A.A. KISELEV
Saint Petersburg State University
Introduction
Justification of the inertial coordinate system in astronomy
The discovery of the movements of "fixed" stars belongs to the famous English astronomer Edmund Halley, who discovered in 1718 that some bright stars from the Hipparchus-Ptolemy catalog noticeably changed their positions among other stars. These were Sirius, which had shifted to the south by almost one and a half diameters of the Moon, Arcturus - by two diameters to the south, and Aldebaran, which had shifted by 1/4 of the diameter of the Moon to the east. The observed changes could not be attributed to Ptolemy's catalog errors, which, as a rule, did not exceed 6 "(1/5 of the moon's diameter). Halley's discovery was soon (1728) confirmed by another English astronomer, James Bradley, who is better known as the discoverer of the annual star. In Tobias Mayer (1723-1762), Nicola Lacaille (1713-1762) and many other astronomers up to Friedrich Bessel (1784-1846) were engaged in further definitions of the movements of stars, who laid the foundation for the modern fundamental system of star positions.
It is curious that it took almost 2000 years to destroy the prevailing idea of fixed stars in order to start looking for and find the movements of stars. This revolution in astronomy, of course, was due to the triumph of Newtonian mechanics, which established the laws of motion celestial bodies, including stars that astronomers already knew in the 18th century to be bodies similar to the Sun. But the main interest for astronomers of that time was the Moon (for navigation), the planets and the Earth as a planet. Newtonian mechanics created the conditions for a mathematically rigorous study of the motions of these bodies, it only remained to find a coordinate system that could be recognized as at rest or in a state of uniform rectilinear motion, that is, an inertial coordinate system that satisfies Newton's first law, such a coordinate system to which all the observed movements of the Moon, planets, and the Earth as well could be easily and simply attributed. Such a system of coordinates, it would seem, was embodied by "fixed" stars. And so astronomers began to determine the spherical coordinates of stars, referring them to the equatorial system, where the plane parallel to the earth's equator is taken as the main plane, and the point spring equinox. The development of instrumental technology and the skill of observers (J. Bradley, T. Mayer) contributed to a sharp improvement in the accuracy of determining the coordinates of stars in the equatorial system. Based on such observations, the first catalogs of the positions of a certain number of selected stars were compiled. The accuracy of the positions of the stars in these catalogs already in the 18th century was approaching 1 ", and in the 19th century it still increased noticeably. The difference in the coordinates of the stars in the catalogs compiled and assigned to different epochs revealed that accepted system equatorial coordinates is non-inertial. Newtonian mechanics made it possible to strictly substantiate the causes and nature of changes in the coordinates of stars related to the equatorial coordinate system - to the reference system set by the free rotation of the Earth, circling around the Sun and experiencing disturbances from the Moon and planets. These changes in coordinates are: 1) the phenomenon of precession, which was known to the ancients as "preceding the equinoxes"; 2) the phenomenon of nutation, which was discovered by Bradley. Both of these phenomena, together with aberration, were traced and studied in detail by several generations of astronomers in the 18th and XIX centuries starting with Bradley and ending with Bessel. As a result, the numerical values of the constants and aberrations were reliably determined, that is, those quantities that are still part of the list of the so-called fundamental constants of astronomy. Thus, all the conditions were created for the transition from the visible (instantaneous) coordinates of stars to coordinates referred to some constant (stopped) system of axes, which can be considered inertial with a good approximation. In the language of astronomers - celestial mechanics - this transition is called the transformation from the apparent positions of the stars to their average positions in the system of the equator and equinoxes of a given epoch. This transformation was substantiated in detail and expounded in Bessel's fundamental work "Fundamenta astronomiae" in 1818, which still retains its significance. Rationale inertial system coordinates in astronomy created the necessary conditions to determine and study the real movements of celestial bodies, including stars, in the stellar world surrounding the Earth.
Proper motions of stars
Meridian proper motions
The idea of this project was simultaneously expressed in the 1930s by the American astronomer Wright and B.V. Numerov in the USSR. According to this idea, it was proposed to determine the photographic proper motions of stars directly relative to extragalactic nebulae (galaxies). The Americans intended to use images of galaxies as reference stars, while Soviet astronomers - only as control stars in the process of absolutization. In view of the extreme remoteness of galaxies (most of the observed galaxies are more than 10 6 pc away from our Galaxy), their proper motions can be neglected, which are much less than 0.001 "/year. Therefore, the photographic proper motions of stars determined with respect to galaxies can be considered absolute and from comparison with the meridian proper motions of the same stars, check whether the meridian proper motions of the stars satisfy the inertial condition, that is, whether they are correctly derived.
Twenty years ago, the word "stars" was often used together with the adjective "fixed", preserved from the old opposition of moving planets to "fixed" stars. But the stars move, like everything else in nature. The term "fixed", apparently, will never again find use in astronomy. True, due to the great remoteness of the stars, their apparent displacements on the celestial sphere occur slowly and considerable skill and patience are required to detect them. Astronomers compare the position of the stars on two photographic plates, of which the second was taken many years after the first. Usually the period of time exceeds 20 years, and often the person who made the second record continues the work begun by the person who made the first record. By dividing the detected displacement of the star, expressed in arc seconds, by the number of years that have passed, the so-called proper motion of the star is found - the displacement of the star on the celestial sphere in arc seconds per year, caused by its movement across the line of sight. In table. 5 is a list of ten stars with the largest proper motion. Naturally, all these stars are close to the Sun, otherwise they could not have large proper motions.
The accuracy of determining the proper motion of a star depends mainly on the size of the time interval that has elapsed between two images. The larger it is, the higher the accuracy. Now the best definitions have reached an accuracy of 0.001 per year.
The speeds of stars across the line of sight are usually 20-30 km/s. If the transverse velocity is 30 km/s, then it can be calculated that it will give a displacement of 0″.001 per year if the distance to the star is 6000 ps. This means that this is the limiting distance to which one can still somehow detect the motion of a star across the line of sight. And in order for a definition to be reliable, it must be five times greater than the error that is made in it; This means that proper motions can only be reliable for stars whose distances do not exceed 1200 ps. For more distant stars, there are currently no means for determining their speed across the line of sight. But the radial velocity, i.e., that part of the velocity that is directed towards us or away from us, can be measured.
The radial velocities of the stars were discovered by studying their spectra. If a source propagating some kind of wave motion - light, radio waves, sound, etc. - approaches us, then the number of waves reaching us per unit time increases. We note an increase in frequency wave motion and, consequently, a decrease in its wavelength. Removing the same
Table 5. Ten stars with the largest proper motion
| Star name | Proper movement | Distance in parsecs |
| Barnard's Star | 1011,27 | 1,8 |
| Captain's Star. | 8,79 | 4,0 |
| L&Kail 9352 bb ~ 37°15492 | 6,87 | 3,7 |
| 6,09 | 4,8 | |
| 61 Swans | 5.22 | 3,4 |
| Wolf 389 | 4,84 | 2,5 |
| Lalande 21185 | 4,78 | 2,5 |
| e Indian | 4,67 | 3,4 |
| about Indian | 4,08 | 4,9 |
| a Centauri | 3,85 | 1,3 |
source of wave motion will cause a decrease in the frequency of oscillations and an increase in their: wavelength. The magnitude of these changes is proportional to the radial velocity and is determined by the Doppler law, i.e., the increment in the wavelength DA relates to the wavelength itself in the same way as the radial velocity V of the radiation source O relates to the speed of light c.
To determine the radial velocity of a star, astronomers take the spectrum of the star and the spectrum of elements (located in the laboratory) on the same plate, the lines of which are visible in the spectrum of the star. Comparing the position of the lines in the obtained spectra, one can find the change in the wavelength caused by the radial velocity of the star, and then using the equation to find this radial velocity. If a star moves away from us and its distance increases, the radial velocity was agreed to be considered positive. Accordingly, the radial velocities of stars moving towards us are considered negative.
The accuracy of determining the radial velocities depends on the quality of the spectra, on how sharp and thin the lines present in it are convenient for measuring the position. For spectra with lines convenient for measurements, the accuracy can reach 0.1 km/s. Of course, if the spectrum is weak and the lines in it are not sharp, the accuracy drops significantly. But the distance of an object does not affect the accuracy of determining the radial velocity, since the radial velocity itself does not decrease with increasing distance. Therefore, no matter how far the object is, if a sufficiently good spectrum of it can be obtained, the radial velocity can be reliably determined.
Program questions:
Proper motion and radial velocities of stars;
Peculiar velocities of stars and the Sun in the Galaxy;
Rotation of the Galaxy.
Summary:
Proper motion and radial velocities of stars, peculiar velocities of stars and the Sun in the Galaxy
A comparison of the equatorial coordinates of the same stars, determined at significant intervals of time, showed that and change over time. A significant part of these changes is caused by precession, nutation, aberration and annual parallax. If we exclude the influence of these causes, then the changes are reduced, but do not disappear completely. The remaining displacement of a star on the celestial sphere for a year is called the proper motion of the star. It is expressed in seconds of arc per year.
To determine these movements, photographic plates taken at long intervals of 20 or more years are compared. By dividing the resulting displacement by the number of years that have passed, the researchers get the movement of the star per year. The accuracy of the determination depends on the amount of time elapsed between two images.
Proper motions are different for different stars in magnitude and direction. Only a few dozen stars have proper motions greater than 1″ per year. The largest known proper motion of Barnard's “flying” star is = 10″.27. Most of the stars have their own motion, equal to hundredths and thousandths of an arc second per year. The best modern definitions reach 0 "001 per year. Over long periods of time, equal to tens of thousands of years, the patterns of the constellations change greatly.
The proper motion of the star is in an arc great circle at a constant speed. Direct motion changes by the value , called proper right ascension motion, and declination - by , called proper declination motion.
The proper motion of a star is calculated by the formula:
E 
If the proper motion of the star for a year and the distance to it r in parsecs are known, then it is not difficult to calculate the projection of the spatial velocity of the star onto the plane of the sky. This projection is called tangential speed V t and is calculated by the formula:
Where r is the distance to the star, expressed in parsecs.
To find the spatial velocity V of a star, it is necessary to know its radial velocity V r , which is determined from the Doppler shift of the lines in the spectrum, and V t , which is determined from the annual parallax u. Since V t and V r are mutually perpendicular, the space velocity of the star is:
V = V t + V r ).
To determine V, the angle , found by its functions, must be indicated:
sin \u003d V t / V,
cos = V t /V.
The angle lies in the range from 0 to 180.
System centauri Solar system True movement in spaceV
sin = /,
cos = /
taking into account the signs of both functions. The spatial speed of a star remains virtually unchanged in magnitude and direction over many centuries. Therefore, knowing V and r of the star at the present epoch, it is possible to calculate the epoch of closest approach of the star to the Sun and to determine for it the distance r min , parallax, proper motion, spatial velocity components, and apparent magnitude. The distance to the star in parsecs is r = 1/, 1 parsec = 3.26 sv. of the year.
Z
System movement
centauri
To judge the movements of stars, one should find the speed of the Sun and exclude it from the observed speeds of the stars.
The point on the celestial sphere, to which the velocity vector of the Sun is directed, is called the solar apex, and the opposite point is called the anti-apex.
Apex of the solar system is located in the constellation Hercules, has the coordinates: = 270 , = +30 . In this direction, the Sun moves at a speed of about 20 km/s, relative to the stars located no further than 100 ps from it. During the year, the Sun travels 630,000,000 km, or 4.2 AU.
Rotation of the Galaxy
If some group of stars moves at the same speed, then being on one of these stars, it is impossible to detect a common movement. The situation is different if the velocity changes as if a group of stars were moving around common center. Then the speed of stars closer to the center will be less than those farther from the center. Observed radial velocities distant stars show such movement. All stars, together with the Sun, move perpendicular to the direction towards the center of the Galaxy. This movement is a consequence of the general rotation of the Galaxy, the speed of which varies with distance from its center (differential rotation).
The rotation of the Galaxy has the following features:
1. It occurs clockwise if you look at the Galaxy from its north pole, located in the constellation Coma Veronica.
2. The angular velocity of rotation decreases with distance from the center.
3. Line speed rotation initially increases with distance from the center. Then, at about the distance of the Sun, it reaches the greatest value about 250 km/s, after which it slowly decreases.
4. The sun and stars in its vicinity make a complete revolution around the center of the Galaxy in about 230 million years. This period of time is called a galactic year.
Control questions:
What is the proper motion of stars?
How is the proper motion of stars detected?
Which star has the largest proper motion?
What formula is used to calculate the proper motion of a star?
Into what components is the space velocity of a star decomposed?
What is the name of the point on the celestial sphere towards which the sun moves?
What constellation is the apex in?
How fast is the sun moving relative to nearby stars?
How far does the sun travel in a year?
What are the features of the rotation of the Galaxy?
What is the rotation period of the galaxy?
Tasks:
1. Radial velocity of the star Betelgeuse 
= 21 km/s, proper motion= 0.032per year, and parallax R= 0.012. Determine the total spatial speed of the star relative to the Sun and the angle formed by the direction of motion of the star in space with the line of sight.
Answer:= 31.
2. Star 83 Hercules is at a distance from us D= 100 pc, its own motion is = 0.12. What is the tangential velocity of this star?
Answer:57 km/s.
3. The proper motion of Kaptein's star, located at a distance of 4 pc, is 8.8 per year, and the radial velocity is 242 km/s. Determine the spatial velocity of the star.
Answer: 294 km/s.
4. At what minimum distance will the star 61 Cygnus approach us if the parallax of this star is 0.3 and its own motion is 5.2. The star is moving towards us with a radial velocity of 64 km/s.
Answer:2.6 pc.
Literature:
1. Astronomical calendar. permanent part. M., 1981.
2. Kononovich E.V., Moroz V.I. General astronomy course. M., Editorial URSS, 2004.
3. Efremov Yu.N. Into the depths of the universe. M., 1984.
4. Tsesevich V.P. What and how to observe in the sky. M., 1979.
Proper motion and radial velocities of stars. Peculiar velocities of stars and the Sun in the Galaxy. Rotation of the Galaxy.
A comparison of the equatorial coordinates of the same stars, determined at significant intervals of time, showed that a and d change over time. A significant part of these changes is caused by precession, nutation, aberration and annual parallax. If we exclude the influence of these causes, then the changes are reduced, but do not disappear completely. The remaining displacement of the star on the celestial sphere per year is called the proper motion of the star m. It is expressed in seconds. arcs per year.
Proper motions are different for different stars in magnitude and direction. Only a few dozen stars have proper motions greater than 1” per year. Barnard's “flying” star has the largest known proper motion m = 10”,27. Most of the stars have their own motion equal to hundredths and thousandths of an arc second per year.
Over long periods of time, equal to tens of thousands of years, the patterns of the constellations change greatly.
The proper motion of the star is along a great circle arc with constant speed. The right ascension changes by the value m a , called the right ascension proper motion, and the declination by the value m d , called the proper declination motion.
The proper motion of the star is calculated by the formula:
m = r(m a 2 + m d 2).
If the proper motion of the star for a year and the distance to it r in parsecs are known, then it is not difficult to calculate the projection of the spatial velocity of the star onto the picture plane. This projection is called the tangential velocity V t and is calculated by the formula:
V t \u003d m "r / 206265" ps / year \u003d 4.74 m r km / s.
to find the spatial velocity V of a star, it is necessary to know its radial velocity V r , which is determined from the Doppler shift of the lines in the spectrum of the star. Since V t and V r are mutually perpendicular, the space velocity of the star is:
V = r(V t 2 + V r 2).
The fastest stars are RR Lyrae variables. Their average speed relative to the Sun is 130 km/s. However, these stars move against the rotation of the Galaxy, so their speed is low (250 -130 = 120 km/s). Very fast stars, with speeds of about 350 km/s relative to the center of the Galaxy, are not observed, because the speed of 320 km/s is enough to leave the field of attraction of the Galaxy or rotate in a highly elongated orbit.
Knowing the proper motions and radial velocities of stars allows us to judge the motions of stars relative to the Sun, which also moves in space. Therefore, the observed movements of stars are composed of two parts, of which one is a consequence of the movement of the Sun, and the other is the individual movement of the star.
In order to judge the motions of the stars, one should find the speed of the Sun and exclude it from the observed speeds of the stars.
The point on the celestial sphere to which the Sun's velocity vector is directed is called the solar apex, and the opposite point is called the anti-apex.
Apex of the solar system is located in the constellation Hercules, has the coordinates: a = 270 0 , d = +30 0 . In this direction, the Sun moves at a speed of about 20 km / s, relative to the stars located no further than 100 ps from it. During the year, the Sun travels 630,000,000 km, or 4.2 AU.
If some group of stars moves at the same speed, then being on one of these stars, it is impossible to detect a common movement. The situation is different if the velocity changes as if a group of stars were moving around a common center. Then the speed of stars closer to the center will be less than those farther from the center. The observed radial velocities of distant stars demonstrate such motion. All stars, together with the Sun, move perpendicular to the direction towards the center of the Galaxy. This movement is a consequence of the general rotation of the Galaxy, the speed of which varies with distance from its center (differential rotation).
The rotation of the Galaxy has the following features:
1. It occurs clockwise if you look at the Galaxy from its north pole, located in the constellation Coma Veronica.
2. The angular velocity of rotation decreases with distance from the center.
3. The linear speed of rotation first increases with distance from the center. Then, approximately at the distance of the Sun, it reaches its maximum value of about 250 km/s, after which it slowly decreases.
4. The sun and stars in its vicinity make a complete revolution around the center of the Galaxy in about 230 million years. This period of time is called a galactic year.
24.2 Stellar populations and galactic subsystems.
Stars located near the Sun are very bright and belong to the I type of population. they are usually found in the outer regions of the Galaxy. Stars located far from the Sun, located near the center of the Galaxy and in the corona belong to the II type of population. The division of stars into populations was carried out by Baade when studying the Andromeda Nebula. Most bright stars Population I - blue and have absolute values to -9 m , and the brightest population II stars are red with abs. -3 m . In addition, population I is characterized by an abundance of interstellar gas and dust, which are absent in population II.
A detailed division of stars in the Galaxy into populations includes 6 types:
1. Extreme population I - includes objects contained in spiral branches. This includes interstellar gas and dust concentrated in the spiral arms from which stars form. The stars of this population are very young. Their age is 20 - 50 million years. The region of existence of these stars is limited by a thin galactic layer: a ring with an inner radius of 5000 ps, an outer radius of 15,000 ps, and a thickness of about 500 ps.
These stars include stars of spectral types from O to B2, supergiants of late spectral types, Wolf-Rayet type stars, class B emission stars, stellar associations, T Tauri type variables.
2. The stars of the ordinary population I are slightly older, their age is 2-3 space years. They moved away from spiral arms and are often located near the central plane of the Galaxy.
These include stars of subclasses from B3 to B8 and normal stars of class A, res. clusters with stars of the same classes, class A to F stars with strong metal lines, less bright red supergiants.
3. Stars of the disk population. Their age is from 1 to 5 billion years; 5-25 space years. These stars include the Sun. This population includes many low-observable stars located within 1000 ps from the central plane in the galactic belt with an inner radius of 5000 ps and an outer radius of 15,000 ps. These stars include ordinary giants of classes from G to K, main sequence stars of classes from G to K, long-period variables with periods of more than 250 days, semi-regular variables, planetary nebulae, new stars, old open clusters.
4. Intermediate population II stars include objects located at distances greater than 1000 pc on either side of the central plane of the Galaxy. These stars rotate in elongated orbits. These include most old stars, with ages from 50 to 80 space years, stars with high velocities, with weak lines, long-period variables with periods from 50 to 250 days, Virgo W-type Cepheids, RR Lyrae variables, white dwarfs, globular clusters .
5. Population of the galactic crown. include objects that have arisen on early stages the evolution of the Galaxy, which was at that time less flat than it is now. These objects include subdwarfs, coronal globular clusters, RR Lyrae stars, stars with extremely faint lines, and stars with the highest velocities.
6. Core population stars include the least known objects. In the spectra of these stars observed in other galaxies, the sodium lines are strong, and the cyanide (CN) bands are intense. These can be class M dwarfs. Such objects include RR Lyrae stars, globular stars. metal-rich clusters, planetary nebulae, M-class dwarfs, G- and M-class giant stars with strong cyanide bands, infrared objects.
The most important elements of the structure of the Galaxy are the central cluster, spiral arms, and disk. The central cluster of the Galaxy is hidden from us by dark opaque matter. Its southern half is best seen as a bright star cloud in the constellation Sagittarius. In infrared rays, it is possible to observe the second half. These halves are separated by a powerful band of dusty matter, which is opaque even to infrared rays. The linear dimensions of the central cluster are 3 by 5 kiloparsecs.
The region of the Galaxy at a distance of 4-8 kpc from the center is distinguished by a number of features. It is concentrated largest number pulsars and gas remnants from supernova explosions, intense non-thermal radio emission, young and hot O and B stars are more common. Hydrogen molecular clouds exist in this area. The diffuse matter in this area has an increased concentration cosmic rays.
At a distance of 3-4 kpc from the center of the Galaxy, radio astronomy methods discovered a neutral hydrogen sleeve with a mass of about 100,000,000 solar masses, expanding at a speed of about 50 km/s. on the other side of the center, at a distance of about 2 kpc, there is a sleeve with a mass 10 times smaller, moving away from the center at a speed of 135 km/s.
In the region of the center there are several gas clouds with masses of 10,000 - 100,000 solar masses, moving away at a speed of 100 - 170 km/s.
The central region with a radius less than 1 kpc is occupied by a ring of neutral gas, which rotates at a speed of 200 km/s around the center. Inside it, there is a vast disk-shaped H II region with a diameter of about 300 ps. In the region of the center, non-thermal radiation is observed, which indicates an increase in the concentration of cosmic rays and the strength of magnetic fields.
The totality of phenomena observed in the central regions of the Galaxy indicates the possibility that more than 10,000,000 years ago, gas clouds with a total mass of about 10,000,000 solar masses and a speed of about 600 km / s were ejected from the center of the Galaxy.
In the constellation Sagittarius, near the center of the Galaxy, there are several powerful sources of radio and infrared radiation. One of them - Sagittarius-A is located in the very center of the Galaxy. It is surrounded by an annular molecular cloud with a radius of 200 ps, expanding at a speed of 140 km/s. In the central regions, there is an active process of star formation.
At the center of our Galaxy, there is most likely a nucleus, similar to a globular star cluster. infrared receivers detected an elliptical object with dimensions of 10 ps there. It may contain a dense star cluster with a diameter of 1 ps. It may also be an object of unknown relativistic nature.
24.3 Spiral structure of the Galaxy.
The nature of the spiral structure of the Galaxy is associated with spiral density waves propagating in the stellar disk. These waves are similar to sound waves, but due to rotation, they take on the appearance of spirals. The medium in which these waves propagate consists not only of gas-dust interstellar matter, but also of the stars themselves. Stars also form a kind of gas, different from the usual topics that there are no collisions between its particles.
Spiral density wave, just like a regular one longitudinal wave, is an alternation of successive densification and rarefaction of the Medium. Unlike gas and stars, the spiral pattern of waves rotates in the same direction as the entire Galaxy, but noticeably slower and with a constant angular velocity, like a solid body.
Therefore, the substance constantly catches up with the spiral branches with inside and goes through them. However, for stars and gas, this passage through the spiral arms occurs in different ways. Stars, like gas, condense in a spiral wave, their concentration increases by 10 - 20%. Accordingly, the gravitational potential also increases. But since there are no collisions between the stars, they conserve momentum, slightly change their path within the spiral arm and exit it in almost the same direction in which they entered.
Gas behaves differently. Due to collisions, entering the arm, it loses momentum, slows down, and begins to accumulate at the inner boundary of the arm. Increasing new portions of gas lead to the formation at this boundary shock wave with a large density difference. As a result, gas sealing edges are formed near the spiral branches and thermal instability. The gas quickly becomes opaque, cools down and passes into a dense phase, forming gas-dust complexes favorable for star formation. Young and hot stars excite the glow of the gas, which gives rise to bright nebulae, which, together with hot stars, outline a spiral structure that repeats the spiral density wave in the stellar disk.
The spiral structure of our galaxy has been studied by examining other spiral galaxies. Studies have shown that the spiral arms of neighboring galaxies are composed of hot giants, supergiants, dust and gas. If you remove these objects, the spiral branches will disappear. Red and yellow stars fill evenly the areas in the branches and between them.
To clarify the spiral structure of our galaxy, we need to observe hot giants, dust and gas. It is quite difficult to do this, because the Sun is in the plane of the Galaxy and various spiral branches are projected onto each other. Modern methods do not allow accurately determining the distances to distant giants, which makes it difficult to create a spatial picture. In addition, large masses of dust of inhomogeneous structure and different densities lie in the plane of the Galaxy, which makes it even more difficult to study distant objects.
Great hopes are given by the study of hydrogen at a wavelength of 21 cm. With their help, it is possible to measure the density of neutral hydrogen in various places Galaxies. This work was done by the Dutch astronomers Holst, Muller, Oort, and others. As a result, a picture of the distribution of hydrogen was obtained, which outlined the contours of the spiral structure of the Galaxy. Hydrogen is in large quantities next to young hot stars that determine the structure of the spiral arms. The radiation of neutral hydrogen is long-wavelength, is in the radio range, and for it the interstellar dusty matter is transparent. The 21-centimeter radiation comes from the most distant regions of the Galaxy without distortion.
The galaxy is constantly changing. These changes are slow and gradual. It is difficult for researchers to find them because human life very short compared to the life of stars and galaxies. Turning to cosmic evolution, one must choose a very long unit of time. Such a unit is the cosmic year, i.e. time full turn Sun around the center of the Galaxy. It is equal to 250 million earth years. The stars of the Galaxy are constantly intermixed, and in one cosmic year, moving even at a low speed of 1 km/s relative to each other, two stars will move away by 250 ps. During this time, some stellar groups may break up, while others may form again. The appearance of the Galaxy will change dramatically. In addition to mechanical changes, the space year changes physical state Galaxies. Stars of classes O and B can shine brightly only for a time equal to some part space year. The age of the brightest observable giants is about 10 million years. However, despite this, the configuration of the helical arms can remain quite stable. Some stars will leave these regions, others will arrive in their place, some stars will die, others will be born from a huge mass of gas-dust complexes of spiral branches. If the distribution of positions and movements of objects in any galaxy is not subject to big changes, then this star system is in a state of dynamic equilibrium. For a certain group of stars, the state of dynamic equilibrium can be maintained for 100 cosmic years. However, over a longer period equal to thousands of cosm. years, the state of dynamic equilibrium will be disturbed due to random close passages of stars. It will be replaced by a dynamically quasi-permanent state of statistical equilibrium, more stable, in which the stars are more thoroughly mixed.
25. Extragalactic astronomy.
25.1 Classification of galaxies and their spatial distribution.
The French comet seekers Messier and Mesham compiled a catalog of nebulous objects observed in the sky in 1784. naked eye or through a telescope in order to further work not to be confused with incoming comets. The objects of the Messier catalog turned out to be of the most diverse nature. Part of them - star clusters and nebulae belong to our Galaxy, the other part - objects more distant and are the same star systems as our Galaxy. Understanding the true nature of galaxies did not come immediately. It wasn't until 1917 that Ritchie and Curtis, observing a supernova in the galaxy NGC 224, calculated that it was at a distance of 460,000 ps, i.e. 15 times the diameter of our Galaxy, which means far beyond its borders. The issue was finally clarified in 1924-1926, when E. Hubble, using a 2.5-meter telescope, obtained photographs of the Andromeda Nebula, where the spiral branches decomposed into individual stars.
Today, a lot of galaxies are known, located at a distance from us from hundreds of thousands to billions of light years. years.
Many galaxies are described and catalogued. The most commonly used is Dreyer's New General Catalog (NGC). Each galaxy has its own number. For example, the Andromeda Nebula is designated NGC 224.
Observation of galaxies has shown that they are very diverse in shape and structure. In appearance, galaxies are divided into elliptical, spiral, lenticular and irregular.
elliptical galaxies(E) have the shape of ellipses in photographs without sharp boundaries. The brightness gradually increases from the periphery to the center. Internal structure usually missing. These galaxies are built from red and yellow giants, red and yellow dwarfs, a certain number of white stars of low luminosity, i.e. mostly from population type II stars. There are no blue-white supergiants, which usually create the structure of spiral arms. Outwardly, elliptical galaxies differ in greater or lesser compression.
The compression indicator is the value
easily found if the large a and small b semiaxes are measured in the photograph. The compression index is added after the letter indicating the shape of the galaxy, for example, E3. It turned out that there are no highly compressed galaxies, so the largest indicator is 7. A spherical galaxy has an indicator of 0.
Obviously, elliptical galaxies have the geometric shape of an ellipsoid of revolution. E. Hubble posed the problem of whether the variety of observed forms is a consequence of the different orientation of equally oblate galaxies in space. This problem was solved mathematically and the answer was obtained that in the composition of galaxy clusters, galaxies with a compression index of 4, 5, 6, 7 are most often found and there are almost no spherical galaxies. And outside the clusters, almost only galaxies with exponents 1 and 0 are found. Elliptical galaxies in clusters are giant galaxies, and outside clusters they are dwarf galaxies.
spiral galaxies(S). They have a structure in the form of spiral branches that emerge from the central core. The branches stand out against a less bright background due to the fact that they contain the hottest stars, young clusters, luminous gaseous nebulae.
Edwin Hubble broke down spiral galaxies into subclasses. The measure is the degree of development of the branches and the size of the core of the galaxy.
In Sa galaxies, the branches are tightly twisted and relatively smooth, and poorly developed. The nuclei are always large, usually about half the observed size of the entire galaxy. Galaxies of this subclass are most similar to elliptical ones. There are usually two branches emerging from opposite parts of the nucleus, but there are rarely more.
In Sb galaxies, the spiral arms are noticeably developed, but do not have branchings. The core is smaller than the previous class. Galaxies of this type often have many spiral arms.
Galaxies with highly developed branches dividing into several arms and a nucleus small in comparison with them belong to the Sc type.
Despite the diversity appearance, spiral galaxies have a similar structure. Three components can be distinguished in them: a stellar disk, the thickness of which is 5-10 times less than the diameter of the galaxy, a spheroidal component, and a flat component, which is several times smaller in thickness than the disk. The flat component includes interstellar gas, dust, young stars, and spiral branches.
The compression ratio of spiral galaxies is always greater than 7. At the same time, elliptical galaxies are always less than 7. This indicates that a spiral structure cannot develop in weakly compressed galaxies. For it to appear, the system must be strongly compressed.
It is proved that a strongly compressed galaxy cannot become weakly compressed during evolution, as well as vice versa. This means that elliptical galaxies cannot turn into spiral ones, and spiral ones into elliptical ones. Different compression due different amount rotation systems. Those galaxies that received a sufficient amount of rotation during formation took a highly compressed shape, spiral branches developed in them.
There are spiral galaxies in which the core is located in the middle of a straight bar and spiral branches begin only at the ends of this bar. Such galaxies are designated SBa, SBb, SBc. The addition of the letter B indicates the presence of a jumper.
lenticular galaxies(S0). Outwardly similar to elliptical, but have a stellar disk. They are similar in structure to spiral galaxies, but differ from them in the absence of a flat component and spiral arms. Lenticular galaxies differ from edge-on spiral galaxies by the absence of a band dark matter. Schwarzschild proposed a theory according to which lenticular galaxies can form from spiral galaxies in the process of sweeping out gas and dust matter.
Irregular galaxies(ir). They have an asymmetrical appearance. They do not have spiral branches, and hot stars and gas-dust matter are concentrated in individual groups or scattered all over the disk. There is a spheroidal component with low brightness. These galaxies are characterized by a high content of interstellar gas and young stars.
The irregular shape of the galaxy may be due to the fact that it did not have time to take correct form because of the low density of matter in it or because of its young age. A galaxy can also become irregular due to shape distortion as a result of interaction with another galaxy.
Irregular galaxies are divided into two subtypes.
The Ir I subtype is characterized by high surface brightness and irregular structure complexity. In some galaxies of this subtype, a destroyed spiral structure is found. Such galaxies often occur in pairs.
Subtype Ir II is characterized by low surface brightness. This property interferes with the detection of such galaxies, and only a few are known. The low surface brightness indicates a low stellar density. This means that these galaxies must very slowly move from irregular shape to the correct one.
In July 1995, a study was conducted on space telescope them. Hubble to search for irregular faint blue galaxies. It turned out that these objects, located at distances from us at distances from 3 to 8 billion light years, are the most common. Most of them have an extremely saturated blue color, which indicates that they are actively undergoing the process of star formation. At close distances corresponding to the modern Universe, these galaxies do not occur.
Galaxies are much more diverse than the considered species, and this diversity concerns shapes, structures, luminosity, composition, density, mass, spectrum, radiation features.
We can distinguish the following morphological types of galaxies, approaching them from different points of view.
Amorphous, structureless systems- including E galaxies and most of S0. They have no or almost no diffuse matter and hot giants.
Haro galaxies- bluer than the others. Many of them have narrow but bright lines in the spectrum. Maybe they are very rich in gas.
Seyfert galaxies - different kind, but characterized by a very large width of strong emission lines in their spectra.
Quasars- quasi-stellar radio sources, QSS, indistinguishable in appearance from stars, but emitting radio waves, like the most powerful radio galaxies. They are characterized by a bluish color and bright lines in the spectrum that have a huge redshift. Supergiant galaxies are superior in luminosity.
Quazagi- QSG quasi-stellar galaxies - differ from quasars in the absence of strong radio emission.
On the celestial sphere during the year due to its movement in space.
The Doppler effect is as follows. Let the wavelength of light received from a stationary source be equal to λ 0. Then from an identical source moving relative to the observer, light will come with a wavelength λ = λ 0 (l + v/c), Where v— speed along the line of sight; c is the speed of light. The radial velocity is positive if the source is moving away from us; in this case, all spectral lines are shifted towards longer wavelengths, i.e., towards the red end of the spectrum.
By photographing the spectrum of a star (or any other object), measuring the wavelengths and comparing them with the wavelengths in the standard spectrum of a stationary source, one can determine its radial velocity.
If somehow it is possible to determine the angle between the directions to the star and the full speed v(and this is sometimes possible, and immediately for a group of stars), then the above formula makes it possible to determine the distances to these stars.
