The reasons for the formation of red giants, what and how red giants are created

When a star passes its long prime and enters old age, it will first become a red giant. Red giants are a shorter period of instability that a star undergoes from burning to the later stages, lasting only millions of years, which is very short compared to the billions or even tens of billions of years of stabilization of stars.

When a star enters old age, it will first become a red giant, and the reason why it is called a red giant star is first of all the color of its appearance is red, and it is very largeKnowledge of horoscopesLet's tell you more about what a red superstar is? How did it come about? Red Giant is what and how it comes aboutAs the name suggests, red giants are very large stars that are red in appearance (glowing red).

Red giants are a type of luminous giant that evolves into a large, small-mass luminous giant at the end of a star's life. Red giants usually have a mass of about one sun, but due to the expansion of the outer atmosphere, red giants are often tens or hundreds of times larger than the sun. The surface temperature of a red giant is about 5000 K (K is the temperature at 4700 degrees Celsius) or less.

When the star depletes the hydrogen fuel in the inner core, the thermonuclear reaction no longer takes place, then the inner core of the star will begin to shrink inward due to gravity, and the combustion will be transferred to the outer hydrogen layer. At this time, the outer layer of the star will continue to expand, the heat generated by combustion will spread into space, and the temperature of the surface will begin to decrease. At the same time, the visible light output of the star will gradually shift to red, and the life stage of the red giant star will begin.

When a star is in the main-sequence phase, a nuclear fusion reaction will take place inside, consuming its own hydrogen fuel, and the outward expansion force generated by the reaction will balance with the gravitational force that collapses inward, so that the star remains stable. The thermonuclear fusion reaction that takes place inside the Sun, the closest star to our Earth, turns about 600 million tons of hydrogen into helium every second. When all the hydrogen in the star becomes helium, the nuclear fusion reaction rate inside the star decreases sharply, and the inner core of the star will collapse inward due to gravity, and then form a white dwarf, neutron star or collapse into a black hole.

The slag leaks and the outside of the star continues to burn. The burning on the outside of a star and the thermonuclear fusion reaction on the inside can be very different, especially the degree of reaction, which can be quite violent. As a result, stars have to adjust their star structure accordingly to accommodate this reaction.

After about 1 million years, the star's inner core energy outflow gradually stabilizes, and in the following hundreds of millions of years, the star will temporarily enter a stable state, the helium in the inner core will continue to be consumed, the hydrogen will continue to burn outward, and the outer gas shell of the star will expand larger and larger to adapt to its structure. At this time, the size of the star can increase by up to 1 billion times, and the bright color will gradually change to red in the process. The red giant does not have enough mass in its interior, and when the carbon and oxygen elements produced are further fused, these two elements form a core inside the star, which is a white dwarf.

At this time, the expanding gas shell of the red giant star will gradually break away from the gravitational constraints of the inner core and spread into the universe, forming a nebula - of course, if the mass of the star's inner core is large enough for carbon and oxygen to further fuse into iron, then the inner core of the star will also become a neutron star, or even a black hole.

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. The process of star evolution after this is:

Inner Shell Contraction, Shell Expansion – The helium nuclei inside the burning shell shrink inward and become hot, while the outer shell of a star expands outward and keeps getting colder, and the surface temperature decreases considerably. This process lasted only a few hundred thousand years, and the star became a red giant in rapid expansion: when all the hydrogen was depleted.

There is no longer excess hydrogen left. and when the core temperature reaches 100 million degrees Celsius. Induces helium fusion--- from the fusion of three helium atoms into one carbon atom.

and emitted more powerful heat, and the red giant began to shrink. Become a white dwarf. Extremely high density.

At this time, the internal environment is stable. White dwarfs can stay at this stage for 10 billion to 20 billion years. Finally, it becomes a black dwarf that does not emit light and emits heat.

The stars are game over

Stars between 7 times the mass of the Sun, after depleting the hydrogen fuel in the core, will burn to the hydrogen layer on the periphery of the core. Because the inert helium nucleus has no energy of its own, it shrinks and is heated by gravity, and the hydrogen on it shrinks with it, so the speed of fusion increases, producing more energy, causing the star to become brighter (1,000 to 10,000 times brighter) and expand in volume. The degree of volume expansion exceeds the increase in luminescence capacity, so the effective temperature of the surface drops.

The drop in surface temperature causes the color of the star to tend to red, hence the name red giant. Theoretically, main-sequence stars with a to K spectra would evolve into red giants and red supergiants, while stars of type O and B would evolve into blue supergiants (in many ways different from red giants).

Helium fusion kicks in when the core of the star continues to shrink enough to be dense and temperature enough to ignite the 3-helium process.

For stars with masses less than a double the Sun, the helium core needs to contract continuously to combat the helium build-up in the growing core, and the only way to fight gravity is the electron degeneracy pressure. Therefore, when the temperature rises to an ignition temperature of 100 million degrees, it is already a degenerate dense core similar to a "white dwarf". In about 1 minute, most of the helium nuclei fuse into carbon nuclei (and the oxygen nuclei that are extinct later), and a huge amount of energy is transmitted to the outer layers of the star, causing the star to brighten suddenly for a short period.

Then, the core no longer produces energy, and the outer layer of hydrogen continues to fuse into helium in a more complex manner at a shallower position. The stellar core slowly accumulates helium again, and after a long period of time, a similar helium flash occurs again in the helium envelope outside the carbon-oxygen rich core. The star is located on the asymptotic giant branch on the Herrault diagram, and after each helium flash, it moves from one red giant branch to another.

For stars with a mass of the Sun, because the fusion of hydrogen nuclei is faster and the core is hotter, helium fusion can ignite before the core shrinks to the degenerate state of the density of the white dwarf, and the entire nuclear reaction will proceed relatively smoothly and continuously. When such stars initially have a low initial content of heavy elements ("metal-lean" stars), they will enter a horizontal branch – the position of these stars on the Herrault diagram is a horizontal distribution. At this stage, metal-rich stars cluster into red clusters on the Hérault diagram.