Why is the temperature of the red giant low, but the luminosity is large?

It's not because the area is large, to put it simply.

During the red giant phase, the surface temperature decreases and the luminosity increases dramatically, because the expansion of their outer layers consumes less energy and produces more energy.

Let's go into more detail.

When the hydrogen in the central region of the star is depleted to form a nuclear sphere of helium, the thermonuclear reaction of hydrogen fusion cannot continue in the central region. At this time, the gravitational weight is not balanced by the radiation pressure, and the central region of the star will be compressed, and the temperature will rise sharply. When the temperature of the central helium nucleus rises, the layer of hydrogen-helium mixture immediately attached to it is heated to the temperature at which hydrogen fusion is initiated, and the thermonuclear reaction resumes.

As the helium sphere grows, the hydrogen combustion layer expands outward, causing the outer layer of the star to expand and transform into a red giant or red supergiant. During the transformation, the hydrogen combustion layer may produce more energy than during the main-sequence period, but the surface temperature of the star decreases instead of rising. Here's why:

The cohesive gravitational pull on the expansion of the outer layer decreases, and even when the temperature decreases, the expansion pressure can still resist or exceed the gravitational force, and the radius and surface area of the star increase more than the increase in the production rate, so that the total luminosity may increase, but the surface temperature will decrease. When a large star with a mass higher than 4 times the mass of the Sun re-initiates hydrogen fusion outside the helium nucleus, the energy released from the nucleus does not increase significantly, but the radius increases by many times, so the surface temperature drops from tens of thousands of cents.

Three or four thousand Kelvin, become a red superstar. Small and medium-sized stars with masses less than 4 times the mass of the Sun enter the red giant phase when their surface temperature decreases, but their luminosity increases dramatically, because their outer expansion consumes less energy and produces more energy.

Luminosity is not only related to temperature, low temperature and large luminosity indicate that energy is converted into more light energy.

Red giants are large in size, so the luminous area is large, the total brightness is large, but the luminosity per unit area is weak.

Red supergiants are more than 25 times larger than Earth, and their internal temperatures are more than a billion degrees Celsius, which is unimaginable.

Red giants and blue giants are both liquid numbers that evolve after the star runs out of hydrogen, and some become red giants and some become blue giants, mainly depending on the mass of the starKnowledge of horoscopesWhat is the difference between a red giant star and a blue giant star? Who is hotter? What is the difference between a red giant and a blue giant1. When a star passes its long period of young adulthood, the main sequence stage, and enters old age, it will first become a red giant.

2. Blue giants are high-quality main-sequence stars, and their internal nuclear reaction rates are very large, and they are oversized stars. 3. Red giants are orange-red in color, have a low temperature (k m type), and are usually old stars; Blue giants, on the other hand, have extremely high temperatures and are a model for young stars. Who has a higher temperature between red and blue giantsIn astronomy, there are red giants and blue giants, the former is dark red, cooler, and usually belongs to older stars; The latter is extremely hot and is a model of a young star.

Blue GiantBlue giants are high-quality main-sequence stars with large rates of nuclear reactions inside them, making them oversized stars. Red GiantWhen a star passes its long prime of youth, main-sequence star, and enters old age, it will first become a red giant. Call it a giant star because it stands out for its huge size.

During the giant phase, the size of the star will expand to 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 bit, the red giant star is so large that its luminosity has become very large and extremely bright.

Many of the brightest stars seen to the naked eye are red giants. Once the red giant 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.

Summary. Blue giants are high-temperature, luminosity type II or III stars with a large rate of nuclear reactions inside them, making them overmassive stars. This is reflected in the Herault diagram, which is the position on the upper right and left of the main sequence zone, and they have all left the main sequence zone.

Their duration is relatively short, only tens of millions of years, but the reasons are not exactly the same. In the usual sense, a blue giant star is a massive star that has just left its main sequence, and its internal nuclear reaction rate is very large, and its evolution is very fast. Blue giants usually have a spectral type earlier than A0 and a surface temperature above 10,000 K.

Due to their higher mass and large radius, blue giants generally have higher brightness, usually more than 500 times brighter than the Sun. However, there is also a class of Lyra RR variable stars, although they are also classified as blue giants, but they are actually the terminal evolution stage of low- and medium-mass stars, with much lower temperature and luminosity than normal blue giants, and the mass is only twice that of the Sun, so they should not be confused with ordinary blue giants.

Blue giants are high-temperature, luminosity type II or III stars with a large rate of nuclear reactions inside them, making them overmassive stars. This is reflected in the Herault diagram, which is the position on the upper right and left of the main sequence zone, and they have all left the main sequence zone. Their duration is relatively short, only tens of millions of years, but the reasons are not exactly the same.

In the usual sense, a blue giant star is a massive star that has just left its main sequence, and its internal nuclear reaction rate is very large, and its evolution is very fast. Blue giants usually have a spectral type earlier than a0 and a surface temperature above 10,000 K. Due to their higher mass and larger radius, blue giants generally have a higher brightness, usually more than 500 times brighter than too coarse skin.

However, there is also a class of Lyrid RR variable stars, although they are also classified as blue giants, but they are actually the terminal evolution stage of low- and medium-mass stars, with much lower temperature and luminosity than normal blue giants, and the mass is only twice that of the Sun, so they should not be confused with ordinary blue giants.

The surface temperature of a blue giant star is about 9800

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If the same substance is burned under the same conditions, the temperature of the blue flame is higher than that of the red flame.

As the temperature of the red giant decreases, the light emitted is red because the outer surface of the star is getting farther and farther away from the center as the star expands rapidly, so the temperature will decrease accordingly, and the light emitted will become more and more reddish.

Stars glow red when they cool down, and we can say the same. The process of stellar evolution is that the inner core shrinks and the outer shell expands – the helium nuclei inside the burning shell shrink inward and become hot, while the stellar shell expands outward (due to gravity) and keeps cooling, and the surface temperature decreases considerably.