Accretion disk in the context of Protoplanetary disc

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.

Sagittarius A*, abbreviated as Sgr A* (/ˈsædʒ ˈeɪ stɑːr/ SADGE-AY-star), is the supermassive black hole at the Galactic Center of the Milky Way. Viewed from Earth, it is located near the border of the constellations Sagittarius and Scorpius, about 5.6° south of the ecliptic, visually close to the Butterfly Cluster (M6) and Lambda Scorpii. Sagittarius A* is a bright and very compact astronomical radio source.

In May 2022, astronomers released the first image of the accretion disk around the event horizon of Sagittarius A*, using the Event Horizon Telescope, a world-wide network of radio observatories. This is the second confirmed image of a black hole, after Messier 87's supermassive black hole in 2019. The black hole itself is not seen; as light is incapable of escaping the immense gravitational force of a black hole, only nearby objects whose behavior is influenced by the black hole can be observed. The observed radio and infrared energy emanates from gas and dust heated to millions of degrees while falling into the black hole.

A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may not be considered an accretion disk; while the two are similar, an accretion disk is hotter and spins much faster; it is also found on black holes, not stars. This process should not be confused with the accretion process thought to build up the planets themselves. Externally illuminated photo-evaporating protoplanetary disks are called proplyds.

In astrophysics, accretion is the accumulation of particles into a massive object by gravitationally attracting more matter, typically gaseous matter, into an accretion disk. Most astronomical objects, such as galaxies, stars, and planets, are formed by accretion processes.

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.

An astrophysical jet is an astronomical phenomenon where ionised matter is expelled at high velocity from an astronomical object, in a pair of narrow streams aligned with the object's axis of rotation. When the matter in the beam approaches the speed of light, astrophysical jets become relativistic jets as they show effects from special relativity.

Astrophysical jets are associated with many types of high-energy astronomical sources, such as black holes, neutron stars and pulsars. Their causes are not yet fully understood, but they are believed to arise from dynamic interactions within accretion disks. One explanation is that as an accretion disk spins, it generates a rotating, tangled magnetic field which concentrates material from the disk into the jets and then drives it away from the central object. Jets may also be influenced by a general relativity effect known as frame-dragging.

The origin of the Moon is usually explained by a Mars-sized body, known as Theia, striking the Earth, creating a debris ring that eventually collected into a single natural satellite, the Moon, but there are a number of variations on this giant-impact hypothesis, as well as alternative explanations, and research continues into how the Moon came to be formed. Other proposed scenarios include captured body, fission, formed together (accretion, synestia), planetesimal collisions (formed from asteroid-like bodies), and collision theories.

The standard giant-impact hypothesis suggests that a Mars-sized body called Theia impacted the proto-Earth, creating a large debris ring around Earth, which then accreted to form the Moon. This collision also resulted in the 23.5° tilted axis of the Earth, thus causing the seasons. The Moon's oxygen isotopic ratios seem to be essentially identical to Earth's. Oxygen isotopic ratios, which may be measured very precisely, yield a unique and distinct signature for each Solar System body. If Theia had been a separate protoplanet, it probably would have had a different oxygen isotopic signature than proto-Earth, as would the ejected mixed material. Also, the Moon's titanium isotope ratio (Ti/Ti) appears so close to the Earth's (within 4 parts per million) that little if any of the colliding body's mass could have been part of the Moon.

A circumstellar disc (or circumstellar disk) is a torus-, pancake- or ring-shaped accretion disk of matter composed of gas, dust, planetesimals, asteroids, or collision fragments in orbit around a star. Around the youngest stars, they are the reservoirs of material out of which planets may form. Around mature stars, they indicate that planetesimal formation has taken place, and around white dwarfs, they indicate that planetary material survived the whole of stellar evolution. Such a disc can manifest itself in various ways.

In astronomy, cataclysmic variable stars (CVs) are stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state. They were initially called novae (from Latin 'new'), since those with an outburst brightness visible to the naked eye and an invisible quiescent brightness appeared as new stars in the sky.

Cataclysmic variable stars are binary stars that consist of two components; a white dwarf primary, and a mass transferring secondary. The stars are so close to each other that the gravity of the white dwarf distorts the secondary, and the white dwarf accretes matter from the companion. Therefore, the secondary is often referred to as the donor star, and it is usually less massive than the primary. The infalling matter, which is usually rich in hydrogen, forms in most cases an accretion disk around the white dwarf. Strong UV and X-ray emission is often detected from the accretion disc, powered by the loss of gravitational potential energy from the infalling material. The shortest currently observed orbit in a hydrogen-rich system is 51 minutes in ZTF J1813+4251.

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."

A microquasar, a smaller version of a quasar, is a compact region surrounding a stellar black hole with a mass several times that of its companion star, observable in sufficient detail, in our own or nearby galaxy. The matter being pulled from the companion star forms an accretion disk around the black hole. This accretion disk may become so hot, due to friction, that it begins to emit X-rays. The disk also projects narrow streams or "jets" of subatomic particles at near-light speed, generating a strong radio wave emission.

Cygnus X-1 (abbreviated Cyg X-1) is a galactic X-ray source in the constellation Cygnus and was the first such source widely accepted to be a black hole. It was discovered in 1964 during a rocket flight and is one of the strongest X-ray sources detectable from Earth, producing a peak X-ray flux density of 2.3×10 W/(m⋅Hz) (2.3×10 jansky). It remains among the most studied astronomical objects in its class. The compact object is now estimated to have a mass about 21.2 times the mass of the Sun and has been shown to be too small to be any known kind of normal star or other likely object besides a black hole. If so, the radius of its event horizon has 300 km "as upper bound to the linear dimension of the source region" of occasional X-ray bursts lasting only for about 1 ms.

Cygnus X-1 is a high-mass X-ray binary system located about 7,000 light-years away, that includes a blue supergiant variable star. The supergiant and black hole are separated by about 0.2 AU, or 20% of the distance from Earth to the Sun. A stellar wind from the star provides material for an accretion disk around the X-ray source. Matter in the inner disk is heated to millions of degrees, generating the observed X-rays. A pair of relativistic jets, arranged perpendicularly to the disk, are carrying part of the energy of the infalling material away into interstellar space.

An Interacting binary star is a type of binary star in which one or both of the component stars has filled or exceeded its Roche lobe, also known as a semidetached binary. When this happens, material from one star (the donor star) will flow towards the other star (the accretor). If the accretor is a compact star, an accretion disk may form. The physical conditions in such a system can be complex and highly variable, and they are common sources of cataclysmic outbursts.

A common type of interacting binary star is one in which one of the components is a compact object which is well within its Roche lobe, while the other is an evolved giant star. If the compact object is a white dwarf, then accretion of material from the evolved star onto the white dwarf's surface may result in its mass increasing to beyond the Chandrasekhar limit. This can lead to runaway thermonuclear reactions and the massive explosion of the star in a Type I supernova.

A hypernova is a very energetic supernova which is believed to result from an extreme core collapse scenario. In this case, a massive star (>30 solar masses) collapses to form a rotating black hole emitting twin astrophysical jets and surrounded by an accretion disk. It is a type of stellar explosion that ejects material with an unusually high kinetic energy, an order of magnitude higher than most supernovae, with a luminosity at least 10 times greater. Hypernovae release such intense gamma rays that they often appear similar to a type Ic supernova, but with unusually broad spectral lines indicating an extremely high expansion velocity. Hypernovae are one of the mechanisms for producing long gamma ray bursts (GRBs), which range from 2 seconds to over a minute in duration. They have also been referred to as superluminous supernovae, though that classification also includes other types of extremely luminous stellar explosions that have different origins.