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 garland. With a telescope, color differences become more apparent. The reason for this diversity lies in the temperature of the photosphere. And, contrary to the 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 do not really flicker. It is very easy to verify this: just 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. Color is another matter. It is a consequence of heating the shells (especially the photosphere) to certain temperatures. True color may vary from visible, but the difference is usually small.
Today, the Harvard spectral classification of stars is used throughout the world. It is temperature based on the form and relative intensity of the spectrum lines. Each class has stars of a certain color. The classification was developed at the Harvard Observatory in 1890-1924.
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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). There are special mnemonic formulas for remembering it. In Russian, one of them sounds like this: "One Shaved Englishman Dates Chewed Like Carrots." Two more are added to these classes. The letters C and S denote cold luminaries with bands of metal oxides in the spectrum. Consider 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 luminosity, they are ahead of our star more than a million times. The blue star HD93129A, belonging to this class, is characterized by one of the largest luminosity indicators among the known cosmic bodies. According to this indicator, it is 5 million times ahead of the Sun. 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 higher than the similar 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 the corresponding parameters of the Sun, respectively. These are white stars.
- Class F: temperature 6000-7500 K. The mass is 1.7 times more than the sun, the radius is 1.3. From the 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 the solar, are 0.9 and 0.8 of the corresponding parameters of the body. The color of these stars visible from Earth is yellowish-orange.
- Class M: temperature 2-3.5 thousand Kelvin. The mass and radius are 0.3 and 0.4 from similar parameters of the Sun. From the surface of our planet, they look red-orange. Class M includes Beta Andromeda and Alpha Chanterelles. A bright red star familiar to many is Betelgeuse (alpha of Orion). It is best to look for her in the sky in winter. The red star is located above and slightly to the left of the Orion 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 white
Thus, classes of stars B through F from Earth may appear white. And only objects belonging to the A-type have such a coloring in fact. So, the star Saif (constellation Orion) and Algol (beta Perseus) to an observer who is not armed with a telescope will appear white. They belong to spectral class B. Their true color is white-blue. Also white are Mithrac and Procyon, the brightest stars in the celestial drawings of Perseus and Lesser Dog. However, their true color is closer to yellow (class F).
Why are stars white for the earth observer? Color is distorted due to the huge distance separating our planet from similar objects, as well as volumetric 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 class O and B. 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, the hydrogen lines of the Balmer series are strongly pronounced. The lines of the other elements are noticeably weaker, but they become more significant as they move from subclass A0 to A9. Giants and supergiants belonging to spectral class A are characterized by slightly less pronounced hydrogen lines than by stars of the main sequence. In the case of these luminaries, the lines of heavy metals become more noticeable.
Spectral class A includes many peculiar stars. Such a term refers to luminaries that have noticeable features in the spectrum and physical parameters, which complicates their classification. For example, fairly rare stars such as Bootes’s lambda are characterized by a lack of heavy metals and very slow rotation. The number of peculiar luminaries includes white dwarfs.
Class A belongs to such bright objects of the night sky as Sirius, Menkalinan, Aliot, Castor and others. 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. To the earth observer, it seems so bright because it has impressive dimensions and yet it is not removed as far as many other large and bright objects. The closest star to the Sun is the Alpha Centauri. Sirius is in fifth place on this list.
It refers to the constellation Canis Major and is a system of two components. Sirius A and Sirius B are separated by a distance of 20 astronomical units and rotate with a period of slightly less than 50 years. The first component of the system - the main sequence star, belongs to the spectral class A1. Its mass is two times higher than the sun, and its radius is 1.7 times. It 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 practically equal in mass to our luminary, which is not typical for such objects. Usually white dwarfs are characterized by a mass of 0.6-0.7 solar. In this case, the size of Sirius B is close to earthly. It is estimated that the white dwarf stage began for this star approximately 120 million years ago. When Sirius B was located on the main sequence, it probably represented a star with a mass of 5 solar and belonged to spectral class B.
Sirius A, according to scientists, will move to the next stage of evolution after about 660 million years. Then he will turn into a red giant, and even a little later - into a white dwarf, like his companion.
Alpha eagle
Like Sirius, many white stars, the names of which are given below, due to the brightness and the frequent mention on the pages of science fiction literature, are familiar not only to people who are fond of astronomy. Altair is one of such stars. Alpha Orla is found, for example, in Ursula le Guin and Steve King. In the night sky, this star is clearly visible due to its brightness and relatively close proximity. The distance between the Sun and Altair is 16.8 light years. Of the stars of spectral class A, only Sirius is closer to us.
Altair in mass exceeds the Sun by 1.8 times. Its characteristic feature is a very fast rotation. A star makes one revolution around its axis in less than nine hours. The rotation speed at the equator is 286 km / s. As a result, the “smart” Altair will be flattened from the poles. In addition, due to the elliptical shape from the poles to the equator, the temperature and brightness of the star decreases. This effect is called "gravitational darkening."
Another feature of Altair is that its brilliance changes over time. It refers to the Shield Delta type variables.
Alpha lira
Vega is the most studied star after the Sun. Alpha Lira is the first star to have a spectrum identified. She 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 parallax method. For a long period, the luminosity of the luminary was taken to be 0 when determining the stellar magnitudes of other objects.
Alpha Lyra is well known to both the amateur astronomer and the simple observer. She is the fifth brightest among the stars, is included in the asterism of the Summer Triangle 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 greater than the similar parameters of our luminary, respectively. Star shape is far from perfect ball. The diameter at the equator is noticeably larger than that of the poles. The reason is the huge speed of rotation. At the equator, it reaches 274 km / s (for the Sun, this parameter is a little more than two kilometers per second).
One of Vega's features is the surrounding dust disk. Presumably, it arose as a result of a large number of collisions of comets and meteorites. The dust disk rotates around the star and warms up under the influence of its radiation. As a result, the intensity of Vega's infrared radiation increases. Not so long ago, asymmetries were detected in the disk. Their likely explanation is the presence of at least one planet in the star.
Alpha Twins
The second brightest object in the constellation Gemini is Castor. He, like the previous luminaries, belongs to the spectral class A. Castor - one of the brightest stars in the night sky. In the corresponding list, it is located at 23 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 spectral double. The components of Castor A and Castor B are less bright and presumably belong to the spectral class M.
Castor C was not immediately connected to the system. Initially, it was designated as an independent star of YY Gemini. In the process of researching this area of the sky, it became known that this star was physically connected with the Castor system. A star revolves around a common center of mass for all components with a period of several tens of thousands of years and is also a spectral double.
Beta Charioteer
The charioteer’s heavenly figure includes approximately 150 “points”, many of which are white stars. The names of the luminaries will say little to a person who is far from astronomy, but this does not detract from their significance for science. The most striking object of the heavenly figure, belonging to the spectral class A, is Menkalinan or beta of the Ascendant. The name of the star in Arabic means "shoulder of the holder of the reins."
Menkalinan is a triple system. Its two components are subgiants of spectral class A. The brightness of each of them exceeds the analogous 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, distant from the pair at 330 a. 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. Aliot enters into the asterism of the Big Dipper and is located closer to other cups.
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 influence of the magnetic field of the star. Fluctuations in the spectrum, according to recent data, can occur due to the proximity of a cosmic body with a mass of almost 15 Jupiter masses. Is this so far a mystery. It, like other secrets of the stars, astronomers try to understand every day.
White dwarfs
The story of the white stars will be incomplete, if not to mention the stage of evolution of the stars, which is referred to as the "white dwarf". Such objects got their name due to the fact that the first ones discovered belonged to spectral class A. It was Sirius B and 40 of Eridan B. Today, white dwarfs call one of the options for the final stage of star life.
Let us dwell in more detail on the life cycle of 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 shape of a ball, while the energy of gravity turns into heat - the temperature of the object rises. At that moment, when it reaches a value of 20 million Kelvin, the nuclear fusion reaction begins. This stage is considered the beginning of the life of a full star.
Most of the time the luminaries spend on the main sequence. In their bowels there are constant reactions of the hydrogen cycle. The temperature of stars may vary. When all hydrogen ends in the nucleus, a new stage of evolution begins. Now helium becomes fuel. In this case, the star begins to expand. Its luminosity increases, and the surface temperature, on the contrary, decreases. A star goes off 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 stage of the red giant begin to swell. As a result of this process, the object drops shells. A planetary nebula and the bare core of a star are formed. In such a nucleus all synthesis reactions were completed. It is called a helium white dwarf. More massive red giants (to a certain limit) evolve into carbon white dwarfs. In their nuclei, heavier elements are present than helium.
Specifications
White dwarfs - bodies, by mass, as a rule, very close to the Sun. Moreover, their size corresponds to the earth. The colossal density of these cosmic bodies and the processes occurring in their depths are inexplicable from the point of view of classical physics. Secrets of stars helped to reveal quantum mechanics.
The substance of white dwarfs is an electron-nuclear plasma. It is almost impossible to construct it even in a laboratory. Therefore, many characteristics of such objects remain incomprehensible.
Even if you study the stars all night, you will not be able to detect even one white dwarf without special equipment. Their luminosity is much less than solar. According to scientists, white dwarfs make up about 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 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 its formation, according to theory, a white dwarf turns into a black dwarf - a body that does not emit visible light.
The white, red or blue star for the observer is primarily distinguished by color. The astronomer looks deeper. Color for him immediately tells a lot about the temperature, size and mass of the object. A blue or light blue star is a giant hot ball, in all respects far ahead of the sun. White luminaries, examples of which are described in the article, are somewhat smaller. The numbers of stars in various catalogs also tell a lot to professionals, but not all. A large amount of information about the life of distant space objects either has not yet received an explanation, or even remains undetected.