The red giant, as well as the 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 greater than the solar radius. 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 line of the main sequence, their absolute magnitude varies within a little above zero or has a negative value.
The area of such a star exceeds the area of the Sun by at least 1, 500 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. Solar temperature is twice the performance of the red giant, and therefore per unit surface area the luminosity of our system emits sixteen times more light.
The visible color of the star depends on the surface temperature. Our Sun is white-hot and relatively small, so it is called the yellow dwarf. Cooler stars have orange and red light. Each star in the process of its evolution can reach the last spectral classes and become a red giant at two stages of development. This occurs 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 when it is compressed.
As the star contracts, its temperature rises. Moreover, due to the reduction in surface size, the luminosity of the star decreases significantly. It is fading. If it is a “young” red giant, then ultimately in its depths the reaction of thermonuclear fusion of helium hydrogen from hydrogen will start. After which the young star will come to the main sequence. Old stars have a different fate. At the later stages of evolution, hydrogen in the bowels of the sun burns out completely. After which the star comes off the main sequence. According to the Hertzsprung-Russell diagram, she moves to the region of supergiants and red giants. But before moving on to this stage, it goes through an intermediate stage - a subgiant.
Stars are called subgiants, in the core of which hydrogen thermonuclear reactions have already ceased, but the burning of helium has not yet begun. This happens because the core has not warmed up enough. An example of such a subgiant is Arthur, located in the constellation Bootes. He is an orange s
driving with an apparent magnitude of -0.1. It is located at a distance of about 36 to 38 light years from the Sun. You can observe it in the Northern Hemisphere in May, if you look directly to the south. Arthur’s diameter is 40 times that of the sun.
The yellow dwarf Sun is a relatively young star. Her age is estimated at 4.57 billion years. In the main sequence, it will remain for approximately another 5 billion years. But scientists 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 by this time will be already impossible. At this stage, the Sun will exist for approximately another 100 million years, after which it will turn into a planetary nebula and become a white dwarf.
