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NEBELS. Previously, astronomers used this name for any celestial objects that are motionless relative to the stars, which, in contrast to them, have a diffuse, blurry appearance, like a small cloud (the Latin term used in astronomy for "nebula" nebula means "cloud"). Over time, it turned out that some of them, for example, the nebula in Orion, are composed of interstellar gas and dust and belong to our Galaxy. Other "white" nebulae, as in Andromeda and Triangulum, turned out to be gigantic star systems similar to the Galaxy. Here we will talk about gaseous nebulae.
Until the middle of the 19th century. astronomers believed that all nebulae were distant clusters of stars. But in 1860, using the spectroscope for the first time, W. Hoggins showed that some nebulae are gaseous. When light passes through a spectroscope ordinary star, there is a continuous spectrum in which all colors from violet to red are represented; in some parts of the spectrum of the star there are narrow dark absorption lines, but it is rather difficult to notice them - they are visible only in high-quality photographs of the spectra. Therefore, when observed by the eye, the spectrum of a star cluster looks like a continuous color bar. The emission spectrum of a rarefied gas, on the contrary, consists of individual bright lines, between which there is practically no light. This is exactly what Hoggins saw when observing certain nebulae through a spectroscope. More recent observations have confirmed that many nebulae are indeed clouds of hot gas. Often astronomers call "nebulae" and dark diffuse objects - also clouds of interstellar gas, but cold.
types of nebulae.
Nebulae are divided into the following main types: diffuse nebulae, or H II regions, such as the Orion Nebula; reflection nebulae, like the Merope nebula in the Pleiades; dark nebulae, like the Coal Sack, which are usually associated with molecular clouds; supernova remnants like the Reticulum Nebula in Cygnus; planetary nebulae, like the Ring in Lyra.
diffuse nebulae.
Wide notable examples diffuse nebulae - this is the Orion Nebula in the winter sky, as well as the Lagoon and Triple (Triple) - in the summer. The dark lines that cut the Triple Nebula apart are the cold dust clouds that lie in front of it. The distance to this nebula is approx. 2200 St. years, and its diameter is slightly less than 2 St. years. The mass of this nebula is 100 times that of the sun. Some diffuse nebulae, such as Lagoon 30 Doradus and the Orion Nebula, are much larger and more massive.
Unlike stars, gaseous nebulae do not have own source energy; they glow only if there are hot stars with a surface temperature of 20,000–40,000 ° C inside or nearby. These stars emit ultraviolet radiation, which is absorbed by the gas of the nebula and re-emitted by it in the form visible light. Passed through a spectroscope, this light is split into characteristic emission lines various elements gas.
Reflection nebulae.
A reflection nebula is formed when a cloud of light-scattering dust particles is illuminated by a nearby star that is not hot enough to cause the gas to glow. Small reflection nebulae are sometimes seen near forming stars.
Dark nebulae.
Dark nebulae are clouds composed mainly of gas and partly of dust (in a mass ratio of ~ 100:1). In the optical range, they cover the center of the Galaxy from us and are visible as black spots along the entire Milky Way, for example, Big Failure in Swan. But in the infrared and radio ranges, these nebulae radiate quite actively. Some of them are now forming stars. The gas density in them is much higher than in the intercloud space, and the temperature is lower, from - 260 to - 220 ° C. They mainly consist of molecular hydrogen, but other molecules are also found in them up to amino acid molecules.
Supernova remnants.
When an aged star explodes, its outer layers are shed at a rate of approx. 10,000 km/s. This fast-moving substance, like a bulldozer, scoops up interstellar gas in front of it, and together they form a structure similar to the Cygnus Net Nebula. In a collision, moving and stationary substances are heated in a powerful shock wave and glow without additional sources energy. The temperature of the gas in this case reaches hundreds of thousands of degrees, and it becomes a source x-ray radiation. In addition, the interstellar magnetic field increases in the shock wave, and charged particles - protons and electrons - are accelerated to energies much higher than the energy of thermal motion. The movement of these fast charged particles in a magnetic field causes radiation in the radio range, called non-thermal.
The most interesting supernova remnant is the Crab Nebula. In it, the ejected supernova gas has not yet mixed with interstellar matter.
In 1054, an outburst of a star in the constellation Taurus was visible. The picture of the outbreak, reconstructed from the Chinese chronicles, shows that it was a supernova explosion, which at its maximum reached a luminosity 100 million times higher than that of the sun. The Crab Nebula is located just at the site of that outbreak. By measuring the angular size and speed of the expansion of the nebula and dividing one by the other, they calculated when this expansion began - almost exactly 1054 turned out. There is no doubt: the Crab Nebula is the remnant of a supernova.
In the spectrum of this nebula, each line is bifurcated. It is clear that one component of the line, shifted to the blue side, comes from the part of the shell approaching us, and the other, shifted to the red side, comes from the part of the shell moving away. Using the Doppler formula, we calculated the expansion velocity (1200 km / s) and, comparing it with the angular expansion velocity, determined the distance to crab nebula: OK. 3300 St. years.
The Crab Nebula has complex structure: its outer fibrous part radiates individual emission lines characteristic of hot gas; inside this shell amorphous body, whose radiation has a continuous spectrum and is strongly polarized. In addition, powerful non-thermal radio emission comes from there. This can only be explained by the fact that inside the nebula, fast electrons move in a magnetic field, while emitting synchrotron radiation in wide range spectrum - from radio to X-ray. For many years, the source of fast electrons in the Crab Nebula remained mysterious, until in 1968 it was possible to discover a rapidly rotating neutron star in its center - a pulsar, the remnant of a massive star that exploded about 950 years ago. Making 30 revolutions per second and having a huge magnetic field, the neutron star throws streams of fast electrons responsible for the observed radiation into the surrounding nebula.
It turned out that the mechanism of synchrotron radiation is very common among active astronomical objects. In our Galaxy, one can point out many supernova remnants emitting as a result of the motion of electrons in a magnetic field, for example, the powerful radio source Cassiopeia A, with which an expanding fibrous shell is associated in the optical range. A thin jet of hot plasma with a magnetic field is ejected from the core of the giant elliptical galaxy M 87, radiating in all ranges of the spectrum. It is not clear whether active processes in the nuclei of radio galaxies and quasars are associated with supernovae, but physical processes radiation in them are very similar.
planetary nebulae.
The simplest galactic nebulae are planetary. There are about two thousand of them, and in total there are approx. 20,000. They are concentrated in the galactic disk, but do not gravitate, like diffuse nebulae, to spiral arms.
When viewed through a small telescope, planetary nebulae look like fuzzy disks without much detail and therefore resemble planets. Many of them have a blue color near the center. hot star; typical example The Ring Nebula in Lyra. Like diffuse nebulae, their glow comes from the ultraviolet radiation of the star inside.
Spectral analysis.
To analyze spectral composition nebula radiation often use a slitless spectrograph. In the simplest case, a concave lens is placed near the focus of the telescope, which turns the converging beam of light into a parallel one. It is directed to a prism or grating, splitting the beam into a spectrum, and then focusing the light on a photographic plate with a convex lens, while obtaining not one image of the object, but several - according to the number of emission lines in its spectrum. However, the image of the central star is stretched into a line, since it has a continuous spectrum.
In spectra gas nebulae lines of all essential elements: hydrogen, helium, nitrogen, oxygen, neon, sulfur and argon. Moreover, as elsewhere in the universe, hydrogen and helium are much more than others.
The excitation of hydrogen and helium atoms in a nebula does not occur in the same way as in a laboratory gas-discharge tube, where a stream of fast electrons, bombarding atoms, transfers them to a higher energy state, after which the atom returns to normal condition, emitting light . There are no such energetic electrons in the nebula that could excite an atom with their impact, i.e. "throw" its electrons into higher orbits. In the nebula, the "photoionization" of atoms occurs by the ultraviolet radiation of the central star, i.e. the energy of the incoming quantum is enough to completely tear off the electron from the atom and let it “free flight”. On average, 10 years pass until a free electron meets an ion, and they recombine (recombine) into a neutral atom, releasing binding energy in the form of light quanta. Recombination emission lines are observed in the radio, optical and infrared spectral ranges.
The strongest emission lines in planetary nebulae belong to oxygen atoms that have lost one or two electrons, as well as to nitrogen, argon, sulfur, and neon. Moreover, they emit such lines that are never observed in their laboratory spectra, but appear only under conditions characteristic of nebulae. These lines are called "forbidden". The fact is that the atom is usually located in excited state less than a millionth of a second, and then goes back to normal, emitting a quantum. However, there are some energy levels between which the atom makes transitions very "reluctantly", remaining in an excited state for seconds, minutes and even hours. During this time, under conditions of a relatively dense laboratory gas, an atom necessarily collides with a free electron, which changes its energy, and the transition is excluded. But in an extremely rarefied nebula, an excited atom does not collide with other particles for a long time, and, finally, a "forbidden" transition occurs. That is why the forbidden lines were first discovered not by physicists in laboratories, but by astronomers, observing nebulae. Since these lines were not in the laboratory spectra, for some time it was even believed that they belonged to an element unknown on Earth. They wanted to call him "nebulium", but the misunderstanding was soon cleared up. These lines are visible in the spectra of both planetary and diffuse nebulae. The spectra of such nebulae also contain a weak continuous emission arising from the recombination of electrons with ions.
On spectrograms of nebulae obtained with a slit spectrograph, the lines often look broken and split. This is the Doppler effect, indicating the relative movement of parts of the nebula. Planetary nebulae usually expand radially from the central star at a speed of 20–40 km/s. The shells of supernovae expand much faster, exciting a shock wave in front of them. In diffuse nebulae, instead of a general expansion, turbulent (chaotic) movement of individual parts is usually observed.
An important feature of some planetary nebulae is the stratification of their monochromatic radiation. For example, the emission of singly ionized atomic oxygen (having lost one electron) is observed in a vast region, at a great distance from the central star, while doubly ionized (i.e., having lost two electrons) oxygen and neon are visible only in the inner part of the nebula, while four-fold ionized neon or oxygen are noticeable only in its central part. This fact is explained by the fact that the energetic photons necessary for stronger ionization of atoms do not reach the outer regions of the nebula, but are absorbed by the gas not far from the star.
The chemical composition of planetary nebulae is very diverse: the elements synthesized in the interior of the star, some of them turned out to be mixed with the substance of the ejected shell, while others did not. The composition of supernova remnants is even more complicated: the matter ejected by the star is largely mixed with interstellar gas and, in addition, different fragments of the same remnant sometimes have a different chemical composition (as in Cassiopeia A). Probably, this substance is ejected from different depths of the star, which makes it possible to test the theory of stellar evolution and supernova explosions.
Origin of nebulae.
Diffuse and planetary nebulae have completely different origins. Diffuse ones are always found in star-forming regions - usually in the spiral arms of galaxies. They are usually associated with large and cold gas and dust clouds in which stars form. A bright diffuse nebula is a small piece of such a cloud heated by a nearby hot massive star. Since such stars form infrequently, diffuse nebulae do not always accompany cold clouds. For example, there are such stars in Orion, so there are some diffuse nebulae, but they are tiny compared to the invisible dark cloud that occupies almost the entire constellation of Orion. There are no bright hot stars in the small star-forming region in Taurus, and therefore no noticeable diffuse nebulae (there are only a few faint nebulae near active young T Tauri stars).
Planetary nebulae are shells dropped by stars on final stage their evolution. A normal star shines due to flowing in its core thermonuclear reactions that convert hydrogen into helium. But when the hydrogen reserves in the core of a star are depleted, rapid changes occur to it: the helium core contracts, the shell expands, and the star turns into a red giant. Usually these are variable stars like Mira Ceti or OH / IR with huge pulsating shells. They eventually shed the outer parts of their shells. The unenveloped inner part of the star has a very high temperature, sometimes over 100,000 ° C. It gradually contracts and turns into a white dwarf, devoid of a nuclear source of energy and slowly cooling. Thus, planetary nebulae are ejected by their central stars, while diffuse nebulae such as the Orion Nebula are material that was left unused in the process of star formation.
- this is types of nebulae. They are beautiful, majestic, bewitching, and despite the fact that they are difficult to detect with a telescope, observing enthusiasts spend a lot of time looking for them. They are unique, each is not like the other. The dimensions in space are relatively small and are removed from us by small distances (in terms of astronomical values). They consist mainly of hydrogen - 90% and helium - 9.9%. We will not consider the belonging to one or another of each of the nebulae within the framework of this article, our task is different. And let me no longer rant, but proceed directly to the point.
1. Diffuse nebula
Diffuse Lagoon Nebula
Diffuse nebulae, unlike stars, do not have their own source of energy. The glow inside them is due to the hot stars that are inside or next to it. Such nebulae are more common on the "branches" of galaxies, where active star formation occurs and are a substance that has not been included in the composition of the star.
Diffuse nebulae are predominantly red in color - this is due to the abundance of hydrogen inside them. Green and blue colors tell us about other chemical elements such as helium, nitrogen, heavy metals.
These nebulae include the most popular and accessible for observation in devices with a small increase - Orion Nebula in the constellation of Orion, which I mentioned in the article.
Diffuse nebulae are often called emission.
2. Reflection Nebula
Reflection Nebula "Witch's Head"
The reflection nebula does not emit any own light. It is a cloud of gas and dust that reflects light from nearby stars. As well as diffuse nebulae, reflection nebulae are located in regions of active star formation. To a greater extent, they have a bluish tint, because. it spreads better than the others.
Today, not many nebulae of this type are known - about 500.
Some sources do not distinguish the reflection nebula separately, but classify it as a diffusion nebula.
3. Dark Nebula
Dark Nebula "Horsehead"
Such a nebula occurs due to the overlap of light from objects located behind it. This is a cloud. The composition is almost identical to the previous reflecting nebula, differing only in the location of the light source.
As a rule, a dark nebula is observed together with a reflective or diffuse nebula. Great example in the photo above. "Horse Head"- here the dark region blocks the light from the much larger diffuse nebula behind it. In an amateur telescope, such nebulae will be extremely difficult or almost impossible to see. However, in the radio range, even such nebulae actively radiate electromagnetic waves.
4 Planetary Nebula
Planetary nebula M 57
Perhaps the most beautiful type of nebulae. As a rule, such a nebula is the result of the end of the life of a star, i.e. its explosion and scattering of gas into outer space. Despite the fact that the star explodes, it is called planetary. This is due to the fact that when observed, such nebulae look like planets. Most of them are round or oval in shape. The shell of gas located inside is illuminated by the remnants of the star itself.
In total, about two thousand planetary nebulae have been discovered, although there are more than 20,000 of them in our Milky Way galaxy alone.
5 Supernova Remnant
Crab Nebula M 1
Supernova- this is a sharp increase in the brightness of a star as a result of its explosion and ejection huge amount energy to the outer space environment.
The photo above shows great example the explosion of a star in which the ejected gas has not yet mixed with interstellar matter. Based on Chinese chronicles, this explosion was captured in 1054. But we must understand that the distance to the Crab Nebula is about 3300 light years.
That's all. There are 5 types of nebulae that you need to know and be able to recognize. I hope I managed to convey the information to you in an accessible form and in simple language. If you have any questions - ask, write in the comments. Thank you.
Watching from the depths of space mysterious objects a long time ago attracted the interest of people watching the sky. Even the ancient Greek scientist Hipparchus in his catalog noted the presence of several foggy objects in the night sky. His colleague Ptolemy added five more nebulae to the list. In the 17th century, Galileo invented the telescope and with its help he was able to see the nebulae of Orion and Andromeda. Since then, as telescopes and other instruments have improved, new discoveries have begun in outer space. And nebulae were classified as a separate class of stellar objects.
Over time, there were a lot of known nebulae. They began to interfere with scientists and astronomers in search of new objects. AT late XVIII century, studying certain objects - comets, Charles Messier compiled a "catalog of diffuse stationary objects" that looked like comets. But due to the lack of sufficient technical support, this catalog includes both nebulae and galaxies, along with globular star clusters.
Just as telescopes improved, so did astronomy itself. The concept of "nebula" took on new colors and was constantly refined. Some types of nebulae were identified as star clusters, some were classified as absorbing, and in the 20s of the last century, Hubble was able to establish the nature of nebulae and highlight regions of galaxies.
The portal site will tell about theories of the origin of nebulae, their approximate number, types and distance from our planet. The portal operates purely scientifically proven facts and the most popular ideas.
Classification and types of nebulae on the portal website
The primary principle by which nebulae are classified is whether they absorb or scatter (emit) light. This criterion divides nebulae into light and dark. The radiation of light depends on their origin. And the sources of energy that excite their radiation depend on their own nature. Very often, not one, but two radiation mechanisms can operate in a nebula. Dark ones can only be seen through the absorption of radiation sources located behind them.
But if the first principle of classification is accurate, then the second (the division of nebulae into dusty and gaseous) is a conditional principle. Every nebula contains dust and gas. This division is due to different mechanisms of radiation and methods of observation. The presence of dust is best observed when the radiation is absorbed by dark nebulae, which are located behind the sources. The intrinsic radiation of the gaseous components of a nebula is seen when it is ionized by ultraviolet light or when the interstellar medium is heated. The latter process is possible after a wave hits it, which was formed after the explosion of a supernova.
The dark nebula is represented as a dense, most often molecular cloud of interstellar dust and gas. By absorbing light, the cloud becomes opaque. Most often, dark nebulae are seen against a background of light ones. It is extremely rare for scientists to notice them against the background of the Milky Way. They are called giant globules.
The absorption of light Av in the dark ones varies within wide limits. It can reach indicators: from 1–10 m to 10–100 m. The structure of nebulae with high absorption can only be studied using the methods of submillimeter astronomy and radio astronomy, when observing infrared radiation and molecular radio lines. Individual seals are often found in the nebula itself, with an Av value of up to 10,000 m. According to the theories of advanced astrophysicists, stars form there.
In the translucent parts of nebulae, a fibrous structure is clearly visible in the optical range. The general elongation and fibers are associated with the presence of magnetic fields, which hinder the movement of matter across magnetohydrodynamic instabilities and field lines. This connection is due to the fact that the dust particles are charged with electricity.
Another bright type Nebulae is a reflection nebula. These are gas and dust clouds illuminated by stars. If the stars are located in or near an interstellar cloud, but are not very hot in order to reduce the amount of hydrogen around them, then the main source optical radiation the nebula itself becomes the light of stars scattered by interstellar dust. A striking example a similar phenomenon is found around the stars of the Pleiades.
Most of the reflection nebulae are located near the plane of the Milky Way. In some cases, the presence of such nebulae is observed at high galactic latitudes. These molecular clouds are different sizes, shape, density and mass and are illuminated by the combined radiation of the stars of the Milky Way. They are difficult to study because the surface brightness is very low. Sometimes, appearing on images of galaxies, non-existent details are visible in the photographs - jumpers, tails, etc.
A small part of the reflection nebulae has a comet-like appearance. They are called comets. In the name of such a nebula, as a rule, there is a variable star of the Taurus type. It illuminates the nebula. They are variable in brightness and are small in size, about hundredths of a parsec.
The light echo is the rarest type of reflection nebula. A striking example is the resulting flash new star in the constellation Perseus. This flash illuminated the dust, causing the resulting nebula to be visible for several years. And while in space, she moved at the speed of light. In addition to light echoes, gaseous nebulae are formed after such incidents.
Most reflection nebulae have a fine-fibrous structure, that is, a system of almost parallel filaments. Their thickness can reach several hundredths of a parsec. These filaments result from the penetration of the magnetic field into the flute instability of the nebula. Fibers of dust and gas push apart lines of force in a magnetic field and seep between them.
Dust properties such as albedo, shape, grain orientation, scattering indicator, and size have enabled scientists and astronauts to study the distribution of light polarization and brightness across the surface of reflection nebulae.
Radiation-ionized nebulae are patches of interstellar gas that are highly ionized by stellar radiation. This radiation can also come from other sources. Most of all, such nebulae are studied in regions of ionized hydrogen, as a rule, this is the H II zone. In such zones, the matter is completely ionized. Its temperature is about 104 K. It heats up due to internal ultraviolet radiation. Inside the H II zones, stellar radiation in the Lyman continuum transforms into subordinate serial radiation (corresponding to the Rosseland theorem). Because of this, the spectrum of nebulae contains bright lines of the Belmer series and Lyman-alpha lines.
These nebulae also include zones of ionized carbon - C II. The carbon in them is completely ionized by starlight. Zones C II, as a rule, are located around zones H II. They are produced due to the low ionization potential of carbon compared to hydrogen. They can also form around stars with a high spectral type in the densities of the interstellar medium. Nebulae ionized by radiation also arise around strong X-ray sources. They have more high temperatures than in the H II zones, and a relatively high degree of ionization.
Planetary nebulae are the most common type of emission nebulae. They are created by the outflowing upper atmospheres of stars. Such a nebula glows and expands in the optical range. They were first discovered in the 17th century by Herschel and called them that because of their resemblance to the disks of the planets. But not all planetary nebulae are disk-shaped; some are rounded rings. Inside such nebulae, a fine structure is observed in the form of spirals, jets, and small globules. Such nebulae expand at a speed of 20 km/s, and their mass is equal to 0.1 solar masses. They live for about 10 thousand years.
The portal site provides only verified and up-to-date information. We will take you to mysterious world space. And thanks to astronomers and astrophysicists, nebulae are no longer such a huge mystery as they used to be.
In addition to the usual, long-lived, foggy formations, there are short-term ones created by shock waves. They disappear when the kinetic energy of the moving gas disappears. There are several sources for the occurrence of such shock waves. Most often - this is the result of the explosion of a star. Less often - stellar wind, flashes of new and supernovae. In any case, there is one source of emission similar substance- star. Nebulae of this origin have the shape of an expanding shell or the shape of a sphere. The material released from the explosion may have various speeds from hundreds to thousands of km / s, because of this, the temperature of the gas behind the shock wave reaches not millions, but billions of degrees.
Gas heated to enormous temperatures radiates in the X-ray range as in spectral lines, as well as in the continuous spectrum. It glows weakly in spectral optical lines. Upon encountering the inhomogeneity of the interstellar medium, the shock wave bends around the seals. Inside the seal itself, its own shock wave propagates. It also causes radiation in the lines of the spectrum of the optical range. As a result, bright fibers are created that are perfectly visible in photographs.
The brightest post-shock nebulae are created by supernova explosions. They are called remnants of starbursts. They play an important role in shaping the shape of the interstellar gas. They are characterized by small size, weakness and fragility.
There is another type of nebulae. This type is also created after the appearance of the shock wave. But the main reason is the stellar wind from the Wolf-Rayet stars. Wolf stars have a fairly powerful wind mass flow and outflow velocity. They form medium-sized nebulae with very bright filaments. Comparing them with the remnants of supernova explosions, scientists argue that the radio emission of such nebulae has a thermal nature. The nebulae that are located around the Wolf stars do not live long. Their existence directly depends on the duration of the star's presence in the stage of the Wolf-Rayet star.
Absolutely similar nebulae are found around O-stars. These are very bright hot stars that belong to the spectral class O. They have a strong stellar wind. Unlike the nebulae located around the Wolf-Rayet stars, O-star nebulae are less bright, but have a much larger size and duration of existence.
The most common nebulae are found in star-forming regions. Slow-speed shock waves are created in regions of the interstellar medium. This is where star formation takes place. Such a process entails heating the gas to hundreds and even thousands of degrees, partial destruction of molecules, heating of the dust itself, and excitation of molecular levels. Such shock waves look like elongated nebulae and, as a rule, glow in the infrared range. A striking example of this phenomenon is seen in the constellation of Orion.
Gas and dust nebulae - the palette of the universe
The universe is essentially almost empty space. The stars take up only a tiny fraction of it. However, gas is present everywhere, albeit in very small quantities. It's mostly hydrogen, the lightest chemical element. If you "scoop" with an ordinary tea cup (volume about 200 cm3) matter from interstellar space at a distance of 1-2 light years from the Sun, then it will contain about 20 hydrogen atoms and 2 helium atoms. In the same volume in the usual atmospheric air contains 1022 oxygen and nitrogen atoms. Everything that fills the space between stars inside galaxies is called the interstellar medium. And the main thing that makes up the interstellar medium is the interstellar gas. It is rather evenly mixed with interstellar dust and permeated with interstellar magnetic fields, cosmic rays and electromagnetic radiation.
Stars are formed from the interstellar gas, which in the later stages of evolution again give up part of their matter to the interstellar medium. Some of the stars, when they die, explode as supernovae, throwing back into space a significant proportion of the hydrogen from which they were once formed. But it is much more important that during such explosions a large number of heavy elements formed in the interiors of stars as a result of thermonuclear reactions. Both the Earth and the Sun condensed in interstellar space from a gas enriched in this way with carbon, oxygen, iron, and others. chemical elements. To comprehend the laws of such a cycle, one must know how new generations of stars successively condense from the interstellar gas. Understand how stars form important goal research on interstellar matter.
200 years ago it became clear to astronomers that in addition to planets, stars and occasionally comets, other objects are observed in the sky. These objects, due to their foggy appearance, were called nebulae. French astronomer Charles Messier (1730-1817) was forced to create a catalog of these nebulous objects to avoid confusion when looking for comets. His catalog contained 103 objects and was published in 1784. It is now known that the nature of these objects, first combined in common group called "nebulae", is completely different. The English astronomer William Herschel (1738-1822), observing all these objects, discovered two thousand more new nebulae in seven years. He also singled out a class of nebulae that, from an observational point of view, seemed to him different from the rest. He called them "planetary nebulae" because they bore some resemblance to the greenish disks of the planets. Thus, we will consider the following objects: interstellar gas, interstellar dust, dark nebulae, light nebulae (self-luminous and reflective), planetary nebulae.
About a million years after the expansion began, the universe was still a relatively homogeneous mixture of gas and radiation. There were no stars or galaxies. Stars were formed somewhat later as a result of compression of gas under the influence of its own gravity. This process is called gravitational instability. When a star collapses under the influence of its own gravitational attraction, its inner layers are continuously compressed. This compression leads to heating of the substance. At temperatures above 107 K, reactions begin leading to the formation of heavy elements. Modern chemical composition solar system is the result of thermonuclear fusion reactions occurring in the first generations of stars.
The stage when the substance ejected during the explosion of the Supernova mixes with the interstellar gas and contracts, forming stars again, is the most complex and less understood than all the other stages. First, the interstellar gas itself is heterogeneous, it has a ragged, cloudy structure. Secondly, expanding from great speed The supernova shell sweeps out the rarefied gas and compresses it, increasing the inhomogeneities. Thirdly, already in a hundred years the supernova remnant contains more interstellar gas captured along the way than the matter of the star. In addition, the substance is mixed imperfectly. The picture on the right shows the Cygnus supernova remnant (NGC 6946). It is believed that the fibers are formed by expanding shells of gas. Curls and loops are visible, formed by the luminous gas of the remnant, expanding at a speed of many thousands of kilometers per second. The question may arise, what ends, in the end, the cosmic cycle? Gas reserves are declining. After all, most of the gas remains in low-mass stars that die peacefully and do not eject their matter into the surrounding space. Over time, its reserves will be depleted so much that not a single star can be formed. By then, the Sun and other old stars will have died out. The universe will gradually plunge into darkness. But the ultimate fate of the universe may be different. The expansion will gradually stop and be replaced by contraction. After many billions of years, the universe will shrink again to an unimaginably high density.
interstellar gas
Interstellar gas makes up about 99% of the mass of the entire interstellar medium and about 2% of our Galaxy. The gas temperature ranges from 4 K to 106 K. Interstellar gas also emits in a wide range (from long radio waves to hard gamma radiation). There are areas where the interstellar gas is in a molecular state (molecular clouds) - these are the densest and coldest parts of the interstellar gas. There are regions where the interstellar gas consists of neutral atoms hydrogen (HI regions) and regions of ionized hydrogen (H II zones), which are bright emission nebulae around hot stars.
Compared to the Sun, there are noticeably fewer heavy elements in the interstellar gas, especially aluminum, calcium, titanium, iron, and nickel. Interstellar gas exists in all types of galaxies. Most of it in the wrong (irregular), and least of all in elliptical galaxies. In our Galaxy, the gas maximum is concentrated at a distance of 5 kpc from the center. Observations show that in addition to an ordered movement around the center of the Galaxy, interstellar clouds also have chaotic speeds. After 30-100 million years, the cloud collides with another cloud. Gas-dust complexes are formed. The substance in them is dense enough to prevent the main part of the penetrating radiation from passing to a great depth. Therefore, inside the complexes, the interstellar gas is colder than in interstellar clouds. Complex processes of transformation of molecules, together with gravitational instability, lead to the emergence of self-gravitating clumps - protostars. Thus, molecular clouds should quickly (in less than 106 years) turn into stars. Interstellar gas is constantly exchanging matter with the stars. According to estimates, at the present time in the Galaxy gas passes into stars in the amount of about 5 solar masses per year.
Region M 42 in the constellation Orion, where in our time runs active process star formation. The nebula glows when the gas is heated by hot radiation from nearby bright stars. So, in the process of evolution of galaxies, there is a circulation of matter: interstellar gas -> stars -> interstellar gas, leading to a gradual increase in the content of heavy elements in the interstellar gas and stars and a decrease in the amount of interstellar gas in each of the galaxies. It is possible that in the history of the Galaxy there could be delays in star formation by billions of years.
interstellar dust
small particulate matter scattered in interstellar space are almost evenly mixed with interstellar gas. The sizes of large gas-dust complexes, which we discussed above, reach tens of hundreds of parsecs, and their mass is approximately 105 solar masses. But there are also small dense gas-dust formations - globules ranging in size from 0.05 to several pc and weighing only 0.1 - 100 solar masses. Interstellar dust grains are not spherical and their size is approximately 0.1-1 microns. They are made up of sand and graphite. They are formed in the shells of late red giants and supergiants, the shells of new and supernova stars, in planetary nebulae, near protostars. The refractory core is dressed in a shell of ice with impurities, which in turn is enveloped by a layer atomic hydrogen. Dust grains in the interstellar medium either break up as a result of collisions with each other at velocities greater than 20 km/s, or vice versa, stick together if the velocities are less than 1 km/s. 
The presence of interstellar dust in the interstellar medium affects the radiation characteristics of the studied celestial bodies. Dust particles weaken the light from distant stars, change its spectral composition and polarization. In addition, dust grains absorb ultraviolet radiation from stars and process it into radiation with less energy. This radiation, which eventually became infrared, is observed in the spectra of planetary nebulae, H II zones, circumstellar shells, and Seyfert galaxies. On the surface of dust particles can actively form various molecules. Dust grains are usually electrically charged and interact with interstellar magnetic fields. It is to dust grains that we owe such an effect as cosmic maser radiation. It arises in the shells of late cool stars and in molecular clouds (the H I and H II zones). This effect of amplifying microwave radiation "works" when a large number of molecules are in an unstable excited rotational or vibrational state, and then it is enough for one photon to pass through the medium to cause an avalanche-like transition of molecules to the ground state with a minimum energy. As a result, we see a narrowly directed (coherent) very powerful radio emission stream. The figure shows a water molecule. The radio emission from this molecule comes at a wavelength of 1.35 cm. In addition to it, a very bright maser appears on molecules of interstellar OH hydroxyl at a wavelength of 18 cm. .
dark nebulae
Nebulae are areas of the interstellar medium that are distinguished by their emission or absorption on general background sky. Dark nebulae are dense (usually molecular) clouds of interstellar gas and dust that are opaque due to interstellar absorption of light by the dust. Sometimes dark nebulae are visible directly against the background of the Milky Way. Such, for example, are the "Coal Sack" nebula and numerous globules. In those parts that are translucent for the optical range, the fibrous structure is clearly visible. The filaments and the general elongation of dark nebulae are associated with the presence of magnetic fields in them, which impede the movement of matter across magnetic lines of force.
light nebulae
Reflection nebulae are clouds of gas and dust illuminated by stars. An example of such a nebula is the Pleiades. Light from stars is scattered by interstellar dust. Most reflection nebulae are located near the plane of the Galaxy. Some reflection nebulae have a cometary appearance and are called cometary. At the head of such a nebula is usually a T Tauri variable star that illuminates the nebula. A rare type of reflection nebula is the "light echo" observed after the 1901 nova explosion in the constellation Perseus. A bright flash of a star illuminated the dust, and for several years a faint nebula was observed, spreading in all directions at the speed of light. The image on the left above shows the Pleiades star cluster, with stars surrounded by bright nebulae. If a star that is in or near the nebula is hot enough, then it will ionize the gas in the nebula. Then the gas begins to glow, and the nebula is called self-luminous or a nebula ionized by radiation.
The brightest and most common, as well as the most studied representatives of such nebulae are the zones of ionized hydrogen H II. There are also C II zones where the carbon is almost completely ionized by light from the central stars. The C II zones are usually located around the H II zones in the regions of neutral hydrogen H I. They seem to be nested into each other. Supernova remnants (see image on the right above), nova shells and stellar winds are also self-luminous nebulae, since the gas in them is heated to many million K (behind the shock wave front). Wolf-Rayet stars create a very powerful stellar wind. As a result, nebulae several parsecs in size with bright filaments appear around them. Similar are the nebulae around bright hot stars of spectral types O - Of stars, which also have a strong stellar wind.
planetary nebulae
By the middle of the 19th century, it became possible to give serious evidence that these nebulae belonged to an independent class of objects. The spectroscope appeared. Josef Fraunhofer discovered that the Sun emits a continuous spectrum speckled with sharp absorption lines. It turned out that the spectra of planets have many character traits solar spectrum. The stars also showed a continuous spectrum, however, each of them had its own set of absorption lines. William Heggins (1824-1910) was the first to study the spectrum of a planetary nebula. It was a bright nebula in the constellation Draco NGC 6543. Before that, Heggins had been observing the spectra of stars for a whole year, but the spectrum of NGC 6543 was completely unexpected. The scientist found only one single, bright line. At the same time, the bright Andromeda Nebula showed a continuous spectrum characteristic of the spectra of stars. We now know that the Andromeda Nebula is actually a galaxy, and therefore is made up of many stars. In 1865, the same Heggins, using a higher resolution spectroscope, found that this "single" bright line consisted of three separate lines. One of them was identified with the Balmer line of hydrogen Hb, but the other two, longer wavelengths and more intense, remained unrecognized. They were attributed to a new element - nebulium. It was not until 1927 that this element was identified with the oxygen ion. And the lines in the spectra of planetary nebulae are still called nebular.
Then there was a problem with the central stars of the planetary nebulae. They are very hot, putting planetary nebulae in front of early spectral class stars. However, studies of spatial velocities led to the opposite result. Here are data on the spatial velocities of various objects: diffuse nebulae - small (0 km/s), class B stars - 12 km/s, class A stars - 21 km/s, class F stars - 29 km/s, class G stars - 34 km/s, K-class stars - 12 km/s, M-class stars - 12 km/s, planetary nebulae - 77 km/s. Only when the expansion of planetary nebulae was discovered was it possible to calculate their age. It turned out to be about 10,000 years old. This was the first evidence that perhaps most stars are going through a planetary nebula stage. Thus, a planetary nebula is a system of a star, called the core of the nebula, and a luminous star symmetrically surrounding it. gas envelope(sometimes, several shells). The shell of the nebula and its core are genetically related. Planetary nebulae have an emission spectrum that differs from the emission spectra of galactic diffuse nebulae. to a large extent excitation of atoms. In addition to the lines of doubly ionized oxygen, the C IV, O V, and even O VI lines are observed. The mass of the shell of a planetary nebula is approximately 0.1 of the mass of the Sun. All the variety of forms of planetary nebulae probably arises from the projection of their main toroidal structure onto celestial sphere at different angles.
The shells of planetary nebulae expand into the surrounding space at velocities of 20 - 40 km/s under the action of the internal pressure of hot gas. As the shell expands, it becomes thinner, its luminosity weakens, and eventually it becomes invisible. The cores of planetary nebulae are hot stars of early spectral classes that undergo significant changes during the lifetime of the nebula. Their temperatures are usually 50 - 100 thousand K. The nuclei of old planetary nebulae are close to white dwarfs, but at the same time much brighter and hotter than typical objects of this kind. There are also double stars among the cores. The formation of a planetary nebula is one of the stages in the evolution of most stars. Considering this process, it is convenient to divide it into two parts: 1) from the moment of ejection of the nebula to the stage when the energy sources of the star are basically exhausted; 2) evolution of the central star from main sequence before the ejection of the nebula. The evolution after the ejection of the nebula is fairly well studied, both observationally and theoretically. The earlier stages are much less understood. Especially the stage between the red giant and the ejection of the nebula.
The lowest luminosity central stars are usually surrounded by the largest and therefore oldest nebulae. The image on the left shows the planetary nebula M 27 Dumbbell in the constellation Vulpecula. Let us recall a little the theory of the evolution of stars. When moving away from the main sequence, the most important stage in the evolution of a star begins after the hydrogen in the central regions is completely burned out. Then the central regions of the star begin to shrink, releasing gravitational energy. At this time, the area in which the hydrogen is still burning begins to move outward. Convection occurs. Dramatic changes begin in the star when the mass of the isothermal helium core makes up 10-13% of the star's mass. The central regions begin to shrink rapidly, and the shell of the star expands - the star becomes a giant, moving along the red giant branch. The core, shrinking, warms up. In the end, helium combustion begins in it. After a certain period of time, helium reserves are also depleted. Then the second "ascent" of the star begins along the red giant branch. The stellar core, consisting of carbon and oxygen, is rapidly contracting, and the shell expands to gigantic sizes. Such a star is called an asymptotic giant branch star. At this stage, the stars have two layered sources of combustion - hydrogen and helium, and begin to pulsate.
The rest evolutionary path far less studied. In stars with masses greater than 8-10 solar masses, the carbon in the core eventually ignites. Stars become supergiants and continue to evolve until a core is formed from the "iron peak" elements (nickel, manganese, iron). it central core, probably collapses to form a neutron star, and the envelope is ejected as a supernova. It is clear that planetary nebulae are formed from stars with masses less than 8-10 solar masses. Two facts suggest that the ancestors of planetary nebulae are red giants. First, the stars of the asymptotic branch are physically very similar to planetary nebulae. The core of a red giant is very similar in mass and size to the central star of a planetary nebula, if we remove the extended rarefied atmosphere of the red giant. Secondly, if the nebula is thrown off by a star, then it must have a minimum speed sufficient to escape from gravitational field. Calculations show that only for red giants this speed is comparable with the expansion speeds of the shells of planetary nebulae (10-40 km/s). In this case, the mass of the star is estimated at 1 solar mass, and the radius lies within 100-200 solar radii (a typical red giant). In conclusion, we note that the most likely candidates for the role of the ancestors of planetary nebulae are variable stars like Mira Ceti. Symbiotic stars can be representatives of one of the transitional stages between stars and nebulae. And of course, you can not ignore the object, FG Sge (in the image on the right above). Thus most stars less than 6-10 solar masses eventually become planetary nebulae. In the preceding stages they lose most of their original mass; only a core with a mass of 0.4-1 the mass of the Sun remains, which becomes a white dwarf. Mass loss affects not only the star itself, but also conditions in the interstellar medium and future generations of stars.
Previously, nebulae in astronomy were called any motionless extended luminous astronomical objects, including star clusters or galaxies outside the Milky Way that could not be separated into stars.
For example, the Andromeda Galaxy is often referred to as the "Andromeda Nebula". But now nebula called a section of the interstellar medium, distinguished by its radiation or absorption of radiation against the general background of the sky.
The change in terminology occurred because in the 1920s it became clear that there are many galaxies among the nebulae. With the development of astronomy and the resolution of telescopes, the concept of "nebula" became more and more precise: some of the "nebulae" were identified as star clusters, dark (absorbing) gas and dust nebulae were discovered, and in the 1920s, first Lundmark, and then Hubble, succeeded in consider stars in the peripheral regions of a number of galaxies and thereby establish their nature. After that, the term "nebula" began to be understood more narrowly.
Composition of nebulae: gas, dust and plasma (partially or fully ionized gas formed from neutral atoms (or molecules) and charged particles (ions and electrons).
Signs of nebulae
As mentioned above, the nebula absorbs or emits (scatters) light, so it happens dark or light.
dark nebulae- dense (usually molecular) clouds of interstellar gas and interstellar dust. They are not transparent due to interstellar absorption of light by dust. They are usually seen against the background of light nebulae. Less commonly, dark nebulae are visible directly against the background of the Milky Way. These are the Coal Sack Nebula and many smaller ones called giant globules. The picture shows the Horsehead Nebula (photo by Hubble). Often, individual clumps are found inside dark nebulae, in which stars are thought to form.
reflective nebulae usually have a blue tint because scattering blue color more effective than red (this explains the blue color of the sky). These are gas and dust clouds illuminated by stars. Sometimes the main source of optical radiation of the nebula is the light of stars scattered interstellar dust. An example of such nebulae are the nebulae around bright stars in the Pleiades cluster. Most reflection nebulae are located near the plane of the Milky Way.
Nebulae ionized by radiation- areas of interstellar gas, strongly ionized by the radiation of stars or other sources of ionizing radiation. Nebulae ionized by radiation also appear around powerful X-ray sources in the Milky Way and in other galaxies (including active galactic nuclei and quasars). They are often characterized by higher temperatures and more high degree ionization of heavy elements.
planetary nebulae- these are astronomical objects consisting of an ionized gas shell and a central star, white dwarf. Planetary nebulae are formed during the ejection of the outer layers (shells) of red giants and supergiants with a mass of 2.5-8 solar masses at the final stage of their evolution. A planetary nebula is a fast-moving (by astronomical standards) phenomenon lasting only a few tens of thousands of years, while the lifespan of the ancestor star is several billion years. Currently, about 1500 planetary nebulae are known in our galaxy. Planetary nebulae are mostly dim objects and are generally not visible to the naked eye. First open planetary nebula was the Dumbbell Nebula in the constellation Chanterelles: Charles Messier, who was searching for comets, when compiling his catalog of nebulae (stationary objects similar to comets when observing the sky) in 1764 cataloged it under the number M27, and W. Herschel in 1784 with compiling his catalog, he singled them out as a separate class of nebulae and proposed the term "planetary nebula" for them.
Nebulae created by shock waves. Typically, such nebulae are short-lived, as they disappear after exhaustion. kinetic energy moving gas. The main sources of strong shock waves in the interstellar medium are stellar explosions - ejections of shells during explosions of supernovae and new stars, as well as stellar wind.
Supernova remnants and new stars. The brightest nebulae created by shock waves are caused by explosions supernovae and are called supernova remnants. Along with the described features, they are characterized by nonthermal radio emission. The nebulae associated with the explosions of new stars are small, weak, and short-lived.
Nebulae around Wolf-Rayet stars. The radio emission from these nebulae is of a thermal nature. Wolf-Rayet stars are characterized by a very powerful stellar wind. But the lifetime of such nebulae is limited by the duration of the stay of stars in the Wolf-Rayet star stage and is close to 105 years.
Nebulae around O stars. They are similar in properties to the nebulae around Wolf-Rayet stars, but form around the brightest hot stars. spectral type O - Of, possessing a strong stellar wind. They differ from the nebulae associated with Wolf-Rayet stars by their lower brightness, larger size, and, apparently, longer lifespan.
Nebulae in star-forming regions. Star formation occurs in the interstellar medium, and shock waves arise that heat the gas to hundreds and thousands of degrees. Such shock waves are visible as elongated nebulae, glowing predominantly in the infrared range. A number of such nebulae have been found in the star formation center associated with the Orion Nebula.
The Andromeda Galaxy or the Andromeda Nebula is spiral galaxy, closest to Milky Way big galaxy located in the constellation Andromeda. It is removed from us at a distance of 2.52 million light years. The plane of the galaxy is inclined to us at an angle of 15°, so it is very difficult to determine its structure. The Andromeda Nebula is the brightest nebula in the northern hemisphere of the sky. It is visible to the naked eye, but only as a faint misty speck.
The Andromeda Nebula is similar to our galaxy, but larger. It has studied several hundred variable stars, which are mostly Cepheids. It also contains 300 globular clusters, more than 200 new stars and one supernova.
The Andromeda Nebula is interesting not only because it is similar to our Galaxy, but also because it has four satellites - dwarf elliptical galaxies.
