A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit the formation of molecules, most commonly molecular hydrogen (H2). This is in contrast to other areas of the interstellar medium that contain predominantly ionized gas.
Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", collapse and form stars. As a branch of astronomy, star formation includes the study of the interstellar medium (ISM) and giant molecular clouds (GMC) as precursors to the star formation process, and the study of protostars and young stellar objects as its immediate products. It is closely related to planet formation, another branch of astronomy. Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics of binary stars and the initial mass function. Most stars do not form in isolation but as part of a group of stars referred as star clusters or stellar associations.
Cygnus X-1 (abbreviated Cyg X-1) is a galactic X-ray source in the constellation Cygnus, and 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 seen from Earth, producing a peak X-ray flux density of 2.3×10−23 Wm−2 Hz−1 (2.3×103 Jansky). It remains among the most studied astronomical objects in its class. The compact object is now estimated to have a mass about 14.8 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.
An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much higher than normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not produced by stars. Such excess non-stellar emission has 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 radiation from an AGN is believed to result from the accretion of matter by a supermassive black hole at the center of its host galaxy.
An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically a cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light years, and density from a few to about a million particles per cubic cm. The Orion Nebula, now known to be an H II region, was observed in 1610 by Nicolas-Claude Fabri de Peiresc by telescope, the first such object discovered.
The Eddington luminosity, also referred to as the Eddington limit, is the maximum luminosity a body can achieve when there is balance between the force of radiation acting outward and the gravitational force acting inward. The state of balance is called hydrostatic equilibrium. When a star exceeds the Eddington luminosity, it will initiate a very intense radiation-driven stellar wind from its outer layers. Since most massive stars have luminosities far below the Eddington luminosity, their winds are mostly driven by the less intense line absorption. The Eddington limit is invoked to explain the observed luminosity of accreting black holes such as quasars.
The nebular hypothesis is the most widely accepted model in the field of cosmogony to explain the formation and evolution of the Solar System. It suggests that the Solar System is formed from the nebulous material. The theory was developed by Immanuel Kant and published in his Allgemeine Naturgeschichte und Theorie des Himmels, published in 1755. Originally applied to the Solar System, the process of planetary system formation is now thought to be at work throughout the universe. The widely accepted modern variant of the nebular hypothesis is the solar nebular disk model (SNDM) or solar nebular model. It offered explanations for a variety of properties of the Solar System, including the nearly circular and coplanar orbits of the planets, and their motion in the same direction as the Sun's rotation. Some elements of the original nebular hypothesis are echoed in modern theories of planetary formation, but most elements have been superseded.
A protostar is a very young star that is still gathering mass from its parent molecular cloud. The protostellar phase is the earliest one in the process of stellar evolution. For a low mass star, it lasts about 500,000 years The phase begins when a molecular cloud fragment first collapses under the force of self-gravity and an opaque, pressure supported core forms inside the collapsing fragment. It ends when the infalling gas is depleted, leaving a pre-main-sequence star, which contracts to later become a main-sequence star at the onset of Hydrogen fusion.
A protoplanetary disk is a rotating circumstellar disk of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may also be considered an accretion disk for the star itself, because gases or other material may be falling from the inner edge of the disk onto the surface of the star. 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 the field of Big Bang theory, and cosmology, reionization is the process that caused the matter in the universe to reionize after the lapse of the "dark ages".
Photoevaporation denotes the process where energetic radiation ionises gas and causes it to disperse away from the ionising source. This typically refers to an astrophysical context where ultraviolet radiation from hot stars acts on clouds of material such as molecular clouds, protoplanetary disks, or planetary atmospheres.
Planetary migration occurs when a planet or other stellar satellite interacts with a disk of gas or planetesimals, resulting in the alteration of the satellite's orbital parameters, especially its semi-major axis. Planetary migration is the most likely explanation for hot Jupiters: exoplanets with jovian masses but orbits of only a few days. The generally accepted theory of planet formation from a protoplanetary disk predicts such planets cannot form so close to their stars, as there is insufficient mass at such small radii and the temperature is too high to allow the formation of rocky or icy planetesimals.
A pair-instability supernova occurs when pair production, the production of free electrons and positrons in the collision between atomic nuclei and energetic gamma rays, temporarily reduces the internal pressure supporting a supermassive star's core against gravitational collapse. This pressure drop leads to a partial collapse, which in turn causes greatly accelerated burning in a runaway thermonuclear explosion, resulting in the star being blown completely apart without leaving a black hole remnant behind. Pair-instability supernovae can only happen in stars with a mass range from around 130 to 250 solar masses and low to moderate metallicity. The recently observed objects SN 2006gy, SN 2007bi, SN 2213-1745, and SN 1000+0216 are hypothesized to have been pair-instability supernovae.
Gamma-ray burst emission mechanisms are theories that explain how the energy from a gamma-ray burst progenitor is turned into radiation. These mechanisms are a major topic of research as of 2007. Neither the light curves nor the early-time spectra of GRBs show resemblance to the radiation emitted by any familiar physical process.
AM Canum Venaticorum is a hydrogen-deficient cataclysmic variable binary star in the constellation of Canes Venatici. It is the type star of its class of variables, the AM Canum Venaticorum stars. The system consists of a white dwarf gaining matter via an accretion disk from a semi-degenerate or white dwarf companion.
In astronomy, CENBOL is the region of an accretion flow around a black hole.
Leo T is a dwarf galaxy situated in the Leo constellation and discovered in 2006 in the data obtained by Sloan Digital Sky Survey. The galaxy is located at the distance of about 420 kpc from the Sun and moves away from the Sun with the velocity of about 35 km/s. The velocity with respect to the Milky Way is around −60 km/s implying a slow infall onto the Milky Way. Leo T is classified as a transitional object between dwarf spheroidal galaxies (dSph) and dwarf irregular galaxies (dIrr). Its half-light radius is about 180 pc.
Rossby Wave Instability (RWI) is a concept related to astrophysical discs. In non-self-gravitating discs, for example around newly forming stars, the instability can be triggered by an axisymmetric bump, at some radius , in the disc surface mass-density. It gives rise to exponentially growing non-axisymmetric perturbation [ , ] in the vicinity of consisting of anticyclonic vortices. These vortices are regions of high pressure and consequently act to trap dust particles which in turn can facilitate planetesimal growth in proto-planetary discs. The Rossby vortices in the discs around stars and black holes may cause the observed quasi-periodic modulations of the disc's thermal emission.
An accretion disk is a structure formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction causes orbiting material in the disk to spiral inward towards the central body. Gravitational and frictional forces compress and raise the temperature of the material, causing the emission of electromagnetic radiation. The frequency range of that radiation depends on the central object's mass. Accretion disks of young stars and protostars radiate in the infrared; those around neutron stars and black holes in the X-ray part of the spectrum. The study of oscillation modes in accretion disks is referred to as diskoseismology.
A circumstellar disc is a torus, pancake or ring-shaped accumulation 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.
