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Growth of an astronomical object's mass by capture of surrounding matter. During planetary scattering, a collision can lead to accretion if material is retained by the planet; not every collision retains all material. An accretion disk is one particular geometry supplying material.

A hot, comparatively optically thin plasma region associated with an accretion flow. Thermal or nonthermal electrons can transfer energy to softer seed photons through inverse Compton scattering, producing a harder radiation component. Its geometry, heating and coupling to the cooler disc are model dependent; a hard radiation spectrum alone does not uniquely determine these properties.

The circumgalactic medium is gas surrounding a galaxy's main stellar body and interstellar gas, typically associated with its halo. accretion, outflow and recycling exchange matter through it. Its observational boundary and inferred mass depend on the adopted halo convention and the gas phases sampled by absorption or emission.

A dense stellar-scale object such as a white dwarf, neutron star or stellar-mass black hole. accretion releases a large gravitational binding energy per unit mass because the accreting surface or inner orbit is small. A black hole has no material surface, so its radiative yield depends on the accretion flow rather than surface impact.

Integrating $L_{\mathrm{acc}}=GM\dot{M}/r_0$ at fixed radius gives this result. With negligible seed mass it supplies the radiative requirement of monatomic cloud collapse only if ${\Omega }_f=-G{M}^2/r_0$ under the adopted zero-initial-energy approximation. A different density-profile coefficient, retained accretion heat, a wind or a changing radius modifies the budget. Positive binding-energy magnitude and negative potential energy must not be interchanged silently.

A cataclysmic variable with recurrent disk outbursts associated with the hydrogen-ionization disk instability. Enhanced accretion powers the bright phase; the event does not require thermonuclear runaway on the white dwarf.

An outward collapse front and inward central accretion measure different parts of the same mass budget. In the inside-out collapse of a singular isothermal sphere, the unperturbed profile has $\rho =a_s^2/(2\pi G{r}^2)$ and the front is at $r_f=a_{s}t$. Its swept mass rate is therefore $4\pi r_f^2\rho (r_f){\dot{r}}_f=2a_s^3/G$.

The central rate is instead ${\dot{M}}_{\ast }=m_0{a}_s^3/G$, with $m_0$ determined by the dynamical solution. Material already crossed by the front can remain in the infalling envelope, so the two rates need not agree. Equating them would silently assume an empty reservoir between the front and the central object.^[ref]

For a singular isothermal sphere with $\rho =a_s^2/(2\pi G{r}^2)$, the initial mass inside the outward front $r_{\mathrm{ew}}=a_{s}t$ is $M_{\mathrm{ew}}=2a_s^{3}t/G$. This is not all immediately accreted by the central object. The inside-out collapse of a singular isothermal sphere instead has ${\dot{M}}_{\ast }=m_0{a}_s^3/G$, with $m_0$ fixed by its dynamical similarity solution; the remainder can stay in the infalling envelope. Equating front sweeping with central accretion loses that mass budget.

At fixed stellar mass $M_{\star }$, planetary mass density ${\rho }_p$, age $t_{\star }$ and initial ratio $\kappa =a_0/a_p$, the order-unity Safronov number comparison and comet energy diffusion give boundariesTheir logarithmic slopes in planetary mass versus radius are $-3/2$ and $3/4$. Above both, strong kicks and a short estimated diffusion time favour ejection; below the escape boundary, repeated weak encounters favour collision statistically. Below the age boundary, the stated diffusion mechanism has not completed on its characteristic clock. Collision lifetimes require additional cross-section and encounter-rate information, so this map alone cannot establish retention or accretion within the age.

A transient pressure-supported stage of a collapsing cloud after rising optical depth inhibits cooling. Continued accretion and changes of the molecular equation of state can lead to further collapse. It is distinct from the later compact stellar core.

A fragment formed by gravitational instability of an astrophysical disk must remain bound after it forms. Its own gravity competes with the surrounding tidal field, while heating, cooling and accretion change its size and internal support.

For a low-mass clump on a circular orbit around a dominant star, its Hill radius estimates the region over which its attraction can resist the local stellar tide. Cooling and contraction can make a fragment more compact, whereas inward migration can shrink that tidal region. A calculation demonstrating initial fragmentation therefore does not alone establish a long-lived giant planet: survival requires the subsequent thermal and orbital evolution as well.

Gas can shock and radiate during a galaxy encounter, allowing it to change orbital energy in ways unavailable to an ideal collisionless stellar component. Gravitational torques can transport its angular momentum and fuel central star formation or accretion. The outcome depends on orbit, gas content and internal structure.

James B. Pollack led numerical models of gas giant formation that calculated solid and gaseous accretion together. The simulations combined capture of planetesimals, evolution of the planetary gas envelope and dissolution of incoming solid material within that envelope.

The coupling matters because accreting solids both increase the central mass and deposit energy, changing the envelope's subsequent ability to collect gas. The resulting evolutionary phases include rapid solid growth, slower growth of both components and eventually rapid gas accumulation. This gave a physical account of how an initially modest solid core could become a giant planet within the available disk lifetime.^[ref]

Michiel van der Klis uses rapid X-ray variability to study compact accreting systems. With Rudy Wijnands, he discovered persistent 2.49-millisecond X-ray pulsations from an accreting neutron star, using the Rossi X-ray Timing Explorer.

Such a coherent pulse train provides a much more direct rotational clock than a broad or drifting variability peak. The 1998 discovery connected accretion in an X-ray binary with the proposed spinning-up of a neutron star into a millisecond pulsar, providing a missing observational link in that evolutionary picture.^[ref]

A cataclysmic variable whose white dwarf magnetic field does not dominate the large-scale accretion flow. It normally supports an accretion disk fed through Roche-lobe overflow. The term does not rule out a magnetized donor star or magnetic braking of a binary star.

The primary numerical study follows spherical protostellar collapse. A small central core forms before most of the envelope reaches it and subsequently grows by accretion. The abstract also describes infrared reprocessing by the dusty envelope. Spherical symmetry and the chosen radiative and thermodynamic treatment are model assumptions; rotation, magnetic fields and non-spherical accretion require additional physics.

A neutron star merger, or a neutron star tidally disrupted by a black hole outside its capture region, can leave a hot accretion disk around the remnant. Rapid accretion can power a relativistic jet and a short gamma-ray burst. Beaming means that an observer must lie sufficiently near the jet axis, and not every merger produces an observable burst. A clean binary black hole merger has no substantial baryonic fuel and is not ordinarily expected to power this mechanism. Disruption in a neutron-star–black-hole pair depends on binary mass ratio, black-hole spin and the neutron star's stellar radius.

An ultraluminous X-ray source is a compact, apparently off-nuclear source in another galaxy with an inferred isotropic X-ray luminosity typically above $10^{32}$ W, exceeding the classical Eddington luminosity of an ordinary stellar-mass accretor. Imaging X-ray surveys revealed unusually bright sources away from galactic nuclei; distance association with the host converted the observed fluxes into surprisingly high luminosities. Better angular resolution helps exclude blends and background active nuclei. The 2014 discovery of coherent pulsations established a neutron star in M82 X-2, so the class cannot be identified exclusively with black holes.^[ref]

An intermediate-mass black hole can radiate approximately isotropically below its own Eddington luminosity: this avoids extreme super-Eddington emission, but the inferred mass requires dynamical or suitable spectral/timing support and is not proved by luminosity alone. A stellar-mass accretor with a supercritical accretion disk and a wind funnel can combine genuinely super-Eddington output with geometrical beaming; this is compatible with a neutron-star accretor but requires a consistent radiation/flow/torque model. Relativistic beaming from a jet is another possibility for some objects, but orientation, spectral and variability evidence must support it. These mechanisms need not apply identically to every member of the observed class.

For spherical radiation with flux $F=L/(4\pi r^2)$, the outward acceleration is $\kappa F/c$. Equating it to $GM/r^2$ givesThe limit assumes the stated opacity, approximate spherical symmetry and coupling between the radiatively accelerated plasma and the accreting matter. It is not a universal ceiling on isotropic-equivalent luminosity for an anisotropic magnetized flow.

Write the available accretion power as $L=\eta \dot{M}{c}^2$. ThenAccretion as the energy source alone does not fix $\eta$. For radiatively efficient Newtonian accretion onto a slowly rotating stellar surface, $\eta \simeq GM/(R{c}^2)$ and the corresponding surface accretion Eddington rate isThis is the appropriate convention for the numerical neutron-star estimates below, rather than imposing a separate reference efficiency of $0.1$.

Using $M=1.4M_{\odot }$, $R=10^4$ m and the stated opacity givesIf the observed X-rays represent isotropic, radiatively efficient surface accretion power, thenA bolometric correction, beaming, radiation advected into the flow or energy carried by a wind would change the mapping from the measured band luminosity to the supplied mass rate. This estimate keeps the assumptions needed for comparison with the torque calculation.

In magnetically channelled accretion, an outer accretion disk supplies nearly Keplerian material. Near the magnetospheric truncation radius, magnetic stresses disrupt the disk and guide matter along field lines onto polar accretion columns. Shocks and settling near the surface release gravitational binding energy, and an oblique magnetic geometry makes the rotating emission pattern pulsate.

The corotation radius satisfies ${\Omega }_K(r_{\mathrm{co}})={\Omega }_{\ast }=2\pi /P$, henceThe accretion Alfvén radius estimates where magnetic stress becomes comparable with the inflow's dynamical stress; it is the magnetic truncation scale in this application, not the Alfvén critical point of an outward stellar wind. The supplied seventh-root scaling follows from a dipole's rapidly decreasing magnetic pressure and an accretion-pressure estimate. Its coefficient depends on disk geometry. Ordinary inward surface accretion requires the magnetosphere to remain outside the star and usually inside or near corotation, $R\lesssim r_{\mathrm{Alf}}\lesssim r_{\mathrm{co}}$. Beyond corotation, a rapidly moving magnetosphere can transfer angular momentum outward and inhibit inflow through the propeller effect.