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An active galactic nucleus is the compact luminous centre of a galaxy powered mainly by black-hole accretion onto a supermassive black hole. Its radiation can dominate the host at some wavelengths even though the emitting region is small compared with the galaxy. Observers identify activity using combinations of emission lines, a compact radio wave source, energetic X-rays and brightness changes. Different components respond on different timescales: reverberation mapping measures the delay between changes in the central radiation and the response of surrounding line-emitting gas, giving a light-travel estimate of its distance. Combining that distance with the Doppler broadening of the emission lines constrains the central mass if the emitting gas is gravitationally bound. Activity can be episodic; the existence of a central black hole alone does not imply a luminous nucleus.^[ref]

Active-galactic-nucleus feedback is the transfer of energy and momentum from an active galactic nucleus to surrounding gas. Radiation can heat or accelerate the gas, while astrophysical jets and galactic outflows inject mechanical energy. Heating may offset radiative cooling and reduce the cold material available for star formation or further black-hole accretion; compression can instead favour local star formation. Evidence includes cavities in hot X-ray-emitting plasma that coincide with radio wave emission, together with surrounding shock waves. Their sizes and the ambient pressure constrain the mechanical work, while estimated ages convert an energy budget into a mean power. Such measurements test whether nuclear activity can regulate gas on scales much larger than the central black hole.^[ref]

Arpita Roy proposed, with collaborators, a physical explanation for the lunar crustal dichotomy: the Moon's farside crust is thicker on average than its nearside crust. Their primordial Earthshine hypothesis connects this asymmetry to unequal irradiation by a hot post-impact Earth.

If the young Moon was already in synchronous rotation, the permanently Earth-facing side would have received more radiation. The proposed temperature gradient would favor condensation of crust-forming refractory material on the cooler farside. The explanation is a model of how a thermal asymmetry could seed a compositional one; it depends on the early locking, cooling and mixing history.^[ref]

Atmospheric escape is the loss of gas from a planet to space. In Jeans escape, particles in the high-speed tail of a velocity distribution leave a nearly collisionless upper atmosphere if they exceed the local escape velocity. Hydrodynamic escape instead treats a collisional bulk outflow. Photochemical escape supplies another route by creating energetic reaction products.

Which description applies depends on gravity, heating, composition and the collision rate, rather than only on the planetary equilibrium temperature. Absorption of energetic stellar radiation can heat high atmospheric layers far above that temperature. Removing a light envelope changes the measured radius and future exoplanet atmosphere, connecting escape to the interpretation of a planetary mass-radius relation.

Atmospheric photochemistry is the network of chemical reactions initiated when absorbed photons dissociate or ionize atmospheric species. The initial fragments can react with other molecules, so a small abundance of short-lived products can control the chemistry of much more abundant gases. The reaction rates depend on the stellar spectrum, absorption and the amount of shielding above a layer.

The Chapman ozone equilibrium illustrates coupling between photodissociation, molecular formation and destruction. In an exoplanet atmosphere, such reactions can change spectral absorbers or supply material for atmospheric haze. A photochemical steady state balances production and loss while reactions continue; it need not be thermochemical equilibrium, because stellar radiation continuously drives the system.

An atmospheric pressure-temperature profile specifies temperature as a function of pressure through an exoplanet atmosphere. Pressure normally increases inward under hydrostatic equilibrium, so it is a useful vertical coordinate even when the absolute altitude is uncertain. In a radiative equilibrium region, heating and cooling by radiation determine the profile; where that profile would be unstable, convection can carry energy and bring the gradient close to an adiabatic temperature gradient.

The profile explains why a molecular band can appear in absorption or as emission lines. Greater opacity makes the emerging light sample a higher layer. If that layer is colder, the band is dimmer; an atmospheric thermal inversion can reverse this behaviour. An atmospheric retrieval therefore needs both abundances and the profile, rather than identifying composition from band depth alone.

Atomic physics studies the structure and dynamics of atoms, particularly their electrons and their interactions with radiation and external fields. The atomic nucleus supplies charge and mass, but changes within the nucleus belong primarily to nuclear physics.

The hydrogen atom is a useful starting point because one electron moves in an approximately Coulombic potential. Its discrete energy differences produce spectral lines. More complex atoms add electron interactions, angular-momentum coupling and field-induced shifts. Spectroscopy tests those models by comparing predicted and observed transition frequencies.

An inference of a compact object's gravitating mass from specified tracers and a dynamical or radiation model. Resolved stellar or gas orbits, spectroscopic binary dynamics, and reverberation mapping supply complementary approaches with different distance, geometry and equilibrium assumptions. A short variability timescale constrains size but is not a unique mass measurement.

An inference from radiation reflected by an irradiated accretion disc, including iron-line and continuum features. Doppler effects, gravitational redshift and ray propagation determine its profile. Associating the innermost reflecting radius with the innermost stable circular orbit can constrain spin, conditional on illumination, ionization, abundance, inclination and disk structure.

An inference of black-hole angular momentum, usually quoted through $a_{\ast }$. Accretion-based techniques use the dependence of the stable inner orbit and relativistic radiation transfer on spin in the Kerr black hole model. Disk truncation, inclination, illumination and atmosphere assumptions can imitate a change of spin and must enter the uncertainty model.

The Bondi mass measures the total mass-energy remaining in an isolated system at a retarded time, as defined at future null infinity. It is obtained by integrating the Bondi mass aspect over a sphere, with normalization set by the metric convention.

Outgoing gravitational radiation carries energy away, so the Bondi mass decreases by a nonnegative news-squared flux in the vacuum mass-loss formula. Additional outgoing matter contributes its own flux. This is distinct from the ADM mass defined at spatial infinity: the two quantities use different asymptotic cuts and account differently for radiation that has escaped.^[ref]

The Bondi shear is the leading trace-free angular distortion in an asymptotically flat spacetime described by Bondi-Sachs coordinates. In a common expansion, the angular metric is $h_{AB}=q_{AB}+C_{AB}/r+O(r^{-2})$, with $q^{AB}{C}_{AB}=0$.

Its retarded-time derivative determines the Bondi news function, with normalization depending on convention. The distinction matters: a nonzero but time-independent shear need not represent ongoing radiation, while changing shear describes radiative degrees of freedom. Changes between early and late configurations are also connected with gravitational memory.^[ref]

Charles Barkla distinguished characteristic X-ray emission associated with a chemical element from radiation that varied more continuously with the exciting conditions. Comparisons of secondary radiation, absorption and emission showed that different chemical elements produce reproducible characteristic components. The emitting material therefore became an experimentally identifiable part of the spectrum. This work helped establish X-ray spectroscopy as a probe of atomic structure before a complete microscopic explanation of the characteristic lines was available.^[ref][ref]

A physicist.^[ref]

The process by which a stalled outgoing shock in a collapsing stellar core regains sufficient energy to expel outer material. Neutrino heating and multidimensional flow can contribute, while dissociation and radiation drain energy. Collapse does not guarantee a successful ordinary supernova.

An expanding-universe regime in which a positive cosmological constant supplies most of the Friedmann equation. The approximately constant positive expansion rate gives exponential growth of the scale factor. Since matter, radiation, and curvature contributions decay with expansion, a positive cosmological constant eventually dominates a perpetually expanding model containing them; this does not guarantee that curvature dominates at an intermediate time.

Dark-matter indirect detection searches for ordinary particles or radiation produced by hypothetical dark-matter annihilation or decay. Candidate signals include gamma rays, neutrinos and charged cosmic rays.

An annihilation rate is typically proportional to the square of dark-matter density, while a decay rate is proportional to density. Predicted signals therefore depend on both particle properties and the distribution of dark matter along the line of sight. Astrophysical sources can produce similar radiation, so a spectral excess alone is not a unique identification. Consistent spatial, spectral and multi-messenger evidence is needed to distinguish a proposed signal from backgrounds.

David Todd Wilkinson helped construct a receiving horn and radiometer for absolute microwave-background measurements. The 1965 hot-universe paper explicitly credited the Princeton instrument to him and P. G. Roll, and explained the 3-centimetre wavelength as a compromise between atmospheric absorption and longer-wavelength foregrounds. Its measurements were still pending when the paper interpreted the separately reported microwave excess. This separates the team's experimental preparation from the evidence already available for relic radiation.^[ref]

Don N. Page investigated equilibrium black-hole thermodynamics with Stephen Hawking. Their 1983 work showed that sufficiently large black holes in Anti-de Sitter spacetime can have positive heat capacity and coexist in stable thermal equilibrium with radiation.^[ref]

The canonical ensemble distinguishes local stability from global preference: a locally stable black-hole solution need not have lower free energy than the competing thermal state.

At constant radiative efficiency of black-hole accretion $\eta$ and the Eddington luminosity, a black hole grows exponentially with the Salpeter time $t_S=\eta t_E/(1-\eta )$, where $t_E={\sigma }_{T}c/(4\pi G{m}_p)$. Its doubling time is $t_S\log 2$. The factor $1-\eta$ accounts for the supplied rest energy that escapes as radiation instead of increasing the hole's mass.

Emission is the production of electromagnetic radiation by a physical system. Mechanisms include accelerated charges, transitions between quantum states and thermal fluctuations.

For an isolated atomic transition, a photon carries energy $h\nu =E_i-E_f$. In thermal emission, a macroscopic distribution of microscopic motions and states produces a spectrum controlled by temperature and material response. Emission must be distinguished from scattering of incident radiation, though a measured signal can contain both contributions. The process and observation geometry determine the relevant spectrum and angular distribution.