Everyone knows what stars look like in the sky. Tiny lights shining with a cold white light . In ancient times, people could not come up with explanations for this phenomenon. Stars were considered the eyes of the gods, the souls of dead ancestors, guardians and protectors, protecting the human peace in the darkness of the night. Then no one could have thought that the Sun is also a star.
What is a star?
Many centuries passed before people understood what stars were. The types of stars, their characteristics, ideas about the chemical and physical processes taking place there are a new field of knowledge. Ancient astronomers could not even imagine that such a luminary is actually not a tiny light at all, but an unimaginably large ball of hot gas in which reactions take place
thermonuclear fusion. There is a strange paradox in the fact that dim starlight is the dazzling radiance of a nuclear reaction, and cozy solar heat is the monstrous heat of millions of Kelvin.
All the stars that can be seen in the sky with the naked eye are in the Milky Way galaxy. The sun is also part of this star system, and it is located on its outskirts. It is impossible to imagine what the night sky would look like if the Sun were in the center of the Milky Way. After all, the number of stars in this galaxy is more than 200 billion.
A bit about the history of astronomy
Ancient astronomers could also tell unusual and interesting about the stars in the sky. Already the Sumerians singled out individual constellations and the zodiac circle, they also first calculated the division of the full angle by 360 0 . They created the lunar calendar and were able to synchronize it with the solar. The Egyptians believed that the Earth was in the center of the universe, but they knew that Mercury and Venus revolve around the Sun.
In China, astronomy as a science was dealt with already at the end of the 3rd millennium BC. e., a
The first observatories appeared in the XII century. BC e. They studied lunar and solar eclipses, while being able to understand their cause and even calculating the forecast dates, they observed meteor showers and comet trajectories.
The ancient Incas knew the differences between stars and planets. There is indirect evidence that they knew the Galilean satellites of Jupiter and the visual blurring of the outlines of the disk of Venus due to the presence of the atmosphere on the planet.
The ancient Greeks were able to prove the sphericity of the Earth, put forward the assumption of the heliocentricity of the system. They tried to calculate the diameter of the Sun, albeit erroneously. But the Greeks were the first who, in principle, suggested that the Sun is larger than the Earth, before that, everyone, relying on visual observations, thought differently. Greek Hipparchus first created a catalog of luminaries and highlighted different types of stars. The classification of stars in this scientific work was based on the intensity of the glow. Hipparchus identified 6 brightness classes; in total there were 850 luminaries in the catalog.
What did the ancient astronomers pay attention to
The initial classification of stars was based on their brightness. After all, this criterion is the only available for an astronomer armed only with a telescope. The brightest or possessing unique visible properties stars even received their own names, and each nation has its own. So, Deneb, Rigel and Algol are Arabic names, Sirius is Latin, and Antares is Greek. The North Star in each nation has its own name. This is perhaps one of the most important in the "practical sense" of the stars. Its coordinates in the night sky are unchanged, despite the rotation of the earth. If other stars move across the sky, going from sunrise to sunset, the North Star does not change its location. Therefore, it was used by sailors and travelers as a reliable guide. By the way, contrary to popular belief, this is not the brightest star in the sky. A polar star does not externally stand out - neither in size nor in intensity of the glow. You can find it only if you know where to look. It is located at the very end of the “handle of the bucket” of Ursa Minor.
What is the basis of stellar classification
Modern astronomers, answering the question of what types of stars are, are unlikely to mention the brightness of the glow or the location in the night sky. Unless in the order of a historical excursion or in a lecture designed for an audience very far from astronomy.
The current classification of stars is based on their spectral analysis. In this case, the mass, luminosity and radius of the celestial body are usually also indicated. All these indicators are given in relation to the Sun, that is, its characteristics are taken as units of measurement.
The classification of stars is based on such a criterion as absolute magnitude. This is the apparent brightness of a celestial body without an atmosphere, conventionally located at a distance of 10 parsecs from the observation point.
In addition, the brightness variations and the size of the star are taken into account. The types of stars are currently determined by their spectral class and, in more detail, by a subclass. Astronomers Russell and Hertzsprung independently analyzed the relationship between luminosity, absolute
magnitude, temperature surface and the spectral class of luminaries. They built a diagram with the corresponding coordinate axes and found that the result was not at all chaotic. The luminaries on the chart were clearly distinguishable groups. The diagram allows, knowing the spectral class of a star, to determine at least with approximate accuracy its absolute magnitude.
How stars are born
This diagram served as a clear evidence in favor of the modern theory of evolution of these celestial bodies. The graph clearly shows that the most numerous class are related to the so-called main sequence of stars. The types of stars belonging to this segment are at the most common point in the development at the moment in the Universe. This is the stage of development of the luminary, in which the energy spent on radiation is compensated for by the thermonuclear reaction. The length of stay at this stage of development is determined by the mass of the celestial body and the percentage of elements heavier than helium.
The currently accepted theory of star evolution says that at the initial
the stage of development of the luminary is a discharged giant gas cloud. Under the influence of its own gravity, it contracts, gradually turning into a ball. The stronger the compression, the more intensively the gravitational energy goes into heat. The gas is heated, and when the temperature reaches 15-20 million K, a thermonuclear reaction is launched in the newborn star. After this, the process of gravitational compression is suspended.
The main period of a star’s life
Initially, the reactions of the hydrogen cycle predominate in the bowels of the young luminary. This is the longest period of a star’s life. The types of stars at this stage of development are presented in the most massive main sequence of the diagram described above. Over time, hydrogen in the core of the star ends, turning into helium. After this, thermonuclear combustion is possible only at the periphery of the nucleus. The star becomes brighter, its outer layers expand significantly, and the temperature drops. The heavenly body turns into a red giant. This period of the star’s life
much shorter than the previous one. Her further fate has been little studied. There are various assumptions, but no reliable evidence has yet been received. The most common theory is that when helium becomes too much, the stellar core, unable to withstand its own mass, contracts. The temperature rises until helium already enters a thermonuclear reaction. Monstrous temperatures lead to another expansion, and the star turns into a red giant. The further fate of the star, according to scientists, depends on its mass. But theories regarding this are just the result of computer modeling, not confirmed by observations.
Cooling stars
Presumably, the red giants with a small mass will shrink, turning into dwarfs and gradually cooling. Medium-sized stars can transform into planetary nebulae, while in the center of such a formation, the core without outer covers will continue to exist, gradually cooling and turning into a white dwarf. If the central star emitted significant infrared radiation, conditions arise for activation of the cosmic maser in the expanding gas shell of the planetary nebula.
Massive luminaries, contracting, can reach such a pressure level that electrons literally crush into atomic nuclei, turning into neutrons. Since between
these particles have no electrostatic repulsive forces; a star can shrink to a size of several kilometers. At the same time, its density will exceed the density of water by 100 million times. Such a star is called a neutron star and is, in fact, a huge atomic nucleus.
Supermassive stars continue to exist, sequentially synthesizing in the process of thermonuclear reactions from helium - carbon, then oxygen, from it - silicon and, finally, iron. At this stage of the thermonuclear reaction, a supernova explosion occurs. Supernova stars, in turn, can turn into neutron stars or, if their mass is large enough, continue compression to a critical limit and form black holes.
Dimensions
The classification of stars by size can be implemented in two ways. The physical size of a star can be determined by its radius. The unit of measurement in this case is the radius of the sun. There are dwarfs, medium-sized stars, giants and supergiants. By the way, the sun itself is just a dwarf. The radius of neutron stars can reach only a few kilometers. And in the supergiant the whole orbit of the planet Mars will fit. The size of a star can also be understood as its mass. It is closely related to the diameter of the star. The larger the star, the lower its density, and vice versa, the smaller the star, the higher the density. This criterion does not virivate so much. There are very few stars that would be 10 or more times smaller than the Sun. Most of the luminaries fit in the range from 60 to 0.03 solar masses. The density of the Sun, taken as the starting indicator, is 1.43 g / cm 3 . The density of white dwarfs reaches 10 12 g / cm 3 and the density of rarefied supergiants can be millions of times lower than the sun.
In the standard classification of stars, the mass distribution scheme is as follows. To small include luminaries with a mass of from 0.08 to 0.5 solar. To moderate - from 0.5 to 8 solar masses, and to massive - from 8 or more.
The classification of stars . Blue to white
The classification of stars by color is actually based not on the visible glow of the body, but on the spectral characteristics. The emission spectrum of an object is determined by the chemical composition of the star, and its temperature depends on it.
The most common is the Harvard classification, created in the early 20th century. According to the then accepted standards, the classification of stars by color involves dividing into 7 types.
So, the stars with the highest temperature, from 30 to 60 thousand K, are class O stars. They are blue in color, the mass of such celestial bodies reaches 60 solar masses (s.m.), and the radius is 15 solar radii (p. R.). The lines of hydrogen and helium in their spectrum are rather weak. The luminosity of such celestial objects can reach 1 million 400 thousand solar luminosities (s.p.).
The class B stars include luminaries with a temperature of 10 to 30 thousand K. These are celestial bodies of white-blue color, their mass starts from 18 s. m., and the radius is from 7 s. m. The lowest luminosity of objects of this class is 20 thousand s. C., and the lines of hydrogen in the spectrum are amplified, reaching average values.
In class A stars, the temperature ranges from 7.5 to 10 thousand K, they are white. The minimum mass of such celestial bodies starts at 3.1 s. m., and the radius - from 2.1 s. R. The luminosity of objects is in the range from 80 to 20 thousand s. from. The hydrogen lines in the spectrum of these stars are strong, metal lines appear.
Class F objects are actually yellow and white, but look white. Their temperature ranges from 6 to 7.5 thousand K, the mass varies from 1.7 to 3.1 cm, the radius is from 1.3 to 2.1 s. R. The luminosity of such stars varies from 6 to 80 s. from. The hydrogen lines in the spectrum weaken, the metal lines, on the contrary, are amplified.
Thus, all types of white stars fall within the classes from A to F. Then, according to the classification, yellow and orange luminaries follow.
Yellow, orange and red stars
The types of stars are distributed in color from blue to red, as the temperature decreases and the size and luminosity of the object decrease.
Class G stars, which include the Sun, reach temperatures from 5 to 6 thousand K, they are yellow. The mass of such objects is from 1.1 to 1.7 s. m., radius - from 1.1 to 1.3 s. R. Luminosity - from 1.2 to 6 s. from. The spectral lines of helium and metals are intense, the lines of hydrogen are becoming weaker.
The luminaries belonging to class K have a temperature of 3.5 to 5 thousand K. They look yellow-orange, but the true color of these stars is orange. The radius of these objects is in the range from 0.9 to 1.1 s. p., mass - from 0.8 to 1.1 s. m. Brightness ranges from 0.4 to 1.2 s. from. The hydrogen lines are almost invisible, the metal lines are very strong.
The coldest and smallest stars are of class M. Their temperature is only 2.5 - 3.5 thousand K and they seem red, although in reality these objects are orange-red. The mass of stars is in the range from 0.3 to 0.8 s. m., radius - from 0.4 to 0.9 s. R. Luminosity - only 0.04 - 0.4 s. from. These are dying stars. Only recently discovered brown dwarfs are colder than them. A separate MT class was allocated for them.