Nebula

The nebula, also referred to as the "space nebula" is probably one of the most beautiful objects of observation in outer space, which have a variety of shapes and structures within them. Galaxies can compete with nebulae in beauty, but cosmic nebulae still win the championship in beauty and variety of forms. Most astrophotographers love to take pictures of these nebulae and then show other, ordinary people, this unearthly, divine beauty of outer space in our Milky Way galaxy. The most beautiful and most interesting nebulae get their own names, such as the Horsehead Nebula, the Shamrock Nebula, the Crab Nebula. Most of the nebulae are known only by catalog numbers, and visually can look like nondescript dark dips against the background of space, or just beautiful watercolor stains near especially shining stars.

Nebulae are areas of the interstellar medium that are distinguished by their radiation, or absorption of radiation, against the general background of the sky. Nebulae are separate luminescent parts of the interstellar medium. According to their chemical composition, nebulae can consist of ionized, neutral or molecular hydrogen, as well as cosmic dust. Nebulae have a wide variety of chemical molecules inside them, and many nebulae are rich in diverse organics, these are the building blocks of life, from which primitive life in the form of bacteria then begins to synthesize on newly formed planets.

Nebulae are also often regions of star formation, such as in the Pillars of Creation, in the Eagle Nebula. In these huge regions, formations of gas, dust and other materials "stick" together, forming denser regions that attract additional matter and eventually become dense enough to form stars and entire star clusters. After that, planets and other objects of the planetary system, such as asteroids and comets, begin to form around the emerging young stars.

Most nebulae are huge, the largest known gaseous nebulae can be about a hundred thousand light-years in diameter, while other nebulae are about a hundred light-years across. Nebulae also have a significant mass, which is estimated at hundreds of thousands and millions of masses. Most nebulae are located at quite large distances from the Earth - hundreds and thousands of light years, and therefore they are more difficult to see with the naked eye. The exception to this rule is the bright band of nebulae in the Milky Way, which has been visible at all times and has given rise to many legends and myths about itself. Nebulae that are visible to the human eye from Earth would appear larger, but not brighter, from a closer distance, so most photographs of nebulae are the collected light of these nebulae over many hours. For example, the Orion Nebula, which is the brightest nebula in the sky, covers an area twice the angular size of the full moon, can be seen with the naked eye, but the first astronomers did not notice it.

While most nebulae are denser than their surroundings, they are far less dense than any vacuum created on Earth. For example, a nebulous cloud the size of the Earth would have a total mass of only a few kilograms. For example: Earth's air has a density of about 10^19 molecules per cubic centimeter, while at the same time, the densest nebulae can have a density of about 10,000 molecules per cubic centimeter. Many nebulae are visible due to the glow of matter, also called fluorescence, which is caused by hot stars embedded in these nebulae. Other nebulae are so dim and so scattered in outer space that they can only be detected with long exposures and special filters. Some nebulae are differently illuminated by variable stars such as T Tauri.

Originally, the term "nebula" was used to describe any diffuse astronomical object, including galaxies outside the Milky Way. The Andromeda Galaxy, for example, was once referred to as the Andromeda Nebula (and spiral galaxies in general as "spiral nebulae") before the true nature of the galaxies was confirmed in the early 20th century by Westo Slipher, Edwin Hubble and others. Edwin Hubble discovered that most nebulae are associated with stars and illuminated by starlight. He also helped classify nebulae based on the type of light spectrum they produce.

History[]

Initially, most of the nebulae were not visible to mankind, and only with the development and increase of telescopes did a full-fledged study of these nebulae begin.

The very first attention to nebulae was drawn by the ancient astronomer Ptolemy, who, somewhere around 150 AD, wrote down five stars in his books VII-VIII of the Almagest, which seemed to him foggy. Ptolemy also noted the area of the nebula between the constellations Ursa Major and Leo, which was not connected in any way or with any star. In essence, these "stars" noted by Ptolemy turned out to be globular star clusters, the most famous of which is Omega Centauri.

The first real nebula, as opposed to a star cluster, was mentioned by the Muslim Persian astronomer Abd ar-Rahman al-Sufi in his Book of the Fixed Stars (964). Al-Sufi noted He noted a "small cloud" where the Andromeda Galaxy is located. This astronomer added to his catalog the star cluster Omicron Velorum, also referred to as IC 2391, which he designated as a "nebulous star", as well as other nebulous objects, such as the Brocca cluster, also referred to as the Hanger asterism.

The famous Crab Nebula, namely the supernova that created it, SN 1054, was observed by many Arab and Chinese astronomers in 1054.

The first real nebula was discovered by Nicolas-Claude Fabry de Peiresc in 1610 using a telescope. It was the famous Orion Nebula. This nebula was also observed by Johann Baptist Cysat in 1618. However, the first detailed study of the Orion Nebula was not made until 1659 by Christian Huygens, who also considered himself the first person to discover this nebula.

Great discoveries of nebulae were presented by the eighteenth century:

In 1715, Edmond Halley published a list of six nebulae.
In 1746, Jean-Philippe de Chezo compiled a list of 20 nebulae, eight of which were completely unknown before.
From 1751 to 1753, Nicolas-Louis de Lacaille compiled a catalog of 42 nebulae, being in the south, at the Cape of Good Hope, this catalog for the first time included the nebulae of the southern hemisphere of the sky, which had not been observed at all before because of their inaccessibility to observation.
By 1781, Charles Messier had compiled a catalog of 103 "nebulae" that are now called Messier objects. In addition to nebulae, this catalog also included star clusters and galaxies, which were then also called nebulae. This catalog was compiled by an astronomer from practical problems, because Charles Messier was looking for comets in the sky, and these objects could easily be confused with comets at the level of development of telescopic technology in his time.
In 1786, William Herschel and his sister Caroline Herschel published their catalog of thousands of new nebulae, where the number of discovered nebulae was greatly increased.
In 1789 Herschel's second catalog was published
In 1790, Herschel discovered a star that was surrounded by a nebula, and he concluded that it was a real nebula and not a more distant cluster.
In 1802 Herschel's third catalog of 510 nebulae was published. Most of these nebulae turned out to be simply unresolved clusters of stars.

In the 19th century, a more detailed study of these distant nebulae began and their division into different types:

So, starting in 1864, William Huggins examined the spectra of about 70 nebulae, and found that about a third of them had a gas emission spectrum. The rest exhibited a continuous spectrum and were therefore thought to be composed of a mass of stars.
In 1912, reflection nebulae were discovered, this was already the third category of discovered gaseous nebulae. They were discovered by Vesto Slifer, who showed that the spectrum of the nebula surrounding the star Merope coincided with that of the Pleiades open cluster. Thus, the nebula emits reflected starlight.
In 1923, after the Great Debate, it became clear that many "nebulae" were in fact galaxies far from the Milky Way.
Further Slipher and Edwin Hubble continued to collect the spectra of many different nebulae, and found 29 more with an emission spectrum and 33 with a continuous spectrum of starlight.
In 1922, Hubble announced that almost all nebulae are associated with stars, and their illumination comes from starlight. He also found that emission spectrum nebulae are almost always associated with stars having a spectral classification of B or higher (including all O-type main sequence stars), while continuous spectrum nebulae appear with cooler stars. Both Hubble and Henry Norris Russell concluded that the nebulae surrounding hotter stars are transforming in some way.

Formation[]

Note: This part of the article is extracted from Wikipedia!

There are a variety of formation mechanisms for the different types of nebulae. Some nebulae form from gas that is already in the interstellar medium while others are produced by stars. Examples of the former case are giant molecular clouds, the coldest, densest phase of interstellar gas, which can form by the cooling and condensation of more diffuse gas. Examples of the latter case are planetary nebulae formed from material shed by a star in late stages of its stellar evolution.

Star-forming regions are a class of emission nebula associated with giant molecular clouds. These form as a molecular cloud collapses under its own weight, producing stars. Massive stars may form in the center, and their ultraviolet radiation ionizes the surrounding gas, making it visible at optical wavelengths. The region of ionized hydrogen surrounding the massive stars is known as an H II region while the shells of neutral hydrogen surrounding the H II region are known as photodissociation region. Examples of star-forming regions are the Orion Nebula, the Rosette Nebula and the Omega Nebula. Feedback from star-formation, in the form of supernova explosions of massive stars, stellar winds or ultraviolet radiation from massive stars, or outflows from low-mass stars may disrupt the cloud, destroying the nebula after several million years.

Other nebulae form as the result of supernova explosions; the death throes of massive, short-lived stars. The materials thrown off from the supernova explosion are then ionized by the energy and the compact object that its core produces. One of the best examples of this is the Crab Nebula, in Taurus. The supernova event was recorded in the year 1054 and is labeled SN 1054. The compact object that was created after the explosion lies in the center of the Crab Nebula and its core is now a neutron star.

Still other nebulae form as planetary nebulae. This is the final stage of a low-mass star's life, like Earth's Sun. Stars with a mass up to 8–10 solar masses evolve into red giants and slowly lose their outer layers during pulsations in their atmospheres. When a star has lost enough material, its temperature increases and the ultraviolet radiation it emits can ionize the surrounding nebula that it has thrown off. The Sun will produce a planetary nebula and its core will remain behind in the form of a white dwarf.

Types of Nebulae[]

Note: This part of the article is extracted from Wikipedia!

The primary feature used in the classification of nebulae is absorption, or emission or scattering of light by them, that is, according to this criterion, nebulae are divided into dark and light. The former are observed due to the absorption of radiation from sources located behind them, the latter 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. Such a division is historically due to different methods of observation and radiation mechanisms: the presence of dust is most clearly observed when dark nebulae absorb radiation from sources located behind them and when reflection or scattering, or re-emission, contained in the nebula by dust, of radiation from stars located nearby or in the nebula itself; The intrinsic radiation of the gaseous component of 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 stars .

Classical types[]

Objects named nebulae belong to four major groups. Before their nature was understood, galaxies ("spiral nebulae") and star clusters too distant to be resolved as stars were also classified as nebulae, but no longer are.

H II regions, large diffuse nebulae containing ionized hydrogen
Planetary nebulae
Supernova remnant (e.g., Crab Nebula)
Dark nebula
Not all cloud-like structures are named nebulae; Herbig–Haro objects are an example.

Flux Nebula[]

Main article: Integrated Flux Nebula

Diffuse nebulae[]

Most nebulae can be described as diffuse nebulae, which means that they are extended and contain no well-defined boundaries. Diffuse nebulae can be divided into emission nebulae, reflection nebulae and dark nebulae.

Visible light nebulae may be divided into emission nebulae, which emit spectral line radiation from excited or ionized gas (mostly ionized hydrogen); they are often called H II regions, H II referring to ionized hydrogen), and reflection nebulae which are visible primarily due to the light they reflect.

Reflection nebulae themselves do not emit significant amounts of visible light, but are near stars and reflect light from them. Similar nebulae not illuminated by stars do not exhibit visible radiation, but may be detected as opaque clouds blocking light from luminous objects behind them; they are called dark nebulae.

Although these nebulae have different visibility at optical wavelengths, they are all bright sources of infrared emission, chiefly from dust within the nebulae.

Dark Nebulae (Wikipedia)[]

Main article: Dark nebula

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 1-10m to 10-100m in the densest ones. The structure of nebulae with large Av can only be studied by methods of radio astronomy and submillimeter astronomy, mainly from observations of molecular radio lines and from the infrared emission of dust. Often individual densifications with Av up to 10,000m are found inside dark nebulae, in which stars appear to be forming.

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 dust component of nebular matter is associated with magnetic fields due to the fact that moving dust particles are electrically charged.

Reflection Nebulae[]

Main article: Reflection nebula

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 of optical radiation from the 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. 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, being projected on the images of galaxies, they lead to the appearance in the photographs of galaxies of details that do not exist in reality - tails, bridges, etc.

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 type of reflection nebula is the so-called light echo observed after the outburst of a new star in 1901 in the constellation Perseus. A bright flash of a 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, gaseous nebulae, similar to the remnants of supernova explosions, are formed after bursts of new stars.

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 magnetic field lines apart and penetrate between them, forming thin filaments.

The study of the distribution of brightness and polarization of light over the surface of reflection nebulae, as well as the measurement of the dependence of these parameters on the wavelength, makes it possible to establish such properties of interstellar dust as albedo, scattering indicatrix, size, shape, and orientation of dust grains.

Emission Nebulae (Wikipedia)[]

Emission nebulae, or nebulae ionized by radiation, are regions of interstellar gas highly ionized by radiation from 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 about 10,000 K by the ultraviolet radiation of the stars inside them. Inside the H II 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.

The 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. The C II zones are usually located around the H II zones in the regions of neutral hydrogen (HI) and manifest themselves in the recombination radio lines of carbon, similar to the recombination radio lines of hydrogen and helium. C II zones are also observed in the C II infrared line (λ = 156 µm). The C II zones are characterized by a low temperature of 30–100 K and a low degree of ionization of the medium as a whole: Ne/N < 10–3, where Ne and N are the concentrations of electrons and atoms. The C II zones arise because the ionization potential of carbon (11.8 eV) is less than that of hydrogen (13.6 eV). The radiation of stars with photon energies from 11.8 eV to 13.6 eV (λ = 1108...912 Å) goes beyond the H II zone into the H I region, compressed by the ionization front of the H II zone, and ionizes carbon there. C II zones also appear around stars of spectral classes B1–B5 located in dense regions of the interstellar medium. Such stars are practically unable to ionize hydrogen and do not create noticeable H II zones.

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 than in H II zones and a higher degree of ionization of heavy elements.

Planetary Nebulae (Wikipedia)[]

Main article: Planetary nebula

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, a fine structure in the form of jets, spirals, small globules is noticeable. The expansion rate of planetary nebulae is 20–40 km/s, the diameter is 0.01–0.1 pc, the typical mass is about 0.1 M⊙, and the lifetime is about 10 thousand years.

Planetary nebulae are the remnants of the last stages of stellar evolution for medium-mass stars (0.5 to ~8 solar masses in size). Evolved asymptotic giant stars push their outer layers outwards due to strong stellar winds, thus forming gaseous envelopes, leaving behind the stellar core as a white dwarf. Radiation from a hot white dwarf excites the ejected gases, creating emission nebulae with spectra similar to those of emission nebulae found in star forming regions. These are H II regions because most of the hydrogen is ionized, but the planets are denser and more compact than the nebulae found in star forming regions. Planetary nebulae got their name from early astronomical observers who initially couldn't tell them apart from planets and tended to confuse them with planets that were of more interest to them. The Sun is expected to give rise to a planetary nebula about 12 billion years after its formation.

Nebulae created by shock waves (Wikipedia)[]

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 explosions 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 front of the shock wave 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.

Protoplanetary Nebula (Wikipedia)[]

Main article: Protoplanetary nebula

A protoplanetary nebula (PPN) is an astronomical object in a short-lived episode during the rapid stellar evolution of a star between the late asymptotic giant branch (LAGB) phase and the subsequent planetary nebula (PN) phase. During the AGB phase, the star loses mass by emitting a circumstellar envelope of hydrogen gas. When this phase comes to an end, the star enters the PPN phase.

The PPN is powered by the central star, causing it to emit strong infrared radiation and become a reflection nebula. The collimated stellar winds from the central shape of the star shock the shell into an axially symmetrical shape, creating a fast-moving molecular wind in the process. The exact point at which a PPN becomes a planetary nebula (PN) is determined by the temperature of the central star. The PPN phase continues until the central star reaches a temperature of 30,000 K, at which point it becomes hot enough to ionize the surrounding gas.

Supernova Remnants and New Stars (Wikipedia)[]

Main article: Supernova remnant

The brightest nebulae created by shock waves are caused by supernova explosions and are called supernova remnants. They play a very important role in shaping 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 the explosions of new stars are small, weak, and short-lived.

A supernova occurs when a star with a large mass reaches the end of its life. When nuclear fusion ceases in the core of a star, the star collapses. Gas falling inward either bounces off or heats up so much that it expands outward from the core, causing the star to explode. The expanding gas envelope forms a supernova remnant, a special diffuse nebula. While most of the optical and X-ray emission from supernova remnants comes from ionized gas, a large amount of radio emission is a form of non-thermal radiation called synchrotron radiation. This radiation comes from high speed electrons oscillating in magnetic fields.

Nebulae around Wolf stars - Rayet (Wikipedia)[]

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⋅103—3⋅103 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 from these nebulae is of a thermal nature. The lifetime of such nebulae is limited by the duration of the stay of stars in the stage of the Wolf-Rayet star and is close to 105 years.

Nebulae around O-stars (Wikipedia)[]

Similar in properties to nebulae around Wolf-Rayet stars, but they form around the brightest hot stars of the O-O spectral type, which have a strong stellar wind. From the nebulae associated with the Wolf-Rayet stars, they differ in their lower brightness, larger size, and, apparently, longer lifespan.

Nebulae in Star Forming Regions (Wikipedia)[]

Shock waves of lower velocities arise in regions of the interstellar medium in which star formation occurs. They lead to gas heating up to hundreds and thousands of degrees, excitation of 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.

Famous Nebulae (Wikipedia)[]

I propose to place here the most interesting nebulae with their own names

Ant Nebula
Barnard's Loop
Boomerang Nebula
Cat's Eye Nebula
Crab Nebula
Eagle Nebula
Eskimo Nebula
Carina Nebula
Fox Fur Nebula
Helix Nebula
Horsehead Nebula
Engraved Hourglass Nebula
Lagoon Nebula
Orion Nebula
Pelican Nebula
Red Square Nebula
Ring Nebula
Rosette Nebula
Tarantula Nebula

Nebulae by Constellations[]

I propose to place here lists of nebulae, which will be divided between constellations

Catalogs[]

NGC Nebulae
Catalog Barnard
LDN catalog
LBN catalog
Gum catalog
RCW Catalogue
Sharpless catalog
Messier Catalogue
Caldwell Catalogue
Abell Catalog of Planetary Nebulae

Classification of nebulae[]

Nebulae can be classified through illumination:

Diffuse nebulae are illuminated nebulae
- Emission nebulae are internally illuminated clouds of ionized gas. Two of the most common types of emission nebula are H II regions and planetary nebulae
- Reflection nebulae are illuminated by reflections from nearby stars. An example is the nebulosity NGC 1435 surrounding the Pleiades star cluster.
Planetary nebulae are compact shells of gas around a dead star or an intermittently active star.
Supernova remnants are generally moving away from their parent star at high speed, and are heated by colliding with (relatively) slow moving galactic dust and gas. An example is the Crab Nebula in the constellation of Taurus.
Dark nebulae are unilluminated. They can be detected when they obscure stars or other nebulae. Famous examples include the Horsehead nebula in Orion, and the Coalsack Nebula in the Southern Cross.

Astrophysics of nebulae[]

H II regions are the birthplace of stars. They are formed when very diffuse molecular clouds begin to collapse under their own gravity, often due to the influence of a nearby supernova explosion. The cloud collapses and fragments, forming sometimes hundreds of new stars. The newly-formed stars ionize the surrounding gas to produce an emission nebula.

Other nebulae are formed by the death of stars; a star that undergoes the transition to a white dwarf blows off its outer layer to form a planetary nebula. Novae and supernovae can also create nebulae known as nova remnants and supernova remnants respectively.