There are many different stars in the universe. Big and small, hot and cold, charged and uncharged. In this article, we will name the main types of stars, as well as give a detailed description of the Yellow and White dwarfs.

1. yellow dwarf. Yellow dwarf - a type of small star main sequence, having a mass of 0.8 to 1.2 solar masses and a surface temperature of 5000–6000 K. See below for more details about this type of star.
2. red giant. The red giant is big star reddish or orange. The formation of such stars is possible both at the stage of star formation and at the later stages of their existence. The largest of the giants turn into red supergiants. A star called Betelgeuse in the constellation Orion is the most striking example of a red supergiant.
3. white dwarf. A white dwarf is what remains of an ordinary star with a mass not exceeding 1.4 solar masses after it passes through the red giant stage. See below for more details on this type of star.
4. red dwarf. Red dwarfs are the most common stellar-type objects in the universe. Estimates of their abundance range from 70 to 90% of the number of all stars in the galaxy. They are quite different from other stars.
5. brown dwarf. Brown dwarf - substellar objects (with masses in the range of approximately 0.01 to 0.08 solar masses, or, respectively, from 12.57 to 80.35 Jupiter masses and a diameter approximately equal to that of Jupiter), in the depths of which, in contrast from main sequence stars, there is no thermonuclear fusion reaction with the conversion of hydrogen into helium.
6. subbrown dwarfs. Subbrown dwarfs or brown subdwarfs are cold formations that lie below the brown dwarf limit in mass. Their mass is less than about one hundredth of the mass of the Sun or, respectively, 12.57 masses of Jupiter, the lower limit is not defined. They are more commonly considered planets, although the scientific community has not yet come to a final conclusion about what is considered a planet and what is a subbrown dwarf.
7. black dwarf. Black dwarfs are white dwarfs that have cooled down and therefore do not radiate in the visible range. Represents the final stage in the evolution of white dwarfs. The masses of black dwarfs, like the masses of white dwarfs, are limited from above by 1.4 solar masses.
8. double star. A binary star is two gravitationally bound stars revolving around a common center of mass.
9. New star. Stars that suddenly increase in luminosity by a factor of 10,000. The nova is a binary system consisting of white dwarf and a companion star located on the main sequence. In such systems, gas from the star gradually flows into the white dwarf and periodically explodes there, causing a burst of luminosity.
10. Supernova. A supernova is a star that ends its evolution in a catastrophic explosive process. The flare in this case can be several orders of magnitude greater than in the case of a new star. Such a powerful explosion is a consequence of the processes taking place in the star at the last stage of evolution.
11. neutron star. Neutron stars (NS) are stellar formations with masses on the order of 1.5 solar masses and sizes noticeably smaller than white dwarfs, on the order of 10-20 km in diameter. They consist mainly of neutral subatomic particles - neutrons, tightly compressed by gravitational forces. In our Galaxy, according to scientists, there can be from 100 million to 1 billion neutron stars, that is, somewhere around one in a thousand ordinary stars.
12. Pulsars. Pulsars are cosmic sources of electromagnetic radiation coming to Earth in the form of periodic bursts (pulses). According to the dominant astrophysical model, pulsars are spinning neutron stars with magnetic field, which is tilted to the axis of rotation. When the Earth falls into the cone formed by this radiation, it is possible to record a radiation pulse that repeats at intervals equal to the period of revolution of the star. Some neutron stars make up to 600 revolutions per second.
13. cepheid. Cepheids are a class of pulsating variable stars with a fairly accurate period-luminosity relationship, named after the star Delta Cephei. One of the most famous Cepheids is the North Star. The above list of the main types (types) of stars with their brief description, of course, does not exhaust the entire possible variety of stars in the Universe.

yellow dwarf

Being at different stages of their evolutionary development, stars are divided into normal stars, dwarf stars, giant stars. Normal stars are the main sequence stars. One such example is our sun. Sometimes such normal stars are called yellow dwarfs.

Characteristic

Today we will briefly talk about yellow dwarfs, which are also called yellow stars. Yellow dwarfs are, as a rule, stars of average mass, luminosity and surface temperature. They are main-sequence stars, lying roughly in the middle of the Hertzsprung–Russell diagram and following cooler, less massive red dwarfs.

According to the Morgan-Keenan spectral classification, yellow dwarfs correspond mainly to the G luminosity class, but in transitional variations they sometimes correspond to the K class (orange dwarfs) or the F class in the case of yellow-white dwarfs.

The mass of yellow dwarfs is often in the range from 0.8 to 1.2 solar masses. At the same time, the temperature of their surface is for the most part from 5 to 6 thousand degrees Kelvin.

The brightest and most known representative of the yellow dwarfs is our Sun.

In addition to the Sun, among the yellow dwarfs closest to the Earth, it is worth noting:

1. Two components in the Alpha Centauri triple system, among which Alpha Centauri A is similar in luminosity spectrum to the Sun, and Alpha Centauri B is a typical K-class orange dwarf. The distance to both components is just over 4 light years.
2. The orange dwarf is the star Ran, also known as Epsilon Eridani, with a luminosity class of K. Astronomers estimated the distance to Ran at about 10 and a half light years.
3. The binary star 61 Cygni is just over 11 light-years from Earth. Both components of 61 Cygnus are typical K-class orange dwarfs.
4. Sun-like star Tau Ceti, about 12 light-years away from Earth, with a G luminosity spectrum and an interesting planetary system consisting of at least 5 exoplanets.

Education

The evolution of yellow dwarfs is very interesting. The lifespan of a yellow dwarf is approximately 10 billion years.

Like most stars, intense thermonuclear reactions take place in their interiors, in which mainly hydrogen burns out into helium. After the start of reactions involving helium in the core of the star, hydrogen reactions move more and more towards the surface. This becomes the starting point in the transformation of a yellow dwarf into a red giant. The result of such a transformation may be the red giant Aldebaran.

Over time, the surface of the star will gradually cool down, and the outer layers will begin to expand. At the final stages of evolution, the red giant sheds its shell, which forms a planetary nebula, and its core will turn into a white dwarf, which will further shrink and cool.

A similar future awaits our Sun, which is now in the middle stage of its development. After about 4 billion years, it will begin its transformation into a red giant, the photosphere of which, when expanding, can absorb not only the Earth and Mars, but even Jupiter.

The lifetime of a yellow dwarf is on average 10 billion years. After the entire supply of hydrogen burns out, the star increases many times in size and turns into a red giant. most planetary nebulae, and the core collapses into a small, dense white dwarf.

white dwarfs

White dwarfs are stars that have a large mass (of the order of the sun) and a small radius (radius of the Earth), which is less than the Chandrasekhar limit for the selected mass, which are the product of the evolution of red giants. The process of production of thermonuclear energy in them is stopped, which leads to special properties these stars. According to various estimates, their number in our Galaxy ranges from 3 to 10% of the total stellar population.

Discovery history

In 1844, the German astronomer and mathematician Friedrich Bessel, when observing Sirius, discovered a slight deviation of the star from rectilinear motion, and made an assumption that Sirius had an invisible massive satellite star.

His assumption was confirmed already in 1862, when the American astronomer and telescope designer Alvan Graham Clark, while adjusting the largest refractor at that time, discovered a dim star near Sirius, which was later dubbed Sirius B.

The white dwarf Sirius B has a low luminosity, and the gravitational field affects its bright companion quite noticeably, which indicates that this star has an extremely small radius with a significant mass. Thus, for the first time, a type of object called white dwarfs was discovered. The second such object was the star Maanen, located in the constellation Pisces.

How are white dwarfs formed?

After all the hydrogen in an aging star burns out, its core contracts and heats up, which contributes to the expansion of its outer layers. The effective temperature of the star drops, and it turns into a red giant. The rarefied shell of the star, very weakly connected with the core, eventually dissipates in space, flowing to neighboring planets, and a very compact star, called a white dwarf, remains in place of the red giant.

For a long time it remained a mystery why white dwarfs, which have a temperature exceeding the temperature of the Sun, are small compared to the size of the Sun, until it became clear that the density of matter inside them is extremely high (within 10 5 - 10 9 g/cm 3). There is no standard dependence - mass-luminosity - for white dwarfs, which distinguishes them from other stars. A huge amount of matter is “packed” in an extremely small volume, which is why the density of a white dwarf is almost 100 times that of water.

The temperature of white dwarfs remains almost constant, despite the absence of thermonuclear reactions. What explains this? Due to the strong compression, the electron shells of the atoms begin to penetrate each other. This continues until the distance between the nuclei becomes minimal, equal to the radius of the smallest electron shell.

As a result of ionization, electrons begin to move freely relative to the nuclei, and the matter inside the white dwarf acquires physical properties which are characteristic of metals. In such matter, energy is transferred to the surface of the star by electrons, the speed of which increases more and more as it contracts: some of them move at a speed corresponding to a temperature of a million degrees. The temperature on the surface and inside the white dwarf can differ dramatically, which does not lead to a change in the diameter of the star. Here you can make a comparison with a cannonball - cooling down, it does not decrease in volume.

The white dwarf fades extremely slowly: over hundreds of millions of years, the radiation intensity drops by only 1%. But in the end, it will have to disappear, turning into a black dwarf, which may take trillions of years. White dwarfs can be called unique objects Universe. No one has yet succeeded in reproducing the conditions in which they exist in earthly laboratories.

X-ray emission from white dwarfs

The surface temperature of young white dwarfs, isotropic stellar cores after shell ejection, is very high - more than 2 10 5 K, however, it drops quite quickly due to radiation from the surface. Such very young white dwarfs are observed in the X-ray range (for example, observations of the white dwarf HZ 43 by the ROSAT satellite). In the X-ray range, the luminosity of white dwarfs exceeds the luminosity of main-sequence stars: the images of Sirius taken by the Chandra X-ray telescope can serve as an illustration - on them, the white dwarf Sirius B looks brighter than Sirius A of spectral class A1, which in the optical range is ~ 10,000 times brighter than Sirius B.

The surface temperature of the hottest white dwarfs is 7 10 4 K, the coldest is less than 4 10 3 K.

A feature of the radiation of white dwarfs in the X-ray range is the fact that the main source x-ray radiation for them is the photosphere, which sharply distinguishes them from "normal" stars: in the latter, the corona, heated to several million kelvins, emits X-rays, and the temperature of the photosphere is too low to emit X-rays.

In the absence of accretion, the source of luminosity of white dwarfs is the supply of thermal energy of ions in their interiors; therefore, their luminosity depends on age. The quantitative theory of the cooling of white dwarfs was built in the late 1940s by Professor Samuil Kaplan.

With the exception of the Moon and all the planets, any object that seems to be stationary in the sky is a star - a thermonuclear source of energy, and the types of stars vary from dwarfs to supergiants.

Ours is a star, but it seems so bright and big because of its proximity to us. Most stars look like luminous dots even in powerful telescopes, and yet we know something about them. So we know that they come in different sizes and at least half of them are made up of two or more stars bound together by the force of gravity.

What is a star?

Stars are huge gas balls of hydrogen and helium with traces of other chemical elements. Gravity pulls the matter inward, and the pressure of the hot gas pushes it outward, establishing equilibrium. The energy source of a star is located in its core, where every second millions of tons of hydrogen merge to form helium. And although in the depths of the Sun this process has been going on continuously for almost 5 billion years, only a very small part of all hydrogen reserves has been used up.

Star types

Main sequence stars. At the beginning of the XX century. The Dutchman Einar Hertzsprung and Henry Norris Ressell from the United States built the Hertzsprung-Ressell (GR) diagram, along the axes of which the luminosity of a star is plotted depending on the temperature on its surface, which makes it possible to determine the distance to the stars.

Most stars, including the Sun, fall into a band that crosses the GR diagram diagonally and is called the main sequence. These stars are often referred to as dwarfs, although some of them are 20 times the size of the Sun and shine 20,000 times brighter.

red dwarfs

At the cold, dim end of the main sequence are red dwarfs, the most common type of star. Being smaller than the Sun, they frugally spend their fuel reserves in order to extend the time of their own existence by tens of billions of years. If one could see all the red dwarfs, the sky would be literally littered with them. However, red dwarfs shine so faintly that we can only observe those closest to us, such as Proxima Centauri.

white dwarfs

Even smaller than red dwarfs are white dwarfs. Usually their diameter is approximately equal to the Earth, but the mass can be equal to the mass of the Sun. The volume of matter of a white dwarf, equal to the volume of this book, would have a mass of about 10 thousand tons! Their position on the GR diagram shows that they are very different from red dwarfs. Their nuclear source has depleted.

red giants

After main sequence stars, red giants are the most common. They have about the same surface temperature as red dwarfs, but they are much brighter and larger, so they are located above the main sequence in the GR diagram. The mass of these giants is usually approximately equal to the sun, however, if one of them took the place of our luminary, the inner planets solar system would be in his atmosphere.

supergiants

Rare supergiants are located in the upper part of the GR diagram. Betelgeuse in Orion's arm is almost 1 billion km across. Another bright object Oriona - Rigel, one of the brightest stars that is visible to the naked eye. It is almost ten times smaller than Betelgeuse and at the same time almost 100 times the size of the Sun.

Dwarf Stars

Dwarf Stars, the type of star most common in our Galaxy - 90% of the stars belong to it, including the Sun. They are also called main sequence stars, according to their position on the HERZSPRUNG-RUSSELL DIAGRAM. The name "dwarf" refers not so much to the size of the stars as to their LUMINOSITY, so this term is not a connotation of diminutiveness.

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