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. She happens to betemperature and is based on the form and relative intensity of the lines of the spectrum. Each class corresponds to the stars of a certain color. The classification was developed at the Harvard Observatory in 1890-1924.
One Shaved Englishman Chewed 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. Let's take a closer look at star classes:
- 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 that of the Sun. These are white-blue and white stars. Their radius is 7 times larger than that of the Sun.
- Class A is characterized by a temperature of 7.5-10 thousand Kelvin,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. Mass greater than the sun by 1.7 times, radius - by 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 apparent 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. Seen from Earth, the color of these stars is yellowish-orange.
- Class M: temperature 2-3.5 thousand Kelvin. Mass and radius - 0.3 and 0.4 from 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 of Orion's belt.
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 seemwhite. 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 (grade 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 not characterized by such a 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, heavy metal lines become more noticeable.
There are many peculiar stars belonging 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 belongs to such bright objects of the nightheaven, like Sirius, Mencalinan, 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. The distance to it 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 Alpha Centauri. Sirius is in fifth place on this list.
It belongs to the constellation Canis Major 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 class A1. Its mass is twice that of the sun, and its radius is 1.7 times. It is he who can be observed with the naked eye from the 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 type B.
Sirius A, according to scientists, will move to the next stage of evolution in about 660 million years. Thenit 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 Ursula le Guin and 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 refers to variables of the Shield delta type.
Alpha Lyra
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.
Alpha Lyra 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 rotation speed. At the equator, it reaches 274 km / s (for the Sun, this parameter is slightly more than two kilometers per second).
One of Vega's special features 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 brightest stars in the 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) rotatearound a common center of mass with a period of 350 years. Each of the two stars is a spectral binary. The components of Castor A and Castor B are less bright and presumably belong to the spectral type M.
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
Auriga's celestial drawing 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 star's name means "shoulder of the rein holder" in Arabic.
Menkalinan - triple 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 in the northern sky (Ursa Major) is Aliot, also classified as class A. The apparent magnitude is 1.76. ListedThe brightest luminary star takes 33rd place. Alioth enters the Big Dipper asterism and is closer to the bowl than other luminaries.
Aliot's spectrum is characterized by unusual lines fluctuating with a period of 5.1 days. It is assumed that the features are associated with the influence of the magnetic field of the star. 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. It, like other secrets of the stars, astronomers try to understand every day.
White dwarfs
The story about white stars will be incomplete if we do not mention that stage of the evolution of the stars, which is designated as a "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's 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 gravitational forces. 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.
Most of the time the luminaries spend on the main sequence. Reactions are constantly going on in their bowelshydrogen cycle. 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. A planetary nebula and a bare core of a star are 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.
Features
White dwarfs are bodies, in mass, as a rule, very close 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 were revealed by 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 won't 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 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 the formation, according to the theory, the white dwarf turns into a black dwarf - a body that does not emit visible light.
White, red or blue star for the observer differ 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.
