Red giant, as well as supergiant is the name of space objects with extended shells and high luminosity. They belong to the late spectral classes K and M. Their radii are hundreds of times larger than the solar one. The maximum radiation of these stars falls on the infrared and red regions of the spectrum. In the Hertzsprung-Russell diagram, red giants are located above the main sequence line, their absolute magnitude varies slightly above zero or has a negative value.
The area of such a star exceeds the area of the Sun by at least 1500 times, while its diameter is approximately 40 times larger. Since the difference in absolute value with our luminary is about five, it turns out that the red giant emits a hundred times more light. But at the same time it is much colder. The solar temperature is twice that of a red giant, and therefore, per unit surface area, the luminary of our system emits sixteen times more light.
The apparent color of a star directly depends on the surface temperature. Our sun is burning whiteand has a relatively small size, so it is called a yellow dwarf. Cooler stars have orange and red light. Each star in the course of its evolution can reach the last spectral classes and become a red giant in two stages of development. This happens in the process of nucleation at the stage of star formation or at the final stage of evolution. At this time, the red giant begins to radiate energy due to its own gravitational energy, which is released during its compression.
As a star shrinks, its temperature rises. At the same time, due to the reduction in the size of the surface, the luminosity of the star decreases several times. She fades. If this is a “young” red giant, then eventually a thermonuclear fusion reaction from helium hydrogen will start in its depths. After that, the young star will enter the main sequence. Old stars have a different fate. At the later stages of evolution, hydrogen in the bowels of the star burns out completely. Then the star leaves the main sequence. According to the Hertzsprung-Russell diagram, it moves into the region of supergiants and red giants. But before moving on to this stage, it goes through an intermediate stage - a subgiant.
Subgiants are stars in the core of which hydrogen thermonuclear reactions have already stopped, but helium burning has not yet begun. This happens because the core has not warmed up enough. An example of such a subgiant would be Arthur, located in the constellation Bootes. He is an orange z
everywhere with an apparent magnitude of -0.1. It is about 36 - 38 light years away from the Sun. It can be observed in the Northern Hemisphere in May, if you look directly to the south. Arthur is 40 times the diameter of the Sun.
Yellow dwarf The Sun is a relatively young star. Its age is estimated at 4.57 billion years. It will remain on the main sequence for about 5 billion years. But scientists have managed to simulate a world in which the Sun is a red giant. Its dimensions will grow 200 times and reach the Earth's orbit, incinerating Mercury and Venus. Of course, life will be impossible by that time. At this stage, the Sun will last approximately another 100 million years, after which it will turn into a planetary nebula and become a white dwarf.
