Stars are usually born in interstellar gas. In the universe, when the density of interstellar gas increases to a certain level, the gas cloud begins to shrink because the increase in its internal gravity is greater than the increase in gas pressure. At the beginning of such a tendency, its own gravity causes the density of huge amounts of matter to generally increase. The huge mass of interstellar matter begins to become unstable. The collapse of these huge amounts of interstellar gas and dust is proceeding more and more rapidly, and they begin to split into smaller clouds, and their density has also increased a lot. These smaller clouds will eventually each become a star. Since the mass of interstellar matter is usually very huge, usually more than 10,000 times that of the sun, stars are always born in large numbers all at once. If there is a mass of interstellar gas that exceeds the density of ordinary interstellar matter (one hydrogen atom per cubic centimeter), reaching 60,000 hydrogen atoms per cubic centimeter. At the beginning, the gas was transparent, and the photothermal radiation emitted was not restricted by the surrounding materials and spread to the outside unimpeded. Material falls to the center in free fall and accumulates in the central area. A mass of matter that was originally evenly distributed became a ball of gas that became denser as it went inward. As the density increases, the gravitational acceleration near the center becomes larger and larger, and the growth of the movement speed of the material in the inner region is the most prominent. At first, almost all hydrogen existed in the form of molecules, and the temperature of the gas was also very low and never increased. This was because it was still too thin and all radiation could penetrate outward. The heating effect of the collapsing gas ball did not Not significant. Over hundreds of thousands of years, the central region gradually became denser, where the gas became opaque to radiation. At this time, the core begins to heat up. As the temperature rises, the pressure begins to increase and the collapse gradually stops. The radius of this dense central region is usually similar to the radius of Jupiter's orbit, and it contains only 5% of the mass involved in the entire collapse process. Matter continues to fall onto the small core inside, and the energy it brings becomes radiation and is released when the matter hits the core. At the same time, the core is shrinking and getting hotter. When the temperature reaches about 2,000 degrees, hydrogen molecules begin to decompose into atoms. The core begins to shrink again, releasing energy that breaks all the hydrogen molecules into atoms. This new core is slightly larger than today's sun. The peripheral matter that continues to fall toward the center will eventually fall on this core, and a star with the same mass as the sun will be born. Such an object is called a "protostar" and its radiation consumption is mainly replenished by the energy of matter falling onto it. As density and temperature increase, atoms gradually lose their outer electrons. The falling gas and dust creates a thick crust that makes it impossible for light to penetrate. It wasn't until more and more falling matter became integrated with the core that the outer shell became transparent, and the luminous star suddenly emerged. The rest of the cloud material is still falling towards it, the density is still increasing, and the internal temperature is also rising. Fusion occurs until the core temperature reaches 10 million degrees. A primitive star is born. In their perpetual struggle against gravity, a star's main weapon is nuclear energy. At its core is a large nuclear bomb that continues to explode. It is precisely because this nuclear power can adjust itself to almost precise balance with gravity that stars can remain stable for billions of years. Thermonuclear reactions occur between atomic nuclei at extremely high temperatures and thus involve the fundamental structure of matter. At the center of a star like the sun, the temperature reaches 15 million degrees Kelvin and the pressure is 300 billion times that of the Earth's atmosphere. Under such conditions, not only do atoms lose all their electrons and only have nuclei left, but the nuclei also move at such a high speed that they can overcome electrical repulsion and combine. This is nuclear fusion. Stars are formed in the centers of hydrogen molecular clouds and are thus composed primarily of hydrogen. Hydrogen is the simplest chemical element. Its atomic nucleus consists of a positively charged proton and a negatively charged electron orbiting the nucleus. The temperature inside the star is so high that all the electrons are separated from the protons, which act like molecules in the gas and move in all directions. Because like charges repel each other, protons are protected by a kind of electrical "armor" that keeps them at a distance from other protons. But at the 15 million degrees Kelvin temperature of young stars' cores, protons move so fast that when they collide with each other, they break through the armor and stick together, rather than acting like rubber balls. Then pop it back up. Four protons combine to form a helium nucleus. Helium is the second most abundant element in the universe. The mass of a helium nucleus is less than the sum of the masses of the four protons from which it is formed.

This mass difference is only seven thousandths of the total mass, but this mass loss is converted into huge energy. The energy released when one kilogram of hydrogen turns into helium is enough to keep a one-hundred-watt light bulb burning for a million years. Stars like the sun have a huge core where 600 million tons of hydrogen are turned into helium every second. The huge nuclear energy slamming toward the outside of the star can prevent the gravitational contraction. The energy released by the center of the star is radiated as photons, but the photons have to travel a long way to reach the surface of the sun and escape into interstellar space. Although the speed of photons is nearly 300,000 kilometers per second and the radius of the sun is 700,000 kilometers, the time it takes for photons emitted from the center of the sun to reach the surface of the sun is not 2.3 seconds. It would take those photons about 10 million years to travel this distance. The sunlight we now receive on Earth left the surface of the sun eight minutes ago, but when it was produced from the core of the sun, apes and long-extinct stylodon elephants were still walking in Africa, and Africa was not yet connected to Eurasia. However, the "constant" evolutionary process will eventually end, and when the raging flames are extinguished, the stars will turn into embers. When all the hydrogen has turned into helium, there is not enough fuel to maintain the fire in the core, and the star's quiet days in the main sequence phase come to an end, and a period of great turbulence arrives. Once the fuel is used up, the rate of thermonuclear reactions immediately decreases sharply, the balance between gravity and radiation pressure is broken, and gravity takes over. A star with a helium core and a hydrogen shell begins to shrink under its own gravity, and the pressure, density, and temperature all increase. Then the unused hydrogen in the outer layer of the star begins to burn, the shell begins to expand, and the core is shrinking. At a high temperature of about 100 million degrees, the helium nuclei in the core of the star fuse into carbon nuclei. Every three helium nuclei fuse into a carbon nucleus, and the carbon nucleus captures another helium nucleus to form an oxygen nucleus. The speed of these new reactions is completely different from that of slow hydrogen fusion. They explode (helium flashes) as fast as lightning, and the star has to adjust its structure accordingly as much as possible. After about a million years, the outflow of nuclear energy gradually stabilized. In the next few hundred million years, the star was temporarily stable, and the helium in the core region was gradually consumed, and the burning of hydrogen pushed further and further to the outer layers. However, there is a price to pay for adjustment. At this time, the star will expand greatly to adapt its structure to the increase in luminosity. Its volume will increase a billion times. During this process, the star's color changes because its outer layers are farther away from the hot core, causing the temperature to drop. Stars in this state are called red giants. Stars in the red giant stage have relatively low surface temperatures but are extremely bright because of their huge size. Many of the brightest stars that can be seen with the naked eye are red giants, such as Betelgeuse, Aldebaran, Arcturus, Antares, etc. Our sun will also become a red "giant" in five or six billion years. When the hydrogen in the core is exhausted, the sun will begin to expand. At that time, Mercury will turn into steam, Venus's atmosphere will be blown away, and Earth's oceans will boil. Then the sun will continue to expand and bring the earth into its sphere of influence. The charred remains of Earth will continue to circle in the giant sun's hot, thin atmosphere. The density of material in the outer layers of a red giant is much lower than the best vacuum available in a laboratory on Earth. After the star expands to become a red giant and the thermonuclear reaction rate irreversibly decays, the star blows out gas and shrinks to the size of the Earth, a few thousand kilometers in diameter. The concentration of matter causes the surface temperature of the star to increase greatly, to the point where it becomes literally white hot. The two characteristics of small size and high surface temperature give this kind of star the name white dwarf. White dwarfs are the end point of the evolution of intermediate-mass stars and can be found throughout the Milky Way. The greater its mass, the smaller its radius. Since there are no thermonuclear reactions to provide energy, the white dwarf emits radiation and cools at the same rate. However, white dwarfs are frugal by nature and require billions of years of cooling after formation. The dimming process of white dwarfs is so slow that probably no black dwarf has been formed since the creation of the universe and the appearance of the first stars 15 billion years ago. This requires great patience. The Sun is at the midpoint of its main sequence phase. It will take another 5 billion years to reach the "advanced age" of a planetary nebula. It will be briefly active for another 100,000 years, and then become a white dwarf and become a white dwarf star in 10 billion years. It died slowly over the course of the year, and finally lived on forever as a black dwarf.