This article mainly introduces four kinds of celestial bodies: white dwarf, red giant, neutron star and black hole. I hope that after reading this, you will be more interested in reading this book.
White dwarfs are a very special type of celestial body. They are small in size and low in brightness, but large in mass and extremely dense. For example, the companion star of Sirius (which is the first white dwarf to be discovered) is not much larger than the earth, but its mass is about the same as the sun.
!In other words, its density is about 10 million tons/cubic meter.
Based on the radius and mass of the white dwarf, it can be calculated that its surface gravity is equal to 10 million to 1 billion times that of the earth's surface. Under such high pressure, any object no longer exists, and even atoms are crushed: electrons are detached
Atomic orbitals become free electrons.
A white dwarf is a late-stage star. According to modern stellar evolution theory, a white dwarf is formed in the center of a red giant.
When the outer region of the red giant expands rapidly, the helium core shrinks strongly inward under the reaction force, and the compressed material continues to heat up. Eventually, the core temperature will exceed 100 million degrees, and helium begins to fuse into carbon.
After millions of years, the helium core has burned out, and now the structure of the star is no longer that simple: the outer shell is still a mixture mainly composed of hydrogen; there is a helium layer below it, and there is also a helium layer buried inside the helium layer.
Carbon spheres. The nuclear reaction process becomes more complex, and the temperature near the center continues to rise, eventually converting carbon into other elements.
At the same time, unstable pulsating oscillations began to occur outside the red giant: the star's radius sometimes increased and sometimes shrank, and the stable main sequence star turned into an extremely unstable giant fireball. The nuclear reactions inside the fireball became increasingly unstable.
Stable, sometimes strong, sometimes weak. At this time, the density of the core of the star has actually increased 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 high?
We know that an atom is composed of a nucleus and electrons. Most of the mass of an atom is concentrated in the nucleus, and the size of the nucleus is very small. For example, the radius of a hydrogen atom is one hundred millionth of a centimeter, while the radius of a hydrogen nucleus is only ten
One trillionth of a centimeter. If the size of the nucleus is like a glass ball, the electron orbit will be two kilometers away.
Under huge pressure, electrons will break away from the nucleus and become free electrons. This free electron gas will occupy the gaps between the nuclei as much 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.
.To put it figuratively, the atomic nucleus is "immersed" in electrons at this time.
This state of matter is generally called a "degenerate state". The pressure of the degenerate electron gas balances with the strong gravity of the white dwarf, maintaining the stability of the white dwarf. By the way, when the mass of the white dwarf further increases, the pressure of the degenerate electron gas will increase.
It may be unable to resist its own gravitational contraction, and the white dwarf may collapse into a denser object: a neutron star or a black hole.
For a single star system, since there is no thermonuclear reaction to provide energy, the white dwarf emits light and heat while also cooling at the same rate. After ten billion years, the old white dwarf will gradually stop radiating and die.
. Its body becomes a huge crystal that is harder than diamond - a black dwarf and exists forever.
For multi-star systems, the evolution process of white dwarfs may be changed. (
If you are amazed by the huge density of white dwarf stars, here is something that will surprise you even more! We will introduce a denser star here: neutron stars.
The density of a neutron star is 10 to the 11th power kilogram/cubic centimeter, which means that the mass per cubic centimeter is 100 million tons! Compared with the tens of tons/cubic centimeter of a white dwarf star, the latter seems not worth mentioning.
In fact, the mass of neutron stars is so large that the mass of a neutron star with a radius of ten kilometers is equivalent to the mass of the sun.
Like white dwarfs, neutron stars are stars in the later stages of evolution. They are also formed in the centers of old stars. It’s just that stars that can form neutron stars are more massive. According to scientists’ calculations, when the mass of an old star is greater than ten suns
When the mass is higher, it may eventually become a neutron star, while stars with a mass less than ten suns can often only transform into a white dwarf.
However, the difference between neutron stars and white dwarfs is not just the mass of the stars that generate them. Their states of material existence are completely different.
Simply put, although the density of a white dwarf is high, it is still within the maximum density range that a normal material structure can achieve: electrons are still electrons, and nuclei are still nuclei. In a neutron star, the pressure is so great that the degenerate electron pressure in the white dwarf star
It can no longer be tolerated: electrons are compressed into the nucleus and neutralized with protons into neutrons, so that the atom becomes composed only of neutrons. The entire neutron star is formed by such atomic nuclei close together. It can be said that
, a neutron star is a huge atomic nucleus. The density of the neutron star is the density of the atomic 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 shrinks under the reaction force. The core undergoes a series of complex physical changes under the huge pressure and resulting high temperature.
, and finally form a neutron star core. And the entire star will end its life in a very spectacular explosion. This is the famous "supernova explosion" in astronomy.
It is easy for people to imagine a "black hole" as a "big black hole", but it is not the case. The so-called "black hole" is a celestial body whose gravitational field is so strong that even light cannot escape.
According to the general theory of relativity, the gravitational field will bend space-time. When the size of a star is large, its gravitational field has almost no impact on space-time. Light emitted from a certain point on the surface of the star can be emitted in a straight line in any direction. The radius of the star
The smaller it is, the greater its effect on the curvature of surrounding space-time, and the light emitted at certain angles will return to the star's surface along the curved space.
When the radius of the star reaches a certain value (called the "Schwarzschild radius" in astronomy), even the light emitted by the vertical surface is captured. At this time, the star becomes a black hole. It is said to be "black"
, means that it is like a bottomless pit in the universe. Once any matter falls into it, it "seems" that it can no longer escape. In fact, black holes are truly "invisible", which we will talk about 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 introduced the formation process of white dwarfs and neutron stars in more detail. When a star ages, its thermonuclear reaction has exhausted the fuel (hydrogen) in the center, and there is not much energy generated by the center. In this way, it can no longer
There is not enough strength to bear the huge weight of the outer shell. So under the weight of the outer shell, the core begins to collapse until it finally forms a small and dense star, which is able to balance with the pressure again.
Stars with smaller masses will mainly evolve into white dwarfs, while stars with larger masses may form neutron stars. According to scientists' calculations, the total mass of neutron stars cannot be greater than three times the mass of the sun. If it exceeds this value, there will be nothing left
The force can compete with its own gravity, triggering another big collapse.
This time, according to scientists' conjecture, matter will march unstoppably toward the center 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 extent (Schwarzschild radius
), as we introduced above, the huge gravity prevents even light from being emitted outward, thus cutting off all connections between the star and the outside world - a "black hole" was born.
Compared with other celestial bodies, black holes are too special. For example, black holes have "invisibility" and people cannot directly observe them. Even scientists can only make various conjectures about its internal structure. So, how do black holes work?
What is it hiding? The answer is - curved space. We all know that light propagates in straight lines. This is the most basic common sense. But according to the general theory of relativity, space will curve under the action of the gravitational field. At this time,
Although 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 originally going to travel in a straight line, but the strong gravity pulled it away from its original direction.
direction.
On the earth, due to the small gravitational field, this bending is minimal. But around the black hole, the deformation of space is very large. In this way, even though part of the light emitted by the star blocked by the black hole will fall
It enters the black hole and disappears, but the other part of the light will bypass the black hole in the curved space and reach the earth. Therefore, we can effortlessly observe the starry sky on the back of the black hole, as if the black hole does not exist. This is the invisibility of black holes.
.
What’s more interesting is that some stars not only emit light towards the earth directly to the earth, but the light they emit in other directions may also be refracted by the strong gravity of nearby black holes and reach the earth. In this way, we can not only see the star
Its "face" can also be seen from its side and even its back!
"Black hole" is undoubtedly one of the most challenging and exciting astronomical theories of this century. Many scientists are working hard to unveil its mystery, and new theories are constantly being proposed. However, these contemporary
The latest results in astrophysics cannot be explained clearly in a few words here. Friends who are interested can refer to specialized works.
When a star passes through its long young adulthood, the main sequence star stage, and enters old age, it will first become a red giant.
Calling it a "giant star" highlights its huge size. In the giant stage, the star's size will expand to one billion times.
It is called a "red" giant because as the star expands rapidly, its outer surface becomes farther and farther away from the center, so the temperature will decrease accordingly and the light emitted will become increasingly redder. However,
Although the temperature has dropped a bit, the size of the red giant is so large that its luminosity has also become very large and extremely bright. Many of the brightest stars seen with the naked eye are red giants.
In the Herbert-Roulhorn diagram, red giant stars are distributed in a fairly dense area in the upper right of the main sequence region, almost horizontally.
Let's look at the formation of red giants in more detail. We already know that stars burn ragingly by the thermonuclear fusion inside them. The result of nuclear fusion is to combine every four hydrogen nuclei into a helium nucleus and release
A large amount of atomic energy creates radiation pressure.
For a star in the main sequence stage, nuclear fusion mainly occurs in its central (core) part. The radiation pressure is balanced by the gravity of its own contraction.
The burning of hydrogen consumes extremely fast, and a helium nucleus is formed in the center and continues to grow. As time goes by, there is less and less hydrogen around the helium nucleus, and the energy generated by the central nucleus is no longer enough to maintain its radiation, so the balance is broken, and gravity
The star has the upper hand. The star with a helium core and a hydrogen shell shrinks under the influence of gravity, causing its density, pressure and temperature to increase. The burning of hydrogen advances into a shell around the helium core.
After that, the process of stellar evolution is: core contraction, outer shell expansion - the helium core inside the burning shell shrinks inward and becomes hotter, while its stellar outer shell expands outward and continues to cool, and the surface temperature is greatly reduced. This process is just
Lasting for hundreds of thousands of years, the star expanded rapidly and became a red giant.
Once a red giant is formed, it moves towards the next stage of a star - a white dwarf. When the outer region expands rapidly, the helium core shrinks strongly inward due to the reaction force. The compressed material continues to heat up, and the final core temperature will exceed 100 million
degrees, igniting helium fusion. The final outcome will be a white dwarf star in the center.