Giant stars

The results of the definition of stellar diameters turned out to be truly astounding. Astronomers did not suspect before that in Universe can be such giant stars. The first star, the true size of which was determined (in 1920), was the bright star of the constellation Orion, bearing the Arabic name Betelgeuse. Its diameter was greater than the diameter of the orbit of Mars! Another giant star is Antares, the brightest star in the constellation Scorpio: its diameter is about one and a half times larger than the diameter of the earth's orbit. In the series of so far discovered stellar giants, it is necessary to put the so-called Marvelous World, a star in the constellation of Keith, whose diameter is 330 times larger than the diameter of our Sun. Usually giant stars have radii from 10 to 100 solar radii and luminosity from 10 to 1000 luminosities of the Sun. Stars with luminosity greater than those of giants are called supergiants and hypergigants.

Astronomers did not suspect before that in Universe can be such giant stars

The giant stars have an interesting physical structure. The calculation shows that such stars, despite their monstrous dimensions, contain a disproportionately small amount of matter. They are heavier than our Sun a few times; and since by the volume of Betelgeuse, for example, more than 40000000 times the Sun, then the density of this star must be negligible. And if the matter of the Sun on average density approaches water, the substance of the giant stars in this respect is like dilute air. The giant stars, in the words of one astronomer, "resemble a huge balloon of low density, much smaller than the density of air".

The star becomes a giant after all the hydrogen available for reaction in the nucleus of the star was used. A star whose initial mass does not exceed about 0,4 solar masses will not become a giant star. This is because the substance inside such stars is strongly mixed by convection, and therefore hydrogen continues to participate in the reaction until the entire mass of the star is consumed, at this point it becomes a white dwarf consisting mainly of helium. If the star is more massive than this lower limit, then when it consumes all of the hydrogen available in the nucleus for the reaction, the nucleus will begin to contract. Now, hydrogen reacts with helium in the envelope around the helium-rich nucleus and part of the star outside the shell expands and cools. At this point in its evolution, the luminosity of the star remains approximately constant and the temperature of its surface decreases. The star begins to become a red giant. At this point, star temperature, already, as a rule, the red giant, will remain approximately constant, while its luminosity and radius will increase substantially, and the nucleus will continue to contract, increasing its temperature.

If the mass of the star was below about 0,5 solar masses, it is believed that it will never reach the central temperatures necessary for the synthesis of helium. Therefore, it will remain a red giant star with hydrogen synthesis, until it begins to turn into a helium white dwarf.