was born.
Compared to other celestial bodies, black holes are too special. For example, black holes have "invisibility", people can't directly observe it, and even scientists can only make various conjectures about its internal structure. So how does the black hole hide itself? The answer is - the space of bending. We all know that light travels in a straight line. This is a basic common sense. However, according to general relativity, space will bend under the action of gravitational field. At this time, although the light still travels along the shortest distance between any two points, it is not a straight line but a curve. In an image, it seems that the light is going to go straight, but the strong gravity pulls it away from the original direction.
On Earth, this curvature is minimal due to the small effect of the gravitational field. And around the black hole, this deformation of the space is very large. In this way, even if the light emitted by the star blocked by the black hole disappears into the black hole, another part of the light will pass through the curved space and pass through the black hole to reach the earth. Therefore, we can effortlessly observe the starry sky on the back of the black hole, just as the black hole does not exist. This is the stealth of the black hole.
What's more interesting is that some stars not only directly transmit light to the earth, but also the light emitted in other directions may be refracted by the strong attraction of nearby black holes to reach the earth. In this way, we can not only see the "face" of this star, but also see its side, even the back!

Black Hole
is undoubtedly one of the most challenging and exciting astronomical theories of the century. Many scientists are working hard to uncover its mystery, and new theories are constantly being proposed. However, the latest achievements of these contemporary astrophysics are not clear here in a few words. Interested friends can refer to the special discussion.
When a stellar spends its long young adulthood, the main sequence, entering the old age, it will first become a red giant.
Calling it a "superstar" is to highlight its huge size. In the superstar phase, the volume of the star will expand to a billion times.
It is called a "red" superstar because the star's outer surface is getting farther and farther away from the center as the star expands rapidly, so the temperature will decrease and the emitted light will become more reddish. However, although the temperature has dropped a little, the size of the red giant star is so large that its luminosity has become very large and extremely bright. Many of the brightest stars seen by the naked eye are red giant stars.
In Her-Roto, the red giant stars are distributed in a fairly dense area on the upper right side of the main star sequence, almost horizontally.
Let's take a closer look at the formation of red giant stars. We already know that stars rely on their internal thermonuclear fusion to burn. As a result of nuclear fusion, each of the four hydrogen nuclei is combined into a helium nuclei, and a large amount of atomic energy is released to form a radiation pressure.
In a star in the main star sequence, nuclear fusion occurs mainly in its central (core) part. The radiation pressure is balanced with the gravitational force of its own contraction.
The combustion of hydrogen is extremely fast, and the center forms a nucleus and increases. As time goes on, the hydrogen around the helium nucleus becomes less and less, and the energy generated by the central nucleus is not enough to sustain its radiation, so the balance is broken and gravity attracts the upper hand. Stars with a helium nucleus and a hydrogen shell contract under gravitation, increasing their density, pressure, and temperature. The combustion of hydrogen advances into a shell around the helium nucleus.
After that, the process of stellar evolution is: the core shrinks and the outer shell expands—the nucleus inside the combustion shell shrinks inward and heats up, while the outer shell of the star expands outward and becomes cold, and the surface temperature is greatly reduced. This process lasted for hundreds of thousands of years, and the star turned into a red giant in rapid expansion.
Once the red giant star is formed, it will move toward the next stage of the star, the white dwarf. When the outer region expands rapidly, the helium nucleus is strongly inwardly contracted by the reaction force, and the compressed material is continuously heated, and the final core temperature will exceed 100 million degrees, igniting and melting. The final ending will form a white dwarf in the center.