Stars: types of stars and their classification by color and size

Every person knows how the stars look in the sky. Tiny lights shining with cold white light. In ancient times, people could not come up with an explanation for this phenomenon. The stars were considered the eyes of the gods, the souls of the dead ancestors, guardians and defenders, protecting the peace of man in the darkness of the night. Then no one could think that the Sun is also a star.

What is a star

Many centuries have passed before people realized what the stars are. Types of stars, their characteristics, representations of the chemical and physical processes that take place there - this is a new field of knowledge. Ancient astronomers could not even imagine that such a star is actually not a tiny flame, but the unimaginable dimensions of a ball of red-hot gas in which reactions occur Thermonuclear fusion. There is a strange paradox in the fact that the dim starlight is a dazzling radiance of nuclear reaction, and cozy sunny 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 stellar 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 little bit about the history of astronomy

Ancient astronomers, too, could tell unusual and interesting about the stars in the sky. Already the Sumerians singled out individual constellations and the zodiacal circle, they also for the first time calculated the division of the total angle by 360 ^° . They also created the lunar calendar and were able to synchronize it with the solar calendar. The Egyptians believed that the Earth is in the center of the universe, but they knew that Mercury and Venus revolve around the Sun.

In China astronomy as a science was engaged already in the end of III millennium BC. E., And 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, meteorite streams and trajectories of comets were observed.

The ancient Incas knew the differences between stars and planets. There is indirect evidence that they were aware of the Galilean satellites of Jupiter and the visual blurriness of the contours 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 heliocentricity of the system. They tried to calculate the diameter of the Sun, even if it was wrong. But the Greeks were the first who in principle assumed that the Sun is larger than the Earth, before that everything, relying on visual observations, was considered differently. The Greek Hipparchus for the first time created a catalog of luminaries and singled out different kinds of stars. The classification of stars in this scientific work relied on the intensity of the glow. Hipparchus identified 6 classes of brightness, total in the catalog was 850 lights.

What did the ancient astronomers pay attention to

The original classification of stars was based on their brightness. After all, this criterion is the only one available for an astronomer armed only with a telescope. The brightest or possessing unique visible properties of the stars even got their own names, and each people has their own. So, Deneb, Rigel and Algol - the names are Arabic, Sirius - Latin, and Antares - Greek. The polar star in each nation has its own name. This, perhaps, is one of the most important stars in the "practical sense". Its coordinates on the night sky are unchanged, despite the rotation of the earth. If the other stars move across the sky, passing the path from sunrise to sunset, the North Star does not change its location. Therefore, it was used by seamen and travelers as a reliable guide. By the way, contrary to the widespread misconception, this is not the brightest star in the sky. The polar star does not externally appear in any way - neither in size, nor in intensity of luminescence. You can find it only if you know where to look. It is located at the very end of the "bucket handle" of the Little Ursa.

What is the basis of the stellar classification

Modern astronomers, answering the question about what kinds of stars are, are unlikely to mention the brightness of the glow or the location on the night sky. Unless in the order of a historical excursion or in a lecture designed for an audience very far removed from astronomy.

The modern classification of stars is based on their spectral analysis. In this case, the mass, luminosity and radius of the celestial body are usually 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 a criterion such as absolute magnitude. This is the apparent degree of brightness of a celestial body without an atmosphere, conventionally located at a distance of 10 parsecs from the observation point. In addition, the variability of the brightness and the size of the star are taken into account. Types of stars are currently determined by their spectral class and already more detailed - a subclass. Astronomers Russell and Hertzsprung independently analyzed the relationship between luminosity, absolute stellar magnitude, temperature surface and the spectral class of luminaries. They constructed a diagram with the corresponding coordinate axes and found that the result is not chaotic at all. The luminaries on the graph were clearly distinguishable groups. The diagram allows, knowing the spectral class of a star, to determine with at least an approximate accuracy its absolute stellar magnitude.

How the stars are born

This diagram served as a clear proof in favor of the modern theory of the evolution of celestial bodies. The graph clearly shows that the most numerous class are those related to the so-called main sequence of a star. Types of stars belonging to this segment are in the most widespread point of development at the moment in the universe. This is the stage of development of the luminary, in which the energy expended for 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 universally accepted theory of the evolution of stars says that at the initial The stage of development of the star represents a discharged giant gas cloud. Under the influence of its own gravitation it contracts, gradually turning into a sphere. The stronger the compression, the more intense the gravitational energy goes into the thermal one. The gas is heated, and when the temperature reaches 15-20 million K, a thermonuclear reaction is triggered in a newborn star. After this, the process of gravitational compression is suspended.

The main period of life of a star

Initially, in the bowels of the young lunar, the reactions of the hydrogen cycle prevail. This is the longest period of a star's life. The types of stars that are at this stage of development are represented in the most massive main sequence of the diagram described above. From time to time, hydrogen in the core of the star ends, turning into helium. After that, thermonuclear combustion is possible only on the periphery of the nucleus. The star becomes brighter, its outer layers expand considerably, and the temperature decreases. The heavenly body turns into a red giant. This period of life of the star Much shorter than the previous one. Its further fate has been studied little. There are various assumptions, but reliable confirmations have not yet been received. The most common theory is that when helium becomes too much, the star core, not sustaining its own mass, shrinks. The temperature rises until helium does not enter a thermonuclear reaction. Monstrous temperatures lead to the next expansion, and the star turns into a red giant. The future fate of the star, according to scientists, depends on its mass. But the theories concerning this are just a result of computer modeling, not confirmed by observations.

Cooling Stars

Presumably, red giants with a small mass will shrink, turning into dwarfs and gradually cooling down. The stars of the average mass can be transformed into planetary nebulae, while at the center of this formation the core, which has no outer shells, will continue to exist, gradually cooling down and turning into a white dwarf. If the central star emits significant infrared radiation, conditions arise for the activation in the expanding gas shell of the planetary nebula of the space maser.

Massive luminaries, squeezing, can reach such a level of pressure that the electrons literally blend into the atomic nuclei, turning into neutrons. Since between These particles do not have the forces of electrostatic repulsion, the star can contract to a size of several kilometers. At the same time, its density will exceed the water density by 100 million times. Such a star is called neutron 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. Supernovae, in turn, can turn into neutron stars, or, if their mass is large enough, continue to contract to a critical limit and form black holes.

Dimensions

The classification of stars in size can be realized 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, stars of medium size, 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 will fit entirely the orbit of the planet Mars. The size of a star can also be understood as its mass. It is closely related to the diameter of the luminary. The higher the star, the lower its density, and vice versa, the smaller the light, the higher the density. This criterion is not very virulent. Stars that would be more or less than the Sun 10 times, very little. Most of the luminaries fit into the interval from 60 to 0.03 solar masses. The density of the Sun, taken for the starting index, 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 less than solar density.

In the standard classification of stars, the mass distribution scheme looks as follows. Small ones include light with a mass from 0.08 to 0.5 solar. To moderate - from 0.5 to 8 solar masses, and to massive - from 8 or more.

Classification of stars . From blue to white

The classification of stars by color is in fact based not on the visible glow of the body, but on the spectral characteristics. The radiation spectrum of an object is determined by the chemical composition of the star, and its temperature also 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 suggests a division into 7 types.

Thus, stars with the highest temperature, from 30 to 60 thousand K, are classed as O-luminaries. They are of a blue color, the mass of such celestial bodies reaches 60 solar masses (m), and the radius is 15 solar radii (p. R.). The hydrogen and helium lines in their spectrum are rather weak. The luminosity of such celestial objects can reach 1 million 400 thousand solar luminosities (pp.).

Class B stars include stars with a temperature of 10 to 30 thousand K. These are celestial bodies of white and 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. And the hydrogen lines in the spectrum are amplified, reaching average values.

Class A stars have temperatures ranging from 7.5 to 10 thousand K, they are white. The minimum mass of such celestial bodies starts from 3.1 seconds. M, and the radius is 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, there are lines of metals.

Objects of class F are actually yellow-white, but they look white. Their temperature fluctuates between 6 and 7,500 K, the mass varies from 1.7 to 3.1 cm, the radius varies from 1.3 to 2.1 seconds. R. The luminosity of such stars varies from 6 to 80 s. from. The hydrogen lines in the spectrum weaken, the lines of metals, on the contrary, are amplified.

Thus, all kinds of white stars fall within the limits of classes from A to F. Further, according to classification, yellow and orange luminaries follow.

Yellow, orange and red stars

Types of stars in color are distributed from blue to red, as the temperature decreases and the size and luminosity of the object decreases.

Stars of class G, to which the Sun belongs, reach a temperature of 5 to 6 thousand K, they are yellow. The mass of such objects is from 1.1 to 1.7 seconds. M, the radius is from 1.1 to 1.3 s. R. The luminosity is from 1.2 to 6 s. from. The spectral lines of helium and metals are intense, hydrogen lines are becoming weaker.

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 with. M. The brightness varies 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 interval from 0.3 to 0.8 s. M, the radius is from 0.4 to 0.9 s. R. The luminosity is only 0.04-0.4 s. from. These are dying stars. Their only recently opened brown dwarfs are colder . For them a separate class of M-T was allocated.