Red stars are the names of the surrounding world 3. Types of stars in the observable universe. Temperature and mass of stars
Stars are very different: small and large, bright and not very bright, old and young, hot and cold, white, blue, yellow, red, etc.
The Hertzsprung-Russell diagram allows you to understand the classification of stars.
It shows the relationship between absolute magnitude, luminosity, spectral type, and surface temperature of a star. The stars in this diagram are not arranged randomly, but form well-defined areas.
Most of the stars are located on the so-called main sequence. The existence of the main sequence is due to the fact that the stage of hydrogen burning is ~90% of the evolutionary time of most stars: the burning of hydrogen in the central regions of the star leads to the formation of an isothermal helium core, the transition to the red giant stage, and the departure of the star from the main sequence. The relatively brief evolution of red giants leads, depending on their mass, to the formation of white dwarfs, neutron stars, or black holes.
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. Our sun is one of them. Sometimes such normal stars as the Sun are called yellow dwarfs.
yellow dwarf
A yellow dwarf is a type of small main sequence star with a mass between 0.8 and 1.2 solar masses and a surface temperature of 5000–6000 K.
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. An example of this type of star is Aldebaran.
The red giant ejects its outer layers of gas, forming planetary nebulae, and the core collapses into a small, dense white dwarf.
A red giant is a large reddish or orange star. The formation of such stars is possible both at the stage of star formation and at the later stages of their existence.
At an early stage, the star radiates due to the gravitational energy released during compression, until the compression is stopped by the onset of a thermonuclear reaction.
At the later stages of the evolution of stars, after the hydrogen burns out in their interiors, the stars descend from the main sequence and move to the region of red giants and supergiants of the Hertzsprung-Russell diagram: this stage lasts about 10% of the time of the “active” life of stars, that is, the stages of their evolution , during which nucleosynthesis reactions take place in the stellar interior.
The giant star has a relatively low surface temperature, about 5000 degrees. A huge radius, reaching 800 solar and due to such large sizes, a huge luminosity. The maximum radiation falls on the red and infrared regions of the spectrum, which is why they are called red giants.
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.
Dwarf stars are the opposite of giants and can be as follows.
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.
Due to the absence of hydrogen, a thermonuclear reaction does not occur in the core of such stars.
White dwarfs are very dense. They are no larger than the Earth in size, but their mass can be compared with the mass of the Sun.
These are incredibly hot stars, reaching temperatures of 100,000 degrees or more. They shine on their remaining energy, but over time, it runs out, and the core cools down, turning into a black 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.
The mass of red dwarfs does not exceed a third of the solar mass (the lower mass limit is 0.08 solar, followed by brown dwarfs), the surface temperature reaches 3500 K. Red dwarfs have a spectral type M or late K. Stars of this type emit very little light, sometimes in 10,000 times smaller than the Sun.
Given their low radiation, none of the red dwarfs are visible from Earth to the naked eye. Even the closest red dwarf to the Sun, Proxima Centauri (the closest star in the triple system to the Sun) and the closest single red dwarf, Barnard's Star, have an apparent magnitude of 11.09 and 9.53, respectively. At the same time, a star with a magnitude of up to 7.72 can be observed with the naked eye.
Due to the low rate of hydrogen combustion, red dwarfs have a very long lifespan - from tens of billions to tens of trillions of years (a red dwarf with a mass of 0.1 solar masses will burn for 10 trillion years).
In red dwarfs, thermonuclear reactions involving helium are impossible, so they cannot turn into red giants. Over time, they gradually shrink and heat up more and more until they use up the entire supply of hydrogen fuel.
Gradually, according to theoretical concepts, they turn into blue dwarfs - a hypothetical class of stars, while none of the red dwarfs has yet managed to turn into a blue dwarf, and then into white dwarfs with a helium core.
Brown dwarfs are substellar objects (with masses in the range of about 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.
The minimum temperature of main sequence stars is about 4000 K, the temperature of brown dwarfs lies in the range from 300 to 3000 K. Brown dwarfs constantly cool down throughout their lives, while the larger the dwarf, the slower it cools.
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.
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.
A binary star is two gravitationally bound stars revolving around a common center of mass.
Sometimes there are systems of three or more stars, in such a general case the system is called a multiple star.
In cases where such a star system is not too far removed from the Earth, individual stars can be distinguished through a telescope. If the distance is significant, then it is possible to understand that a double star is possible before astronomers only by indirect signs - fluctuations in brightness caused by periodic eclipses of one star by another and some others.
New star
Stars that suddenly increase in luminosity by a factor of 10,000. A nova is a binary system consisting of a white dwarf and a main sequence companion star. In such systems, gas from the star gradually flows into the white dwarf and periodically explodes there, causing a burst of luminosity.
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.
neutron star
Neutron stars (NS) are stellar formations with masses of the order of 1.5 solar masses and sizes noticeably smaller than white dwarfs, the typical radius of a neutron star is, presumably, of the order of 10-20 kilometers.
They consist mainly of neutral subatomic particles - neutrons, tightly compressed by gravitational forces. The density of such stars is extremely high, it is commensurate, and according to some estimates, it can be several times higher than the average density of the atomic nucleus. One cubic centimeter of NZ matter would weigh hundreds of millions of tons. The force of gravity on the surface of a neutron star is about 100 billion times greater than on Earth.
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.
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 rotating neutron stars with a magnetic field that 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.
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.
Experts put forward several theories of their occurrence. The most probable of the bottom says that such blue stars were binary for a very long time, and they had a merger process. When 2 stars unite, a new star appears with much greater brightness, mass, temperature.
Blue stars examples:
- Gamma Sails;
- Rigel;
- Zeta Orion;
- Alpha Giraffe;
- Zeta Korma;
- Tau Canis Major.
White stars - white stars
One scientist discovered a very dim white star that was a satellite of Sirius and it was named Sirius B. The surface of this unique star is heated to 25,000 Kelvin, and its radius is small.
White stars examples:
- Altair in the constellation Eagle;
- Vega in the constellation Lyra;
- Castor;
- Sirius.
yellow stars - yellow stars
Such stars have a glow yellow color, and their mass is within the mass of the Sun - this is about 0.8-1.4. The surface of such stars is usually heated to a temperature of 4-6 thousand Kelvin. Such a star lives for about 10 billion years.
Yellow stars examples:
- Star HD 82943;
- Toliman;
- Dabih;
- Hara;
- Alhita.
red stars red stars
The first red stars were discovered in 1868. Their temperature is quite low, and the outer layers of red giants are filled with a lot of carbon. Previously, such stars made up two spectral classes - N and R, but now scientists have been able to identify another common class - C.
Values. By general agreement, these scales are chosen so that a white star, like Sirius, has the same magnitude on both scales. The difference between the photographic and photovisual quantities is called the color index of a given star. For such blue stars as Rigel, this number will be negative, since such stars on an ordinary plate give a greater blackening than on a yellow-sensitive one.
For red stars like Betelgeuse, the color index reaches + 2-3 magnitudes. This measurement of color is also a measurement of the surface temperature of the star, with blue stars being much hotter than red ones.
Since color indices can be obtained quite easily even for very faint stars, they have great importance when studying the distribution of stars in space.
Instruments are among the most important tools for studying stars. Even the most superficial look at the spectra of stars reveals that they are not all the same. The Balmer lines of hydrogen are strong in some spectra, weak in some, and absent altogether in some.
It soon became clear that the spectra of stars can be divided into a small number of classes, gradually passing into each other. The current spectral classification was developed at the Harvard Observatory under the direction of E. Pickering.
At first, the spectral classes were denoted by Latin letters in alphabetical order, but in the process of refining the classification, the following designations for successive classes were established: O, B, A, F, G, K, M. In addition, a few unusual stars are combined into classes R, N and S , and individual individuals who do not fit into this classification at all are designated by the symbol PEC (peculiar - special).
It is interesting to note that the arrangement of stars by class is also an arrangement by color.
- Class B stars, to which Rigel and many other stars in Orion belong, are blue;
- classes O and A - white (Sirius, Deneb);
- classes F and G - yellow (Procyon, Capella);
- classes K and M - orange and red (Arcturus, Aldebaran, Antares, Betelgeuse).
Arranging the spectra in the same order, we see how the maximum of the emission intensity shifts from the violet to the red end of the spectrum. This indicates a decrease in temperature as one moves from class O to class M. A star's place in the sequence is determined more by its surface temperature than by its chemical composition. It is generally accepted that the chemical composition is the same for the vast majority of stars, but different surface temperatures and pressures cause large differences in stellar spectra.
Blue class O stars are the hottest. Their surface temperature reaches 100,000°C. Their spectra are easily recognizable by the presence of some characteristic bright lines or by the propagation of the background far into the ultraviolet region.
They are directly followed class B blue stars, are also very hot (surface temperature 25,000°C). Their spectra contain lines of helium and hydrogen. The former weaken, while the latter strengthen in the transition to class A.
V classes F and G(a typical G-class star is our Sun) the lines of calcium and other metals, such as iron and magnesium, gradually increase.
V class K calcium lines are very strong, and molecular bands also appear.
Class M includes red stars with surface temperatures below 3000°C; bands of titanium oxide are visible in their spectra.
Classes R, N and S belong to the parallel branch of cool stars whose spectra contain other molecular components.
To the connoisseur, however, there is a very big difference between "cold" and "hot" class B stars. In a precise classification system, each class is subdivided into several more subclasses. The hottest class B stars are subclass VO, stars with an average temperature for this class - k subclass B5, the coldest stars - to subclass B9. The stars are directly behind them. subclass AO.
The study of the spectra of stars turns out to be very useful, since it makes it possible to roughly classify stars according to their absolute magnitudes. For example, the VZ star is a giant with an absolute magnitude of approximately -2.5. It is possible, however, that the star will be ten times brighter (absolute value - 5.0) or ten times fainter (absolute value 0.0), since it is impossible to give a more accurate estimate from the spectral type alone.
When establishing a classification of stellar spectra, it is very important to try to separate giants from dwarfs within each spectral class, or, where this division does not exist, to single out from the normal sequence of giants stars that have too high or too low luminosity.
If you look closely at the night sky, it is easy to notice that the stars looking at us differ in color. Bluish, white, red, they shine evenly or flicker like a Christmas tree garland. In a telescope, color differences become more apparent. The reason for this diversity lies in the temperature of the photosphere. And, contrary to a logical assumption, the hottest are not red, but blue, white-blue and white stars. But first things first.
Spectral classification
Stars are huge hot balls of gas. The way we see them from Earth depends on many parameters. For example, stars don't actually twinkle. It is very easy to be convinced of this: it is enough to remember the Sun. The flickering effect occurs due to the fact that the light coming from cosmic bodies to us overcomes the interstellar medium, full of dust and gas. Another thing is color. It is a consequence of the heating of the shells (especially the photosphere) to certain temperatures. The true color may differ from the visible one, but the difference is usually small.
Today, the Harvard spectral classification of stars is used all over the world. It is a temperature one and is based on the shape and relative intensity of the spectrum lines. Each class corresponds to the stars of a certain color. The classification was developed at the Harvard Observatory in 1890-1924.
One Shaved Englishman Chewing Dates Like Carrots
There are seven main spectral classes: O-B-A-F-G-K-M. This sequence reflects a gradual decrease in temperature (from O to M). To remember it, there are special mnemonic formulas. In Russian, one of them sounds like this: "One Shaved Englishman Chewed Dates Like Carrots." Two more are added to these classes. The letters C and S denote cold luminaries with metal oxide bands in the spectrum. Consider the star classes in more detail:
- Class O is characterized by the highest surface temperature (from 30 to 60 thousand Kelvin). Stars of this type exceed the Sun in mass by 60, and in radius by 15 times. Their visible color is blue. In terms of luminosity, they are ahead of our star by more than a million times. The blue star HD93129A, belonging to this class, is characterized by one of the highest luminosity among known cosmic bodies. According to this indicator, it is ahead of the Sun by 5 million times. The blue star is located at a distance of 7.5 thousand light years from us.
- Class B has a temperature of 10-30 thousand Kelvin, a mass 18 times greater than the same parameter of the Sun. These are white-blue and white stars. Their radius is 7 times greater than that of the Sun.
- Class A is characterized by a temperature of 7.5-10 thousand Kelvin, a radius and mass exceeding 2.1 and 3.1 times, respectively, the similar parameters of the Sun. These are white stars.
- Class F: temperature 6000-7500 K. The mass is 1.7 times greater than the sun, the radius is 1.3. From Earth, such stars also look white, their true color is yellowish white.
- Class G: temperature 5-6 thousand Kelvin. The Sun belongs to this class. The visible and true color of such stars is yellow.
- Class K: temperature 3500-5000 K. The radius and mass are less than solar, they are 0.9 and 0.8 of the corresponding parameters of the star. The color of these stars seen from Earth is yellowish-orange.
- Class M: temperature 2-3.5 thousand Kelvin. The mass and radius are 0.3 and 0.4 of the similar parameters of the Sun. From the surface of our planet, they look red-orange. Beta Andromedae and Alpha Chanterelles belong to the M class. The bright red star familiar to many is Betelgeuse (Alpha Orionis). It is best to look for it in the sky in winter. The red star is located above and slightly to the left
Each class is divided into subclasses from 0 to 9, that is, from the hottest to the coldest. The numbers of stars indicate belonging to a certain spectral type and the degree of heating of the photosphere in comparison with other luminaries in the group. For example, the Sun belongs to the class G2.
visual whites
Thus, star classes B through F can look white from Earth. And only objects belonging to the A-type actually have this coloration. So, the star Saif (the constellation Orion) and Algol (beta Perseus) to an observer not armed with a telescope will seem white. They belong to spectral class B. Their true color is blue-white. Also appearing white are Mythrax and Procyon, the brightest stars in the celestial drawings of Perseus and Canis Minor. However, their true color is closer to yellow (class F).
Why are stars white to an earthly observer? The color is distorted due to the vast distance separating our planet from similar objects, as well as voluminous clouds of dust and gas, often found in space.
Class A
White stars are characterized by a not so high temperature as representatives of the O and B classes. Their photosphere heats up to 7.5-10 thousand Kelvin. Spectral class A stars are much larger than the Sun. Their luminosity is also greater - about 80 times.
In the spectra of A stars, hydrogen lines of the Balmer series are strongly pronounced. The lines of other elements are noticeably weaker, but they become more significant as you move from subclass A0 to A9. Giants and supergiants belonging to the spectral class A are characterized by slightly less pronounced hydrogen lines than main sequence stars. In the case of these luminaries, the lines of heavy metals become more noticeable.
Many peculiar stars belong to the spectral class A. This term refers to luminaries that have noticeable features in the spectrum and physical parameters, which makes it difficult to classify them. For example, rather rare stars of the Bootes lambda type are characterized by a lack of heavy metals and very slow rotation. Peculiar luminaries also include white dwarfs.
Class A includes such bright objects in the night sky as Sirius, Menkalinan, Aliot, Castor and others. Let's get to know them better.
Alpha Canis Major
Sirius is the brightest, though not the closest, star in the sky. Its distance is 8.6 light years. For an earthly observer, it seems so bright because it has an impressive size and yet is not as far removed as many other large and bright objects. The closest star to the Sun is Sirius in this list is in fifth place.
It refers to and is a system of two components. Sirius A and Sirius B are separated by 20 astronomical units and rotate with a period of just under 50 years. The first component of the system, a main-sequence star, belongs to the spectral type A1. Its mass is twice that of the sun, and its radius is 1.7 times. It can be observed with the naked eye from Earth.
The second component of the system is a white dwarf. The star Sirius B is almost equal to our luminary in mass, which is not typical for such objects. Typically, white dwarfs are characterized by a mass of 0.6-0.7 solar masses. At the same time, the dimensions of Sirius B are close to those of the earth. It is assumed that the white dwarf stage began for this star about 120 million years ago. When Sirius B was located on the main sequence, it was probably a luminary with a mass of 5 solar masses and belonged to the spectral class B.
Sirius A, according to scientists, will move to the next stage of evolution in about 660 million years. Then it will turn into a red giant, and a little later - into a white dwarf, like its companion.
Alpha Eagle
Like Sirius, many white stars, whose names are given below, are well known not only to people who are fond of astronomy because of their brightness and frequent mention in the pages of science fiction literature. Altair is one of those luminaries. Alpha Eagle is found, for example, in Steven King. In the night sky, this star is clearly visible due to its brightness and relatively close proximity. The distance separating the Sun and Altair is 16.8 light years. Of the stars of spectral class A, only Sirius is closer to us.
Altair is 1.8 times as massive as the Sun. Its characteristic feature is a very fast rotation. The star makes one rotation around its axis in less than nine hours. The rotation speed near the equator is 286 km/s. As a result, the "nimble" Altair will be flattened from the poles. In addition, due to the elliptical shape, the temperature and brightness of the star decrease from the poles to the equator. This effect is called "gravitational darkening".
Another feature of Altair is that its brilliance changes over time. It belongs to the Delta Shield type variables.
Alpha Lyrae
Vega is the most studied star after the Sun. Alpha Lyrae is the first star to have its spectrum determined. She also became the second luminary after the Sun, captured in the photograph. Vega was also among the first stars to which scientists measured the distance using the parlax method. For a long period, the brightness of the star was taken as 0 when determining the magnitudes of other objects.
Lyra's alpha is well known to both the amateur astronomer and the simple observer. It is the fifth brightest among the stars, and is included in the Summer Triangle asterism along with Altair and Deneb.
The distance from the Sun to Vega is 25.3 light years. Its equatorial radius and mass are 2.78 and 2.3 times larger than the similar parameters of our star, respectively. The shape of a star is far from being a perfect ball. The diameter at the equator is noticeably larger than at the poles. The reason is the huge speed of rotation. At the equator, it reaches 274 km / s (for the Sun, this parameter is slightly more than two kilometers per second).
One of the features of Vega is the disk of dust that surrounds it. Presumably, it arose as a result of a large number of collisions of comets and meteorites. The dust disk revolves around the star and is heated by its radiation. As a result, the intensity of the infrared radiation of Vega increases. Not so long ago, asymmetries were discovered in the disk. Their likely explanation is that the star has at least one planet.
Alpha Gemini
The second brightest object in the constellation Gemini is Castor. He, like the previous luminaries, belongs to the spectral class A. Castor is one of the most bright stars night sky. In the corresponding list, he is on the 23rd place.
Castor is a multiple system consisting of six components. The two main elements (Castor A and Castor B) revolve around a common center of mass with a period of 350 years. Each of the two stars is a spectral binary. The Castor A and Castor B components are less bright and presumably belong to the M spectral class.
Castor C was not immediately connected to the system. Initially, it was designated as an independent star YY Gemini. In the process of researching this region of the sky, it became known that this luminary was physically connected with the Castor system. The star revolves around a center of mass common to all components with a period of several tens of thousands of years and is also a spectral binary.
Beta Aurigae
The celestial drawing of the Charioteer includes about 150 "points", many of them are white stars. The names of the luminaries will say little to a person far from astronomy, but this does not detract from their significance for science. The brightest object in the celestial pattern, belonging to the spectral class A, is Mencalinan or Beta Aurigae. The name of the star in Arabic means "shoulder of the owner of the reins."
Mencalinan is a ternary system. Its two components are subgiants of spectral class A. The brightness of each of them exceeds the similar parameter of the Sun by 48 times. They are separated by a distance of 0.08 astronomical units. The third component is a red dwarf at a distance of 330 AU from the pair. e.
Epsilon Ursa Major
The brightest "point" in perhaps the most famous constellation of the northern sky (Ursa Major) is Aliot, also belonging to class A. The apparent magnitude is 1.76. In the list of the brightest luminaries, the star takes 33rd place. Alioth enters the asterism of the Big Dipper and is located closer to the bowl than other luminaries.
The Aliot spectrum is characterized by unusual lines that fluctuate with a period of 5.1 days. It is assumed that the features are associated with the impact magnetic field stars. Fluctuations in the spectrum, according to the latest data, may occur due to the proximity of a cosmic body with a mass of almost 15 Jupiter masses. Whether this is so is still a mystery. Her, like other secrets of the stars, astronomers are trying to understand every day.
white dwarfs
The story about white stars will be incomplete if we do not mention that stage in the evolution of the stars, which is designated as the "white dwarf". Such objects got their name due to the fact that the first discovered of them belonged to the spectral class A. It was Sirius B and 40 Eridani B. Today, white dwarfs are called one of the options for the final stage of a star's life.
Let us dwell in more detail on the life cycle of the luminaries.
Star evolution
Stars are not born in one night: any of them goes through several stages. First, a cloud of gas and dust begins to shrink under the influence of its own. Slowly, it takes the form of a ball, while the energy of gravity turns into heat - the temperature of the object rises. At the moment when it reaches a value of 20 million Kelvin, the reaction of nuclear fusion begins. This stage is considered the beginning of the life of a full-fledged star.
Suns spend most of their time on the main sequence. Hydrogen cycle reactions are constantly going on in their depths. The temperature of the stars may vary. When all the hydrogen in the nucleus ends, a new stage of evolution begins. Now helium is the fuel. At the same time, the star begins to expand. Its luminosity increases, while the surface temperature, on the contrary, decreases. The star leaves the main sequence and becomes a red giant.
The mass of the helium core gradually increases, and it begins to shrink under its own weight. The red giant stage ends much faster than the previous one. The path that further evolution will take depends on the initial mass of the object. Low-mass stars at the red giant stage begin to swell. As a result of this process, the object sheds its shells. The bare core of the star is also formed. In such a nucleus, all fusion reactions are completed. It is called a helium white dwarf. More massive red giants (up to a certain limit) evolve into carbon white dwarfs. They have heavier elements than helium in their cores.
Specifications
White dwarfs are bodies that are usually very close in mass to the Sun. At the same time, their size corresponds to the earth. The colossal density of these cosmic bodies and the processes taking place in their depths are inexplicable from the point of view of classical physics. The secrets of the stars helped to reveal quantum mechanics.
The substance of white dwarfs is an electron-nuclear plasma. It is almost impossible to design it even in a laboratory. Therefore, many characteristics of such objects remain incomprehensible.
Even if you study the stars all night long, you will not be able to detect at least one white dwarf without special equipment. Their luminosity is much less than that of the sun. According to scientists, white dwarfs make up approximately 3 to 10% of all objects in the Galaxy. However, to date, only those of them have been found that are located no further than 200-300 parsecs from the Earth.
White dwarfs continue to evolve. Immediately after formation, they have a high surface temperature, but cool quickly. A few tens of billions of years after formation, according to the theory, a white dwarf turns into a black dwarf - a body that does not emit visible light.
A white, red or blue star for the observer differs primarily in color. The astronomer looks deeper. Color for him immediately tells a lot about the temperature, size and mass of the object. A blue or bright blue star is a giant hot ball, far ahead of the Sun in all respects. White luminaries, examples of which are described in the article, are somewhat smaller. Star numbers in various catalogs also tell professionals a lot, but not all. A large amount of information about the life of distant space objects has either not yet been explained, or remains not even discovered.
With a telescope, you can observe 2 billion stars up to 21 magnitudes. There is a Harvard spectral classification of stars. In it, the spectral types are arranged in order of decreasing stellar temperature. Classes are designated by letters of the Latin alphabet. There are seven of them: O - B - A - P - O - K - M.
A good indicator of the temperature of a star's outer layers is its color. Hot stars of spectral types O and B are blue; stars similar to our Sun (whose spectral type is 02) appear yellow, while stars of spectral classes K and M are red.
Brightness and color of stars
All stars have a color. There are blue, white, yellow, yellowish, orange and red stars. For example, Betelgeuse is a red star, Castor is white, Capella is yellow. By brightness, they are divided into stars 1st, 2nd, ... nth star values (n max = 25). The term "magnitude" has nothing to do with true dimensions. The magnitude characterizes the light flux coming to Earth from a star. Stellar magnitudes can be both fractional and negative. The magnitude scale is based on the perception of light by the eye. The division of stars into stellar magnitudes according to apparent brightness was carried out by the ancient Greek astronomer Hipparchus (180 - 110 BC). Most bright stars Hipparchus attributed the first magnitude; he considered the next in brightness gradation (i.e., about 2.5 times weaker) to be stars of the second magnitude; stars weaker than stars of the second magnitude by 2.5 times were called stars of the third magnitude, etc.; stars at the limit of visibility to the naked eye were assigned a sixth magnitude.
With such a gradation of the brightness of the stars, it turned out that the stars of the sixth magnitude are weaker than the stars of the first magnitude by 2.55 times. Therefore, in 1856, the English astronomer N.K. Pogsoy (1829-1891) proposed to consider as stars of the sixth magnitude those that are exactly 100 times weaker than the stars of the first magnitude. All stars are located at different distances from the Earth. It would be easier to compare magnitudes if the distances were equal.
The magnitude that a star would have at a distance of 10 parsecs is called absolute magnitude. The absolute stellar magnitude is indicated - M, and the apparent stellar magnitude - m.
The chemical composition of the outer layers of stars, from which their radiation comes, is characterized by the complete predominance of hydrogen. In second place is helium, and the content of other elements is quite small.
Temperature and mass of stars
Knowing the spectral type or color of a star immediately gives the temperature of its surface. Since stars radiate approximately like absolutely black bodies of the corresponding temperature, the power radiated by a unit of their surface per unit of time is determined from the Stefan-Boltzmann law.
The division of stars based on a comparison of the luminosity of stars with their temperature and color and absolute magnitude (Hertzsprung-Russell diagram):
- the main sequence (in the center of it is the Sun - a yellow dwarf)
- supergiants (large in size and high luminosity: Antares, Betelgeuse)
- red giant sequence
- dwarfs (white - Sirius)
- subdwarfs
- white-blue sequence
This division is also based on the age of the star.
The following stars are distinguished:
- ordinary (Sun);
- double (Mizar, Albkor) are divided into:
- a) visual double, if their duality is noticed when observing through a telescope;
- b) multiples - this is a system of stars with a number greater than 2, but less than 10;
- c) optical-double - these are stars that their proximity is the result of a random projection onto the sky, and in space they are far away;
- d) physical binaries are stars that form a single system and circulate under the action of forces of mutual attraction around a common center of mass;
- e) spectroscopic binaries are stars that, when mutually revolving, come close to each other and their duality can be determined from the spectrum;
- e) eclipsing binary - these are stars "which, when mutually revolving, block each other;
new stars- these are stars that existed for a long time, but suddenly flared up. Their brightness increased in a short time by 10,000 times (the amplitude of the change in brightness from 7 to 14 magnitudes).
supernovae- these are stars that were invisible in the sky, but suddenly flashed and increased in brightness 1000 times relative to ordinary new stars.
Pulsar- a neutron star that occurs during a supernova explosion.
Data on the total number of pulsars and their lifetimes indicate that, on average, 2-3 pulsars are born per century, which approximately coincides with the frequency of supernova explosions in the Galaxy.
Star evolution
Like all bodies in nature, stars do not remain unchanged, they are born, evolve, and finally die. Astronomers used to think that it took millions of years for a star to form from interstellar gas and dust. But in recent years, photographs have been taken of a region of the sky that is part of the Great Nebula of Orion, where a small cluster of stars has appeared over the course of several years. In the photographs of 1947, a group of three star-like objects was recorded in this place. By 1954 some of them had become oblong, and by 1959 these oblong formations had disintegrated into individual stars. For the first time in the history of mankind, people observed the birth of stars literally before our eyes.
In many parts of the sky, there are conditions necessary for the appearance of stars. When studying photographs of the hazy regions of the Milky Way, it was possible to find small black spots of irregular shape, or globules, which are massive accumulations of dust and gas. These gas and dust clouds contain dust particles that very strongly absorb the light coming from the stars behind them. The size of the globules is huge - up to several light years in diameter. Despite the fact that the matter in these clusters is very rarefied, their total volume is so large that it is quite enough to form small clusters of stars close in mass to the Sun.
In a black globule, under the influence of radiation pressure emitted by surrounding stars, the matter is compressed and compacted. Such compression proceeds for some time, depending on the sources of radiation surrounding the globule and the intensity of the latter. The gravitational forces arising from the concentration of mass in the center of the globule also tend to compress the globule, causing matter to fall towards its center. Falling, particles of matter acquire kinetic energy and heat up the gas and cloud.
The fall of matter can last hundreds of years. At first, it occurs slowly, unhurriedly, since the gravitational forces that attract particles to the center are still very weak. After some time, when the globule becomes smaller and the gravitational field increases, the fall begins to occur faster. But the globule is huge, no less than a light year in diameter. This means that the distance from its outer border to the center can exceed 10 trillion kilometers. If a particle from the edge of the globule starts to fall towards the center at a speed slightly less than 2 km/s, then it will reach the center only after 200,000 years.
The lifespan of a star depends on its mass. Stars With a mass less than that of the Sun use their nuclear fuel very sparingly and can shine for tens of billions of years. The outer layers of stars like our Sun, with masses no greater than 1.2 solar masses, gradually expand and, in the end, completely leave the core of the star. In place of the giant remains a small and hot white dwarf.
