Extinction (astronomy) in the context of Light scattering

A quasar (/ˈkweɪzɑːr/ KWAY-zar) is an extremely luminous active galactic nucleus (AGN). It is sometimes known as a quasi-stellar object, abbreviated QSO. The emission from an AGN is powered by accretion onto a supermassive black hole with a mass ranging from millions to tens of billions of solar masses, surrounded by a gaseous accretion disc. Gas in the disc falling towards the black hole heats up and releases energy in the form of electromagnetic radiation. The radiant energy of quasars is enormous; the most powerful quasars have luminosities thousands of times greater than that of a galaxy such as the Milky Way. Quasars are usually categorized as a subclass of the more general category of AGN. The redshifts of quasars are of cosmological origin.

An active galactic nucleus (AGN) is a compact region at the center of a galaxy that emits a significant amount of energy across the electromagnetic spectrum, with characteristics indicating that this luminosity is not produced by the stars. Such excess, non-stellar emissions have been observed in the radio, microwave, infrared, optical, ultra-violet, X-ray, and gamma ray wavebands. A galaxy hosting an AGN is called an active galaxy. The non-stellar radiation from an AGN is theorized to result from the accretion of matter by a supermassive black hole at the center of its host galaxy. Not every supermassive black hole generates an AGN. For example, our Milky Way galaxy is not an active galaxy even though it has a supermassive black hole in its center.

Active galactic nuclei are the most luminous persistent sources of electromagnetic radiation in the universe and, as such, can be used as a means of discovering distant objects; their evolution as a function of cosmic time also puts constraints on models of the cosmos. The observed characteristics of an AGN depend on several properties such as the mass of the central black hole, the rate of gas accretion onto the black hole, the orientation of the accretion disk, the degree of obscuration of the nucleus by dust, and presence or absence of jets. Numerous subclasses of AGN have been defined on the basis of their observed characteristics; the most powerful AGN are classified as quasars. A blazar is an AGN with a jet pointed toward the Earth, in which radiation from the jet is enhanced by relativistic beaming.

Apparent magnitude (m) is a measure of the brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction of the object's light caused by interstellar dust or atmosphere along the line of sight to the observer.

Unless stated otherwise, the word magnitude in astronomy usually refers to a celestial object's apparent magnitude. The magnitude scale likely dates to before the ancient Roman astronomer Claudius Ptolemy, whose star catalog popularized the system by listing stars from 1st magnitude (brightest) to 6th magnitude (dimmest). The modern scale was mathematically defined to closely match this historical system by Norman Pogson in 1856.

Circumstellar dust is cosmic dust around a star. It can be in the form of a spherical shell or a disc, e.g. an accretion disk. Circumstellar dust can be responsible for significant extinction and is usually the source of an infrared excess for stars that have it. For some evolved stars on the asymptotic giant branch, the dust can be composed of silicate emissions. According to a study, it is still uncertain whether the dust is a result of crystalline silicate or polycyclic aromatic hydrocarbon. However, recent observations revealed that Vega-type stars display broad silicate emission. It is suggested that the circumstellar dust components can depend on the evolutionary stage of a star and is related to the changes in its physical conditions.

The study of the composition of this dust is dubbed astrominerology. The circumstellar dust around aging stars has been observed to include, "almost pure crystalline Mg-rich silicates (forsterite and clinoenstatite), amorphous silicates, diopside, spinel, oxides (corundum and Fe0.9Mg0.1O), and also carbon-rich solids such as: (hydrogenated) amorphous carbons, aromatic hydrocarbons and silicon carbide."

Beta Fornacis (Beta For, β Fornacis, β For) is solitary star in the southern constellation of Fornax. It is visible to the naked eye with an apparent visual magnitude of 4.46. Based upon an annual parallax shift of 18.46 mas, it is located around 177 light years away from the Sun. At that distance, the visual magnitude is reduced by an interstellar extinction factor of 0.1.

This is an evolved, G-type giant star with a stellar classification of G8 III. It is a red clump giant, which means it has undergone helium flash and is currently generating energy through the fusion of helium at its core. Beta Fornacis has 1.33 times the mass of the Sun and, at an age of 3.3 billion years, has expanded to 10.5 times the Sun's radius. It is radiating 51 times the solar luminosity from its outer atmosphere at an effective temperature of 4,790 K.

Sagittarius A (Sgr A) is a complex radio source at the center of the Milky Way, which contains a supermassive black hole. It is located between Scorpius and Sagittarius, and is hidden from view at optical wavelengths by large clouds of cosmic dust in the spiral arms of the Milky Way. The dust lane that obscures the Galactic Center from a vantage point around the Sun causes the Great Rift through the bright bulge of the galaxy.

The radio source consists of three components: the supernova remnant Sagittarius A East, the spiral structure Sagittarius A West, and a very bright compact radio source at the center of the spiral, Sagittarius A* (read "A-star"). These three overlap: Sagittarius A East is the largest, West appears off-center within East, and A* is at the center of West.

A dust lane consists of relatively dense, obscuring clouds of interstellar dust, observed as a dark swath against the background of brighter object(s), especially a galaxy. These dust lanes can usually be seen in spiral galaxies, such as the Milky Way, when viewed from the edge. Due to the dense and relatively thick nature of this dust, observed light from a galaxy can be reduced by dust lanes by up to several magnitudes. In the Milky Way, this attenuation of visible light makes it impossible to see the stars behind the Great Rift through the bulge around the Galactic Center from Earth. This dust, as well as the gasses also found within these lanes, mixes and combines to form stars and planets. The gas in the dust lanes is funneled toward the Central Molecular Zone. Approximately one third of the gas will combine with the CMZ. The rest will overshoot and accrete at a later time.

The presence of a dust lane is most apparent in disc galaxies that are viewed edge on. Although they are absent in many low-mass late-type galaxies. However, the absence of a dust lane does not signify a lack of dust but that it is more dispersed throughout the galaxy. Simulations have shown that in barred spiral galaxies the strength of the bar has an affect on the curvature of the dust lanes. Galaxies with weak bars result in curved dust lanes whereas strong bars result in straight dust lanes.

In astronomy, absolute magnitude (M) is a measure of the luminosity of a celestial object on an inverse logarithmic astronomical magnitude scale; the more luminous (intrinsically bright) an object, the lower its magnitude number. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it were viewed from a distance of exactly 10 parsecs (32.6 light-years), without extinction (or dimming) of its light due to absorption by interstellar matter and cosmic dust. By hypothetically placing all objects at a standard reference distance from the observer, their luminosities can be directly compared among each other on a magnitude scale. For Solar System bodies that shine in reflected light, a different definition of absolute magnitude (H) is used, based on a standard reference distance of one astronomical unit.

Absolute magnitudes of stars generally range from approximately −10 to +20. The absolute magnitudes of galaxies can be much lower (brighter).

A dark nebula or absorption nebula is a type of interstellar cloud, particularly molecular clouds, that is so dense that it obscures the visible wavelengths of light from objects behind it, such as background stars and emission or reflection nebulae. The extinction of the light is caused by interstellar dust grains in the coldest, densest parts of molecular clouds. Clusters and large complexes of dark nebulae are associated with Giant Molecular Clouds. Isolated small dark nebulae are called Bok globules. Like other interstellar dust or material, the things it obscures are visible only using radio waves in radio astronomy or infrared in infrared astronomy.

Dark clouds appear so because of sub-micrometre-sized dust particles, coated with frozen carbon monoxide and nitrogen, which effectively block the passage of light at visible wavelengths. Also present are molecular hydrogen, atomic helium, CO (CO with oxygen as the O isotope), CS, NH₃ (ammonia), H₂CO (formaldehyde), c-C₃H₂ (cyclopropenylidene) and a molecular ion N₂H (diazenylium), all of which are relatively transparent.

Mirach is a prominent star in the northern constellation of Andromeda. It is pronounced /ˈmaɪræk/ and has the Bayer designation Beta Andromedae, which is Latinized from β Andromedae. This star is positioned northeast of the Great Square of Pegasus and is potentially visible to all observers north of latitude 54° S. It is commonly used by stargazers to find the Andromeda Galaxy. The galaxy NGC 404, also known as Mirach's Ghost, is seven arcminutes away from Mirach.

This star has an apparent visual magnitude of around 2.07, varying between 2.01 and 2.10, which at times makes it the brightest star in the constellation. Based upon parallax measurements, it is roughly 197 light-years (60 parsecs) from the Solar System. Its apparent magnitude is reduced by 0.06 by extinction due to gas and dust along the line of sight. The star has a negligible radial velocity of 0.1 km/s, but with a relatively large proper motion, traversing the celestial sphere at an angular rate of 0.208″·yr.

The Alpha Persei Cluster, also known as Melotte 20 or Collinder 39, is an open cluster of stars in the northern constellation of Perseus. To the naked eye, the cluster consists of several blue-hued spectral type B stars. The most luminous member is the ~2nd magnitude yellow supergiant Mirfak, also known as Alpha Persei. Bright members also include Delta, Sigma, Psi, 29, 30, 34, and 48 Persei. The Hipparcos satellite and infrared color-magnitude diagram fitting have been used to establish a distance to the cluster of ~560 light-years (172 pc). The distance established via the independent analyses agree, thereby making the cluster an important rung on the cosmic distance ladder. As seen from the Earth, the extinction of the cluster due to interstellar dust is around 0.30.

The cluster is centered to the northeast of Alpha Persei. It has a core radius of 11.4 ± 1.4 ly, a half-mass radius of 18 ly, and a tidal radius of 70.6 ± 8.5 ly, with 517 members being identified within the latter. The cluster shows solid evidence of having undergone mass segregation, with the mean stellar mass decreasing toward the edge. The age of this cluster is about 50–70 million years. Cluster member stars show a near-solar metallicity, meaning the abundance of elements with atomic numbers higher than 2 are similar to those in the Sun. The cluster shows evidence of tidal tails, which are most likely of galactic origin.

In astronomy, air mass or airmass is a measure of the amount of air along the line of sight when observing a star or other celestial source from below Earth's atmosphere (Green 1992). It is formulated as the integral of air density along the light ray.

As it penetrates the atmosphere, light is attenuated by scattering and absorption; the thicker atmosphere through which it passes, the greater the attenuation. Consequently, celestial bodies when nearer the horizon appear less bright than when nearer the zenith. This attenuation, known as atmospheric extinction, is described quantitatively by the Beer–Lambert law.