Danish astronomer Einar Hertzsprung was deeply interested in stellar evolution. Why do the stars in the sky vary in color? Why are some stars bright and others dim? Why, although most stars emit light evenly, like our Sun, there are also some luminaries whose brightness either increases or decreases?
At the beginning of the 20th century, there was no clear answer to all these questions. Hertzsprung began to compare the physical characteristics of stars known to him in search of any pattern. He soon came to an interesting conclusion: red stars were clearly divided into two types. The first is giant stars with enormous luminosity; Another type of red stars are small, visible only through a telescope, stars that emit hundreds of times less light than our Sun. Red stars that would shine with the same power as our Sun did not exist!
This division into very bright and faint stars fully applied to orange-colored stars, and to a lesser extent to yellow stars like the Sun, but was not detected for blue or white stars.
Later in America an astronomer came to the same conclusion Henry Norris Russell(Russell). He constructed a diagram on which he plotted the temperature of the stars horizontally and luminosity vertically. Most of the stars fell on the diagram within a narrow oblique line, while some luminaries appeared to be on the upper right or lower left.
Hertzsprung-Russell diagram. The spectra (temperature) of stars are marked along the Χ axis. Along the Υ axis is the luminosity in solar luminosities. Drawing: Big Universe
Since then this dependence has been called Hertzsprung-Russell diagram, and many scientists consider it the most important diagram in astrophysics. With its help, you can determine the age of star clusters and entire galaxies, as well as many other interesting things.
The curved line on which 90% of all stars lie is called Main sequence. The sun is also on it. A simple look at it tells us that the radiation power for these stars depends on the temperature of its surface, and that, in turn, on the mass of the star. Yellow, orange and red dwarfs are found on the Main Sequence. The hotter stars on it could also be called dwarfs, if the term “white dwarf” had not been assigned to completely different objects.
Giant stars in the diagram are located at the top right. These are the brightest stars in the Universe. There are few of them, since the giant stage in the life of a star is short-lived, but these stars are the most noticeable, since they shine much brighter than others.
Comparative sizes of the Sun and a typical red giant. Drawing: pics-about-space.com/Big Universe
Finally, at the bottom left we find white dwarfs- hot stars of very low luminosity. They emit little light because their surface area is very small. White dwarfs can be the size of Earth or even smaller. These are the remains of dead stars, their cores, in which thermonuclear fusion processes have already ceased.
What do we see in the sky? Now in the evenings it is visible in the southern sky. Two of the three stars that make up it are Main Sequence stars. This is Altair. And here is a supergiant star. Moreover, it is probably the brightest star that can be seen with the naked eye!
Big summer triangle. Drawing: Stellarium
In general, most of the stars visible in the sky are red, yellow or orange giants. Betelgeuse and Canopus, Capella and Aldebaran, Pollux and Rigel are all giant stars and even supergiants. But the brightest star in the night sky, Sirius, lies, like the Sun, on the Main Sequence. The yellow dwarf Alpha Centauri is also located here. Of course, these stars are only bright because they are very close to the Sun. If they were at the same distance as Deneb, they could only be seen with the most powerful telescopes!
With the exception of the Moon and all planets, every seemingly stationary object in the sky is a star - a thermonuclear energy source, and the types of stars range from dwarfs to supergiants.
Ours is a star, but it appears so bright and large because it is so close to us. Most stars look like luminous points even in powerful telescopes and, nevertheless, we know something about them. So, we know that they come in different sizes and that at least half of them consist of two or more stars bound by gravity.
What is a star?
Stars- These are huge gas balls of hydrogen and helium with traces of other chemical elements. Gravity pulls the substance in, and the pressure of the hot gas pushes it out, establishing equilibrium. The source of a star's energy lies in its core, where millions of tons of hydrogen fuse every second to form helium. And although this process has been going on continuously in the depths of the Sun for almost 5 billion years, only a very small part of all hydrogen reserves has been used up.
Types of stars
Main sequence stars. At the beginning of the 20th century. Dutchman Einar Hertzsprung and Henry Norris Russell from the USA constructed a Hertzsprung-Russell (HR) 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 cuts diagonally across the HR diagram, called the main sequence. These stars are often called dwarfs, although some of them are 20 times larger than the Sun and shine 20 thousand times brighter.
Red dwarfs
At the cool, dim end of the main sequence are red dwarfs, the most common type of star. Being smaller than the Sun, they use their fuel reserves sparingly to extend their existence by tens of billions of years. If all red dwarfs could be seen, the sky would be literally littered with them. However, red dwarfs shine so faintly that we can only observe the closest ones, such as Proxima Centauri.
White dwarfs
Even smaller in size than red dwarfs are white dwarfs. Typically, their diameter is approximately equal to that of the Earth, but their mass can be equal to that of the Sun. A volume of white dwarf matter equal to the volume of this book would have a mass of about 10 thousand tons! Their position on the HR diagram shows that they are very different from red dwarfs. Their nuclear source has become depleted.
Red giants
After main sequence stars, the most common are red giants. 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 on the HR diagram. The mass of these giants is usually approximately equal to the sun, however, if one of them took the place of our star, the inner planets of the solar system would end up in its atmosphere.
Supergiants
At the top of the GR diagram are rare supergiants. Betelgeuse, in Orion's shoulder, is almost 1 billion km across. Another bright object in Orion is Rigel, one of the brightest stars visible to the naked eye. It is almost ten times smaller than Betelgeuse and at the same time almost 100 times larger than the size of the Sun.
A red giant, as well as a supergiant, is the name of cosmic objects with extended shells and high luminosity. They belong to the late spectral classes K and M. Their radii are hundreds of times greater than the solar radius. The maximum radiation from these stars occurs in the infrared and red regions of the spectrum. On the Hertzsprung-Russell diagram, red giants are located above the main sequence line, their absolute fluctuates slightly above zero or has a negative value.
The area of such a star exceeds the area of the Sun by at least 1500 times, and at the same time its diameter is approximately 40 times greater. Since the difference in absolute magnitude with our star is about five, it turns out that the red giant emits a hundred times more light. But at the same time it is much colder. The solar temperature is twice that of the red giant, and therefore, per unit surface area, the star of our system emits sixteen times more light.
The apparent color of a star depends directly on the surface temperature. Our Sun is white-hot and has a relatively small size, which is why it is called a yellow dwarf. Cooler stars have orange and red light. Each star in the process of its evolution can reach the last spectral classes and become a red giant at two stages of development. This occurs during the process of nucleation at the stage of star formation or at the final stage of evolution. At this time, the red giant begins to radiate energy due to its own gravitational energy, which is released during its compression.
As a star contracts, its temperature increases. At the same time, due to the reduction in the size of the surface, it decreases significantly. It attenuates. If this is a “young” red giant, then eventually the fusion of helium from hydrogen will begin in its depths. After which the young star will enter the main sequence. Old stars have a different fate. At the later stages of evolution, hydrogen in the bowels of the star burns out completely. After which the star leaves the main sequence. According to the Hertzsprung-Russell diagram, it moves into the region of supergiants and red giants. But before moving to this stage, it goes through an intermediate stage - subgiant.
Subgiants are called stars in whose core hydrogen thermonuclear reactions have already stopped, but helium combustion has not yet begun. This happens because the core is not warmed up enough. An example of such a subgiant would be Arthur, located in He is orange.
everywhere with an apparent magnitude of -0.1. It is located at a distance of about 36 - 38 from the Sun. It can be observed in the Northern Hemisphere in May, if you look directly south. Arthur's diameter is 40 times that of the sun.
The yellow dwarf Sun is a relatively young star. Its age is estimated at 4.57 billion years. It will remain on the main sequence for about 5 billion more years. But scientists managed to simulate a world in which the Sun is a red giant. Its size will increase 200 times and reach the level of incineration of Mercury and Venus. Of course, life by this time will no longer be possible. At this stage, the Sun will exist for approximately another 100 million years, after which it will turn into and become a white dwarf.
Rigel and the nebula it illuminates, IC 2118.
A blue supergiant is a type of supergiant (luminosity class I) spectral classes O and B.
General characteristics
These are young, very hot and bright stars with a surface temperature of 20,000-50,000 °C. On the Hertzsprung-Russell diagram they are located in the upper left part. Their mass is in the range of 10-50 solar masses (), the maximum radius reaches 25 solar radii (). These rare and mysterious stars are among the hottest, largest and brightest objects in the studied region.
Due to their enormous masses, they have a relatively short lifespan (10-50 million years) and are present only in young cosmic structures such as open clusters, spiral arms and irregular galaxies. They are virtually never found in the cores of spiral and elliptical galaxies or in globular clusters, which are believed to be old objects.
Despite their rarity and their short lives, blue supergiants are often found among stars visible to the naked eye; their inherent brightness compensates for their small numbers.
Interconversion of supergiants
Gamma Orionis, Algol B and the Sun (center).
Blue supergiants are massive stars that are in a certain phase of the “dying” process. In this phase, the intensity of thermonuclear reactions occurring in the star’s core decreases, which leads to the compression of the star. As a result of a significant decrease in surface area, the density of emitted energy increases, and this, in turn, entails heating of the surface. This kind of compression of a massive star leads to the transformation of a red supergiant into a blue one. The reverse process is also possible - the transformation of a blue supergiant into a red one.
While the stellar wind from a red supergiant is dense and slow, the wind from a blue supergiant is fast but thin. If the contraction causes a red supergiant to turn blue, the faster wind collides with the previously emitted slower wind and causes the ejected material to compact into a thin shell. Almost all observed blue supergiants have a similar envelope, confirming that they were all previously red supergiants.
As a star evolves, it can transition several times from a red supergiant (slow, dense wind) to a blue supergiant (fast, thin wind) and vice versa, which creates concentric weak shells around the star. In the intermediate phase, the star may be yellow or white, such as the North Star. Typically, a massive star ends its existence with an explosion, but a very small number of stars, whose mass ranges from eight to twelve solar masses, do not explode, but continue to evolve and eventually turn into oxygen-neon stars. It is not yet clear exactly how and why these white dwarfs are formed from stars, which theoretically should end their evolution with a small supernova explosion. Both blue and red supergiants can evolve into a supernova.
Because massive stars spend much of their time in the red supergiant state, we see more red supergiants than blue supergiants, and most supernovae come from red supergiants. Astrophysicists previously even assumed that all supernovae originate from red supergiants, but supernova SN 1987A was formed from a blue supergiant and, thus, this assumption turned out to be incorrect. This event also led to a revision of some provisions of the theory of stellar evolution.
Examples of blue supergiants
Rigel
The most famous example is Rigel (beta Orionis), the brightest star in the constellation Orion, with approximately 20 times the mass and approximately 130,000 times the luminosity of the Sun, making it one of the most powerful stars in the Galaxy (at any rate). case, the most powerful of the brightest stars in the sky, since Rigel is the closest star with such enormous luminosity). The ancient Egyptians associated Rigel with Sakh, the king of the stars and patron of the dead, and later with Osiris.
Gamma Parusov
Gamma Vela is a multiple star, the brightest in the constellation Vela. It has an apparent magnitude of +1.7m. The distance to the stars of the system is estimated at 800 light years. Gamma Parus (Regor) is a massive blue supergiant. Has a mass 30 times the mass of the Sun. Its diameter is 8 times that of the sun. Regor's luminosity is 10,600 solar luminosities. The unusual spectrum of the star, where instead of dark absorption lines there are bright emission lines, gave the name to the star as the “Spectral Pearl of the Southern Sky”
Alpha Giraffe
The distance to the star is approximately 7 thousand light years, and yet the star is visible to the naked eye. It is the third brightest star in the constellation Giraffe, with Beta Giraffe and CS Giraffe occupying first and second place, respectively.
Zeta Orionis
Zeta Orionis (called Alnitak) is a star in the constellation Orion, which is the brightest O-class star with a visual magnitude of +1.72 (maximum +1.72 and minimum to +1.79), the left and closest star asterism "Orion's Belt". The distance to the star is about 800 light years, its luminosity is approximately 35,000 solar.
Tau Canis Majoris
Spectral double star in the constellation Canis Major. It is the brightest star in the open star cluster NGC 2362, located at a distance of 3200 light. years from . Tau Canis Majoris is a blue supergiant of spectral class O with an apparent magnitude of +4.37m. The Tau Canis Majoris star system consists of at least five components. To a first approximation, Tau Canis Majoris is a triple star in which two stars have apparent magnitudes of +4.4m and +5.3m and are separated by 0.15 arcseconds, and the third star has an apparent magnitude of +10m and are separated from them by 8 arcseconds, orbiting with a period of 155 days around the inner pair.
Zeta Stern
Zeta Puppis as imagined by an artist
Zeta Puppis is the brightest star in the constellation Puppis. The star has its own name Naos. It is a massive blue star with a luminosity of 870,000 times the luminosity of the Sun. Zeta Puppis is 59 times more massive than the Sun. It has a spectral class of O9.
Over the next hundreds of thousands of years, Zeta Puppis is expected to gradually cool and expand, and will go through all spectral classes: B, A, F, G, K, and M as it cools. As this happens, the star’s main radiation will move into the visible range, and Naos will become one of the brightest stars in the future earth’s sky. After 2 million years, Naos will have a spectral class of M5 and will be much larger than Earth's current orbit. Naos will then explode into a supernova. Due to the short distance to Earth, this supernova will be much brighter than the full one, and the core of the star will collapse immediately into . It is possible that this will be accompanied by a strong gamma-ray burst.
The results of determining stellar diameters turned out to be truly amazing. We didn’t suspect before that there could be such giant stars. The first star whose true dimensions were determined (in 1920) was the bright star of the constellation Orion, which bears the Arabic name Betelgeuse. Its diameter turned out to exceed the diameter of the orbit of Mars! Another giant star is Antares, the brightest star in the constellation Scorpio: its diameter is about one and a half times the diameter of the Earth's orbit. Among the currently discovered stellar giants, we must also include the so-called Marvelous “Mira,” a star in the constellation Cetus, the diameter of which is 330 times greater than the diameter of our Sun. Typically, giant stars have radii from 10 to 100 solar radii and luminosities from 10 to 1000 solar luminosities. Stars with luminosities greater than those of the giants are called supergiants and hypergiants.
Giant stars have an interesting physical structure. Calculations show that such stars, despite their monstrous sizes, contain disproportionately little matter. They are only a few times heavier than our Sun; and since the volume of Betelgeuse, for example, is 40,000,000 times larger than the Sun, the density of this star should be negligible. And if the matter of the Sun on average approaches density, then the matter of giant stars in this respect resembles rarefied air. Giant stars, as one astronomer put it, “resemble a huge balloon of low density, much less than the density of air.”
A star becomes a giant after all the hydrogen available for reaction in the star's core has been used up. A star whose initial mass does not exceed about 0.4 solar masses will not become a giant star. This is because the matter inside such stars is highly mixed by convection, and so hydrogen continues to participate in the reaction until all the mass of the star is consumed, at which point it becomes a white dwarf consisting predominantly of helium. If a star is more massive than this lower limit, then when it consumes all the hydrogen available in the core for reaction, the core will begin to contract. The hydrogen now reacts with the helium in the shell around the helium-rich core, and the portion of the star outside the shell expands and cools. At this point in its evolution, the star's luminosity remains approximately constant and its surface temperature decreases. The star begins to become a red giant. At this point, already, as a rule, a red giant, it will remain approximately constant, while its luminosity and radius will increase significantly, and the core will continue to contract, increasing its temperature.
If the star's mass was below about 0.5 solar masses, it is believed that it would never reach the central temperatures required for helium fusion. Therefore, it will remain a red giant star with hydrogen fusion until it begins to turn into a helium white dwarf.