Before talking about the stars, a few words about the structure of our universe and the distances in it.
Light flying at a speed of 300,000 kilometers per second crosses the distance to the nearest star in about four years. And the other stars are even further away. Thus, all these giant distances are measured in light-years. They often ask how many kilometers it is? For lovers of accuracy and imagery, I'll answer - a light year, this is the distance covered by light for the year, ie, about 9,461 billion kilometers or 9.5 trillion kilometers.
In short and simplistic terms, all the stars we see in the sky separately are the stars of our galaxy. The Milky Way, a whitish stripe that crosses all the sky and is now visible to the eye, unfortunately, only in ideal conditions away from the cities, when viewed through a telescope breaks up into separate stars - it is thousands and thousands of stars - the edge of our galaxy, visible from the Earth. Because of this, our galaxy is sometimes called the Milky Way galaxy. The diameter of the galaxy is about 100 thousand light-years. From us to the center of the galaxy about 30 thousand light-years - we see our galaxy as if "from within". Other galaxies are farther away from us - the distance to them is millions of light-years and they are visible in telescopes with foggy spots, the star structure can be distinguished only in the brightest and closest to us of them in the large telescopes.
Stars are huge gas balloons, in the depths of which nuclear reactions are carried out with the release of huge amounts of energy. It is thanks to this that the light of stars is visible for many thousands of light years. Stars in gas and dust clouds are formed from the clots of matter due to gravity compression. The compression takes place until a nuclear reaction begins in the depths of these clots - this is how the star is born. Nebulae consist of hydrogen and, to a lesser extent, helium. These two basic elements are the initial composition of our universe. Accordingly, the stars will have the same composition. The energy in the star is formed by the conversion of hydrogen into helium - a thermonuclear reaction.
All-stars have different sizes and different masses. The latter is the main characteristic of the star - the temperature of the star and the lifetime of the star depend on it.
The stars with the lowest mass are the coldest and longest-lived - nuclear reactions are so slow that they can last millions of years - a hundred times longer than our Sun. For example, red dwarfs weigh in at least a tenth of the solar mass and have an average surface temperature of about 3500 degrees Celsius. A typical example of a red dwarf is the star Barnard, one of our closest neighbors, just 6 light-years away.
Stars like our Sun have a mass equal to one solar and a temperature of about 6000 degrees. The life expectancy of such stars is about 10 billion years. Our Sun is now at the middle stage of life (about 5 billion years).
Stars of twice the mass of our Sun have already lived for about 1 billion years, the example of such a star is the brightest star of our sky, Sirius, the alpha of the Big Dog. The surface temperature of Sirius is about 11000 degrees Celsius.
Spica, the alpha of the Virgin has a mass of 11 solar, and the temperature is about 24000 degrees. Spica's lifetime is "only" 10 million years.
Stars are classified by the spectrum of radiation (or by its color), which corresponds to different temperatures.
The following spectral classification of stars is accepted:
Main spectral classes O, B, A, F, G, K, M. The hottest stars of classes O and B. This is again Spica, Beta Centauri, Alpha and Beta South Cross, Orion Delta. These are stars of blue and bluish-white color.
Next, come the white class A stars. This is the same Sirius, for example. Even colder Class F stars are yellowish-white. An example is Procyon (the alpha of the Little Dog). The G-class stars are yellow. Examples include our Sun, Toliman (Alpha Centauri) and the Tau of China. Class K stars have an orange hue - Epsilon Eridan. And finally, the coldest of them all, the M class stars, are red. Examples are Antares (Alpha Scorpio) and Betelgeuse (Alpha Orion).
Also, each class is divided into 10 subclasses from 0 to 9. For example, the full recording of the spectral class of the Sun is G2.
At the beginning of the twentieth century, a spectrum-luminosity diagram was drawn up - a Herzsprung - Rassel diagram. It clearly shows all the stages of the life of the stars.
At different stages of their lives, a star undergoes different stages of evolution depending on its initial mass. For example, in about four billion years our Sun will be swollen into a red giant, and in the final stage of evolution will turn into a white dwarf. But the stages of the evolution of the stars are already more serious conversation. Our task was to get acquainted with the same stars...