Some examples of this use have survived to this day. For example, the Andromeda Galaxy is often referred to as the "Andromeda Nebula".
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 and, finally, in the 1920s, first Lundmark, and then Hubble, managed to resolve the peripheral regions of a number of galaxies on the stars and thereby establish their nature. Since that time, the term "nebula" has been used in the above sense.
Nebula types
The primary feature used in the classification of nebulae is their absorption or emission (scattering) of light, that is, according to this criterion, nebulae are divided into dark and light. The first are observed due to the absorption of radiation from sources located behind them, the second - due to their own radiation or reflection (scattering) of light from nearby stars. The nature of the radiation of bright nebulae, the sources of energy that excite their radiation, depend on their origin and can be of a diverse nature; often several radiation mechanisms operate in one nebula.
The division of nebulae into gaseous and dusty ones is largely arbitrary: all nebulae contain both dust and gas. This division is historically different ways observations and emission mechanisms: the presence of dust is most clearly observed when radiation is absorbed by dark nebulae of sources located behind them and when reflected or scattered, or re-emitted by dust contained in the nebula, radiation from nearby or in the nebula itself of stars; own radiation gas component A nebula is observed when it is ionized by ultraviolet radiation from a hot star located in the nebula (H II emission regions of ionized hydrogen around stellar associations or planetary nebulae) or when the interstellar medium is heated by a shock wave due to a supernova explosion or the impact of a powerful stellar wind of Wolf-Rayet type stars.
dark nebulae
Dark nebulae are dense (usually molecular) clouds of interstellar gas and interstellar dust that are opaque due to interstellar dust absorption of light. 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 interstellar absorption of light Av in dark nebulae varies widely, from 1-10 m to 10-100 m in the densest ones. The structure of nebulae with large A v can only be studied by methods of radio astronomy and submillimeter astronomy, mainly from observations of molecular radio lines and from infrared radiation dust. Often inside dark nebulae individual densifications with A v up to 10,000 m are found, in which, apparently, stars are formed.
In those parts of the nebulae that are semitransparent in the optical range, a fibrous structure is clearly visible. The filaments and the general elongation of nebulae are associated with the presence of magnetic fields in them, which impede the movement of matter across the lines of force and lead to the development of a number of types of magnetohydrodynamic instabilities. The dusty component of matter in nebulae is associated with magnetic fields due to the fact that dust grains are electrically charged.
reflection nebulae
Reflection nebulae are clouds of gas and dust illuminated by stars. If the star(s) are in or near an interstellar cloud, but are not hot enough (hot) to ionize a significant amount of interstellar hydrogen around them, then the main source optical radiation nebula turns out to be the light of stars scattered by 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. In a number of cases reflection nebulae are observed at high galactic latitudes. These are gas-dust (often molecular) clouds of various sizes, shapes, densities and masses, illuminated by the combined radiation of stars in the disk of the Milky Way. They are difficult to study due to their very low surface brightness (usually much fainter than the sky background). Sometimes, projected on images of galaxies, they lead to the appearance in photographs of galaxies of details that do not exist in reality - tails, bridges, etc.
Reflection nebula "Angel" is located at a height of 300 pc above the plane of the galaxy
Some reflection nebulae have a cometary appearance and are called cometary. In the "head" of such a nebula is usually a T Tauri variable star that illuminates the nebula. Such nebulae often have variable brightness, tracking (with a delay by the time of light propagation) the variability of the radiation of the stars illuminating them. The sizes of cometary nebulae are usually small - hundredths of a parsec.
A rare reflection nebula is the so - called light echo observed after the 1901 nova outburst in the constellation Perseus . Bright flash new star illuminated the dust, and for several years a faint nebula was observed, spreading in all directions at the speed of light. In addition to the light echo after the outbreaks of new stars, gas nebulae are formed, similar to the remnants of supernovae.
Many reflection nebulae have a fine-fibrous structure, a system of nearly parallel filaments a few hundredths or thousandths of a parsec thick. The origin of the filaments is associated with flute or permutation instability in a nebula pierced by a magnetic field. Fibers of gas and dust push the lines of force apart magnetic field and are introduced between them, forming thin threads.
Studying the distribution of brightness and polarization of light over the surface of reflection nebulae, as well as measuring the dependence of these parameters on wavelength, makes it possible to establish such properties of interstellar dust as albedo, scattering indicatrix, size, shape and orientation of dust grains.
Nebulae ionized by radiation
Nebulae ionized by radiation are regions of interstellar gas highly ionized by the radiation of stars or other sources of ionizing radiation. The brightest and most widespread, as well as the most studied representatives of such nebulae, are regions of ionized hydrogen (H II zones). In the H II zones, the matter is almost completely ionized and heated to a temperature of ~10 4 K by the ultraviolet radiation of the stars inside them. Inside the HII zones, all radiation from the star in the Lyman continuum is processed into radiation in the lines of subordinate series, in accordance with the Rosseland theorem. Therefore, in the spectrum of diffuse nebulae, there are very bright lines of the Balmer series, as well as the Lyman-alpha line. Only rarefied H II zones of low density are ionized by the radiation of stars, in the so-called. coronal gas.
Nebulae ionized by radiation also include the so-called zones of ionized carbon (zones C II), in which carbon is almost completely ionized by the light of the central stars. C II zones are usually located around H II zones in neutral hydrogen (HI) regions and manifest themselves in carbon recombination radio lines similar to hydrogen and helium recombination radio lines. C II zones are also observed in the C II infrared line (λ = 156 µm). Zones C II are characterized by low temperature 30-100 K and a low degree of ionization of the medium as a whole: N e /N< 10 −3 , где N e и N концентрации электронов и атомов. Зоны C II возникают из-за того, что потенциал ионизации углерода (11,8 эВ) меньше, чем у водорода (13,6 эВ). Излучение звёзд с энергией E фотонов 11,8 эВ E 13,6 эВ (Å) выходит за пределы зоны H II в область H I, сжатую ионизационным фронтом зоны H II, и ионизует там углерод. Зоны C II возникают также вокруг звёзд спектральных классов B1-B5, находящихся в плотных участках межзвёздной среды. Такие звёзды практически не способны ионизовать водород и не создают заметных зон H II.
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 more high temperatures than in H II zones, and more high degree ionization of heavy elements.
planetary nebulae
A variety of emission nebulae are planetary nebulae formed by the upper outflowing layers of stellar atmospheres; usually it is a shell shed by a giant star. The nebula expands and glows in the optical range. The first planetary nebulae were discovered by W. Herschel around 1783 and so named for their resemblance to planetary disks. However, not all planetary nebulae are disk-shaped: many are ring-shaped or symmetrically elongated along a certain direction (bipolar nebulae). Inside them is noticeable fine structure in the form of jets, spirals, small globules. The expansion rate of planetary nebulae is 20-40 km/s, diameter is 0.01-0.1 pc, typical mass is about 0.1 solar mass, lifetime is about 10 thousand years.
Nebulae created by shock waves
The diversity and multiplicity of sources of supersonic motion of matter in the interstellar medium lead to a large number and variety of nebulae created by shock waves. Typically, such nebulae are short-lived, as they disappear after the kinetic energy of the moving gas is exhausted.
The main sources of strong shock waves in the interstellar medium are stellar explosions - ejections of shells during outbursts of supernovae and new stars, as well as stellar wind (as a result of the action of the latter, so-called stellar wind bubbles are formed). In all these cases, there is a point source of substance ejection (star). The nebulae created in this way have the form of an expanding shell, close to spherical in shape.
The ejected matter has velocities of the order of hundreds and thousands of km/s, so the temperature of the gas behind the shock wave front can reach many millions and even billions of degrees.
A gas heated to a temperature of several million degrees emits mainly in the X-ray range, both in the continuous spectrum and in spectral lines. It glows very weakly in optical spectral lines. When the shock wave encounters inhomogeneities in the interstellar medium, it bends around the seals. A slower shock wave propagates inside the seals, causing radiation in the spectral lines of the optical range. The result is bright fibers that are clearly visible in photographs. The main shock front, compressing the clot of interstellar gas, sets it in motion in the direction of its propagation, but at a slower speed than that of the shock wave.
Supernova remnants and new stars
The brightest nebulae created by shock waves are caused by supernova explosions and are called flare remnants. supernovae. They play very important role in the formation of the structure of interstellar gas. Along with the described features, they are characterized by nonthermal radio emission with a power-law spectrum caused by relativistic electrons accelerated both during the supernova explosion and later by the pulsar, which usually remains after the explosion. The nebulae associated with nova explosions are small, faint, and short-lived.
Nebulae around Wolf-Rayet stars
Thor's helmet - the nebula around Wolf's star - Rayet
Another type of nebulae created by shock waves is associated with stellar wind from Wolf-Rayet stars. These stars are characterized by a very powerful stellar wind with a mass flux per year and an outflow velocity of 1·10 3 -3·10 3 km/s. They create nebulae a few parsecs in size with bright filaments at the edge of the astrosphere of such a star. Unlike the remnants of supernova explosions, the radio emission of these nebulae is of a thermal nature. 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 10 5 years.
Nebulae around O stars
Similar in properties to nebulae around Wolf-Rayet stars, but are formed around the brightest hot stars spectral type O - Of, possessing a strong stellar wind. They differ from the nebulae associated with the Wolf-Rayet stars by their lower brightness, larger size, and, apparently, longer life span.
Nebulae in star forming regions
The Orion A Nebula is a giant star forming region
Shock waves of lower velocities arise in regions of the interstellar medium in which star formation occurs. They lead to gas heating to hundreds and thousands of degrees, excitation molecular levels, partial destruction of molecules, heating of dust. Such shock waves are seen as elongated nebulae that glow predominantly in the infrared range. A number of such nebulae have been discovered, for example, in the star formation center associated with the Orion Nebula.
July 31st, 2010
Nebulae. Part I
Nebulae. Previously, astronomers used this name for any celestial objects that are stationary relative to the stars, which, in contrast to them, have a diffuse, blurred appearance, like a small cloud (the Latin term used in astronomy for "nebula" is the Latin term 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. Therefore, scientists came to the conclusion that nebula - an interstellar cloud, consisting of dust, gas and plasma, emitted by its radiation or absorption in comparison with the surrounding interstellar medium.
Nebula types . 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.
This is NGC 2174, a bright nebula in the constellation Orin.
NGC 2237 - emission nebula in the constellation Unicorn. It is an area of ionized hydrogen where star formation processes take place.
Crescent Nebula. Or another name - NGC 6888 (another designation - LBN 203) - an emission nebula in the constellation Cygnus.
The Medusa Nebula, usually subtle and dim, is captured in this beautiful false-color telescopic image. In the sky, the nebula is located at the feet of the celestial Gemini, and on its sides are the stars μ and η Gemini. The Medusa Nebula itself in the picture is at the bottom right. It is like a luminous crescent of emission gas with dangling tentacles. The Medusa Nebula is part of supernova remnant IC 443, an expanding bubble left over from the explosion. massive star. The first light from that explosion reached the Earth 30,000 years ago. Like its cosmic sea-floating sister, the Crab Nebula, the remnant of IC 443 is home to neutron star the collapsed core of a star. The Medusa Nebula is 5,000 light-years away. The image covers an area of 300 light years. The rest of the field in the image is occupied by the emission nebula Sharpless 249.
The nebula in the constellation Toucan or NGC 346 belongs to the emission class, that is, it is a cloud of hot gas and plasma. Its length is about 200 light years. The reason for the high temperature of NGC 346 is a large number of young stars in the region. Most of the stars are only a few million years old. For comparison, the age of the Sun is about 4.5 billion years.
crab nebula(M1, NGC 1952, col. "Crab") - a gaseous nebula in the constellation Taurus, which is the remnants of a supernova. Located at a distance of about 6500 light years from Earth, has a diameter of 6 light years and is expanding at a speed of 1000 km / s. At the center of the nebula is a neutron star.
NGC 1499 (also known as LBN 756, California Nebula) is an emission nebula in the constellation Perseus. It has a reddish color, and in shape resembles the outlines of the US state of California. The length of the nebula is about 100 light years, the distance from Earth is 1500 light years.
The Veil Nebula, also known as the Loop Nebula or the Fishing Net Nebula, is a diffuse nebula in the constellation Cygnus, a huge and relatively dim supernova remnant. The star exploded about 5000-8000 years ago, during which time the nebula covered an area of 3 degrees in the sky. The distance to it is estimated at 1400 light years. This nebula was discovered on September 5, 1784 by William Herschel.
One of several "dust columns" of the Eagle Nebula, which can be seen as an image mythical creature. It is about ten light years across.
The Eagle Nebula (also known as Messier Object 16, M16 or NGC 6611) is a young open star cluster in the constellation Serpens.
Dust columns in which new stars form in the Eagle Nebula. The picture was taken with Hubble telescope.
NGC 281 (other designations - IC 11, LBN 616) is an emission nebula in the constellation Cassiopeia. It is an area of ionized hydrogen, where processes of active star formation take place. It is located at a distance of about 10 thousand light years from Earth. For its shape, the nebula was named the Pac-Man Nebula in honor of the character of the arcade computer game of the same name. The nebula fluoresces with red light when exposed to ultraviolet irradiation, the source of which is the hot young stars of the open cluster IC 1590. Dark dust structures are also present in the nebula.
You see a known shape in an unknown place! This emission nebula is widely known because it looks like one of the continents of the planet Earth - North America. To the right of the nebula North America, which is also designated NGC 7000, is the less bright Pelican Nebula. These two nebulae are about 50 light-years across and are about 1,500 light-years away from us. They are separated by a dark absorbing cloud.
The Orion Nebula (also known as Messier 42, M42 or NGC 1976) is a luminous greenish emission nebula located below Orion's Belt. It is the brightest diffuse nebula. " Great Nebula Orion, along with the Andromeda Nebula, the Pleiades and the Magellanic Clouds, is one of the famous objects deep space. This is perhaps the most attractive winter object in the northern sky for astronomy lovers. Few astronomical views are as exciting as this nearby stellar nursery known as the Orion Nebula. The glowing gas of the nebula surrounds hot young stars at the edge of a huge interstellar molecular cloud just 1,500 light-years away.
The Dumbbell Nebula (also known as Messier Object 27, M27, or NGC 6853) is a planetary nebula in the constellation Vulpecula, located 1250 light-years from Earth. Its age is estimated at 3,000 to 4,000 years. This planetary nebula is one of the most remarkable objects for amateur observations. M27 is large, relatively bright and easy to find. This photograph was taken on a computer using the narrow-band imaging method, when images taken by telescopes in different wavelength ranges are combined: visible, infrared, ultraviolet, etc.
The Eskimo Nebula was discovered by astronomer William Herschel in 1787. If you look at the NGC 2392 nebula from the surface of the Earth, then it looks like a human head, as if in a hood. If you look at the nebula from space, as the space telescope did. Hubble in 2000, after the upgrade, it is a gas cloud of the most complex internal structure, over the structure of which scientists are still scratching their heads. The Eskimo Nebula belongs to the class of planetary nebulae, i.e. is a shell that 10 thousand years ago were the outer layers of a star like the Sun. The inner shells seen in the picture today have been blown out. powerful wind from a star at the center of the nebula. The "hood" consists of many relatively dense gaseous filaments, which, as shown in the picture, glow orange in the nitrogen line. The Eskimo Nebula is located at a distance of 5 thousand light years from us, and it can be detected with a small telescope in the direction of the constellation of the Gemini constellation.
Against the backdrop of a scattering of stars in the central part of the Milky Way and in the famous constellation of Ophiuchus, dark nebulae writhe. The S-shaped dark feature at the center of this wide-field image is called the Serpens Nebula.
The Carina Nebula is located in southern constellation Kiel is at a distance of 6500-10000 St. from us. years. It is one of the brightest and largest diffuse nebulae in the sky. It has many massive stars and active star formation. This nebula contains an unusually high concentration of young, massive stars, the result of explosive star formation some 3 million years ago. The nebula contains more than a dozen large stars, the mass of which is 50-100 times the mass of our Sun. The brightest of them - Karina - in the near future should end its existence with a supernova explosion.
Blown by the wind from a massive star, this interstellar vision has a surprisingly familiar shape. Cataloged as NGC 7635, it is better known simply as the Bubble Nebula. Although this bubble, 10 light-years in diameter, looks elegant, it is evidence of very violent processes at work. Above and to the right of the center of the bubble is a bright, hot star Wolf-Rayet, the mass of which is from 10 to 20 times more mass Sun. A strong stellar wind and powerful radiation from the star formed this structure from the glowing gas in the surrounding molecular cloud. The attention-grabbing Bubble Nebula lies just 11,000 light-years away in the constellation of Cassiopeia.
In the pictures: the region of the Trapezium cluster in the Orion Nebula, named after four the brightest stars, forming something close to a trapezoid. The left picture was taken in visible light, right - in infrared. In the left picture, only ordinary stars not obscured by dust clouds. Added to the right are stars inside gaseous dust clouds and about 50 faint objects called "brown dwarfs".
Based on materials from Astronet, Wikipedia and Spiritual and Philosophical Forum A108.
The great astronomer of the 18th century, William Herschel, who discovered the planet Uranus, was also famous for the first deep study of the world of nebulae. He divided them into classes, in particular, singling out among them the so-called "planetary nebulae". Herschel proposed this name solely because of their superficial resemblance to the planet Uranus. Small and dim, planetary nebulae reminded astronomers of the past the disk of a distant planet.
Much later, scientists discovered physical nature these objects. The origin of planetary nebulae was first explained in the 1950s by the Soviet astrophysicist I. S. Shklovsky. It turned out that planetary nebulae are generated by dying stars. In the process of turning into a white dwarf, stars shed outer layers into space, which are ionized by ultraviolet radiation and re-emit photons in the optical range. AT recent times it turned out that many planetary nebulae have a very complex structure. This is especially evident in photographs taken with the Hubble telescope.
By astronomical standards, planetary nebulae are very short-lived phenomena: their lifetime is about ten thousand years. Therefore, astronomers know no more than one and a half thousand such objects in our galaxy. We bring to your attention 34 of the most interesting of them.
Variety of planetary nebulae
The magnificent planetary nebula "Snail" is one of the brightest and most beautiful. In the New general directory nebulae, it is numbered 7293. Photo: NASA, ESA, C.R. O "Dell (Vanderbilt University), M. Meixner and P. McCullough (STScI)
The Cat's Eye Nebula, NGC 6543: fantastic gas and dust sculptures photographed by the Hubble Space Telescope. Photo: NASA, ESA, HEIC, and The Hubble Heritage Team (STScI/AURA)
Another false color photo of NGC 6543. The Cat's Eye Nebula is about 1000 years old. Its shape may indicate that it was formed from a binary star system. Photo: J.P. Harrington, K.J. Borkowski (University of Maryland) / NASA
The famous planetary nebula M57 in the constellation Lyra, or the Ring Nebula. Images like this show the complex structure of the nebula. Photo: The Hubble Heritage Team (STScI/AURA/NASA)
Another famous example planetary nebula - object MyCn18, an "hourglass" around a dying star. Photo: Raghvendra Sahai / John Trauger (JPL) / WFPC2 science team / NASA
The Medusa Nebula is a very old planetary nebula. It is located about 1500 light years from Earth in the constellation Gemini. Photo: H. Schweiker/NOAO/AURA/NSF/T. A. Rector/University of Alaska Anchorage
The nebula NGC 3132 is a lake of light. Photo: The Hubble Heritage Team (STScI/AURA/NASA)
The planetary nebula Abell 39 is almost perfectly spherical. Its diameter is almost 5 light years, and the thickness of the walls is a third light year. The Abell 39 nebula lies 7,000 light-years from Earth in the constellation Hercules. Photo: WIYN/NOAO/NSF
When a star dies, it sheds its outer layers, which, dissipating in space, form a planetary nebula. Such nebulae are called planetary nebulae solely because in small telescopes they look like tiny and dim disks. Previously, many astronomers took them for distant planets, hence the name. But big and modern instruments show astronomers many interesting details. NGC 6369 is another example of a magnificent planetary nebula with rich structure. Photo: NASA / The Hubble Heritage Team (STScI/AURA)
The planetary nebula "Dumbbell" in the constellation Vulpecula is one of the brightest objects of its kind. The nebula was first discovered by the French astronomer Charles Messier, who included it in his catalog of nebulous objects at number 27. The distance to M27 is known only approximately and is about 1200 light years. Photo: ESO
Planetary nebula NGC 2346. Credit: NASA/The Hubble Heritage Team (AURA/STScI).
One of recent photos space telescope them. Hubble Nebula "Necklace". Photo: NASA, ESA, Hubble Heritage Team (STScI/AURA)
Eskimo Nebula or NGC 2392. Photo: NASA / Andrew Fruchter / ERO Team
The Spirograph Nebula (IC 418). Photo: NASA / The Hubble Heritage Team (STScI/AURA)
The Jones 1 Nebula, also known as PK 104-29.1, is a very faint, ghost-like nebula in the constellation Pegasus. This image was taken in 2009 by the Mayall Telescope. Photo: T.A. Rector/University of Alaska Anchorage, H. Schweiker/WIYN and NOAO/AURA/NSF
Planetary nebula "Turtle", NGC 6210. Photo: NASA
The Electric Ray Nebula or Hen-1357 is the youngest known planetary nebula. Photo: Matt Bobrowsky (Orbital Sciences Corporation) / NASA
Young planetary nebula Hen 1357. Credit: Matt Bobrowsky (CTA INCORPORATED) / NASA
The very unusual planetary nebula Sharpless 2-188 (Sh2-188). Having an almost spherical shape, the nebula glows unevenly. The brighter glow of the southeastern part (lower left) is due to the collision of gas with interstellar matter, which generated this shock wave. It is in this direction that the dead star that gave rise to the nebula is moving. Sharpless 2-188 is located in the constellation Cassiopeia. Photo: T.A. Rector/University of Alaska Anchorage, H. Schweiker/WIYN and NOAO/AURA/NSF
Twisted like a spiral galaxy, planetary nebula K 4-55. Photo: NASA / ESA / Hubble Heritage Team (STScI / AURA)
The object Mz 3 is the planetary nebula "Ant". Image from the Hubble telescope. Photo: NASA / ESA / The Hubble Heritage Team (STScI / AURA)
The silent cosmic flames of a dying star: the planetary nebula NGC 6302. Photo: NASA / ESA / Hubble SM4 ERO Team
Scattered light from the Boomerang Nebula. In 1995, astronomers using the Hubble telescope measured the temperature of matter inside this nebula. It turned out that the substance of the nebula is only 1 degree warmer than the point absolute zero. The Boomerang Nebula is one of the coldest places in the universe. Photo: NASA / ESA / The Hubble Heritage Team (STScI / AURA)
Nebula NGC 7662 or Blue Snowball. Photo: Volker Wendel, Josef Pöpsel, Stefan Binnewies
planetary nebula Soap bubble". The object PN G75.5+1.7 was found on July 6, 2008 by amateur astronomer Dave Jurasevich. This image was taken with the Kitt Peak Observatory's 4-meter telescope. Photo: T. A. Rector/University of Alaska Anchorage, H. Schweiker/WIYN and NOAO/AURA/NSF
Planetary nebula NGC 5307 as seen by the Hubble Space Telescope. Photo: NASA / ESA / The Hubble Heritage Team (STScI / AURA)
The planetary nebula M76 "Little Dumbbell" in the constellation Perseus. This photo, taken with a 60 cm telescope in Greece, shows that at the center of the nebula is double star. Photo: Stefan Heutz, Stefan Binnewies, Josef Pöpsel
Nebula He 2-47. Photo: NASA / ESA / The Hubble Heritage Team (STScI / AURA)
Distant planetary nebula NGC 6894 in the constellation Cygnus. Photo: Volker Wendel, Stefan Binnewies, Josef Pöpsel
NGC 3242 or "Ghost of Jupiter" is a planetary nebula in the constellation Hydra. Photo: Rainer Sparenberg, Stefan Binnewies, Volker Robering
The planetary nebula NGC 6781 in the constellation Aquila is a favorite subject for astrophotographers. Photo: Adam Block/Mount Lemmon SkyCenter/University of Arizona
Planetary nebula NGC 6751. Credit: NASA/The Hubble Heritage Team (STScI/AURA)
Planetary nebula IC 4406 thanks to complex structure called "Retina". Photo: NASA / The Hubble Heritage Team (STScI/AURA)
Nebula NGC 5315. Photo: NASA / ESA / The Hubble Heritage Team (STScI / AURA)
Nebula NGC 6445 photographed in H-alpha rays in the constellation Sagittarius. Photo: Josef Popsel, Beate Behle
In the Universe, in addition to stars, planets and galaxies, there are also diffuse nebulae. Their role in development outer space huge: it is in the depths of the nebulae that stars are born. Nebulae are made up of two components, gas and dust. The gas is of prehistoric origin, i.e. it was formed at the dawn of the universe, it was at this time that hydrogen and helium were formed - the main components of the first stars. More heavy elements appeared later, when stellar flares and ejections into the interstellar medium began to occur.
The dust that makes up nebulae is a mixture of carbon in different stages adhesion and silicates, there are also traces of other organic matter. The gas is mostly hydrogen.
In principle, nebulae are regions with the interstellar medium compacted under the influence of gravity, in which clouds have formed. Increasing in size, they attracted a part of the matter from environment. Sometimes these clouds become visible due to the fact that the relatively young stars that make up them excite the atoms. As a result, the nebula becomes brighter.
Nebula classification
There are many nebulae in the sky. They are divided into three types: emission nebulae, light (they glow by reflected light) and dark. This division is based on appearance nebulae and phenomena characteristic of them. Emission nebulae are bright because atoms are excited by ultraviolet radiation nearby young stars. The nebulae themselves also turn into a source of radiation.
Light nebulae do not emit radiation, but reflect light from nearby stars. A classic example of a bright nebula is the bluish nebula surrounding the Pleiades open star cluster. Dark nebulae are dense concentrations of dust that actively absorb light. They become visible only if there is a source of brilliance behind them.
Many nebulae are easily visible, sometimes even to the naked eye. It is enough to use binoculars or a small amateur telescope. Such nebulae are recorded in Messier's famous catalog. This French astronomer compiled it in the second half of the 18th century.
The brightest nebula in our hemisphere is the Orion Nebula, in the catalog it has the designation M42. Perhaps this is the first celestial object, at which sky lovers aim their astronomical instruments on long winter nights.
There are many other very beautiful nebulae. Here are some examples.
Nebula in the constellation Sagittarius
The Lagoon Nebula, M8, is located in the constellation Sagittarius. There are many nebulae in this region of the sky. This is a very "populated" region of the Milky Way, there are many gas clouds.
M8 is located near an open star cluster - this combination is not uncommon. As already noted, nebulae are zones of star formation and often clusters of young and bright stars are located within them or nearby. Already with the help of small binoculars, you can see some of the details of the M8, and using more powerful binoculars, you can see characteristics, such as a dark band inside a cloud.
In the open star cluster NGC 6530, about 40 stars are visible, ranging in magnitude from 8 to 13. Their light excites the atoms of the nebula, as a result, it becomes visible.
M8 also contains Bok's globules, dark zones, the diameter of which is equal to tens of thousands of a.u. The distance to M8 is 3000-4000 light years. In the constellation of Sagittarius is also M20, a typical emission nebula. This refers to the Trifid Nebula (“divided into three parts”). The name reflects its shape.
This nebula was discovered by the astronomer Le Gentil in 1750, but its first description did not appear until 1764. Messier did it. William Herschel identified three lines that divide this nebula into three triangular sectors. With binoculars, you can see the brightest part of the nebula. It looks like a round spot with a diameter of up to 10 '. The existence of dark zones that divide the cloud into three parts is associated with the presence of dust and cold gases in its composition.
The distance to M20 is approximately 3200 light years. In the constellation Sagittarius, in the middle of the Milky Way, there is also the M24 nebula, which is visible to the naked eye. It was discovered before, even before Messier included it in his catalogue. This astronomer believed that its diameter was about 1.5 °.
The Eagle Nebula in the constellation Serpens
M16, the Eagle Nebula, was discovered by De Chezo in 1746. Messier recorded it two years later. This nebula is located on the border of the constellations of the Shield and the Serpent. Inside it there is a dark area that extends from the northern to the central part of the cloud.
The star cluster has several dozen stars, some of them are very faint, red. magnitude The brightest stars range from 8 to 11; they belong to the spectral classes O and B, i.e. they are classic hot and young stars. M16 is an emission nebula, but it also has an element of reflection nebula. The distance to it is from 5,000 to 11,000 light years, with an average of about 7500.
In addition to diffuse, there are planetary nebulae. Their name is due to the fact that in the beginning observers often confused them with planets, since they have a round shape.
These nebulae are formed from emissions gas envelope stars for more late stages their evolution.
The most famous planetary nebula, M57, is located in the constellation Lyra. It is difficult to identify because of the low surface illumination. There is also the M27 nebula - Dumbbell, it is located in the constellation of the Fox. This nebula was discovered by Messier in 1764. He, observing it through a telescope, determined the oval shape of the formation. In small amateur telescopes, this nebula appears in the form of “ hourglass". M27 is located at a distance of 500-1000 light years from Earth. Its maximum diameter is about 2.5 light years.
Previously, nebulae in astronomy were called any motionless extended luminous astronomical objects, including star clusters or galaxies outside the Milky Way, which could not be divided into stars.
For example, the Andromeda Galaxy is often referred to as the "Andromeda Nebula". But now nebula called the area of the interstellar medium, which is distinguished by its radiation or absorption of radiation on general background 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 from a nebula is stellar light scattered by 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 a higher degree of 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, with the lifespan of an ancestor star of several billion years. Currently, about 1500 planetary nebulae are known in our galaxy. Planetary nebulae are mostly faint objects and are usually not visible. naked eye. The first discovered planetary nebula was the Dumbbell Nebula in the constellation Chanterelle: 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 In 1784, when 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 supernova explosions 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 nebulae around Wolf-Rayet stars, but are formed around the brightest hot stars of the spectral type O - Of, which have 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. AT interstellar medium star formation occurs, with shock waves, which 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 the 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.
