An exoplanet atmosphere controls the planet's spectrum, heat redistribution, chemistry, clouds, and atmospheric escape.
An irradiated planetary atmosphere receives stellar energy from above while internal heat emerges from below. Their competition creates radiative, convective, and sometimes thermally inverted regions.
A semi-grey atmosphere uses separate mean visible and infrared opacities. Their ratio controls where stellar energy is deposited.
The irradiation temperature is defined from the incident stellar flux, commonly by before redistribution and albedo factors are applied.
The internal effective temperature parametrizes the intrinsic cooling luminosity of a planet through .
An atmospheric thermal inversion is a region where temperature increases outward. In a strongly irradiated atmosphere it can arise when shortwave opacity exceeds thermal infrared opacity and stellar energy is absorbed high in the atmosphere.
The radiative-convective boundary is where radiative transport first requires a temperature gradient steeper than the adiabatic gradient, causing convection below.
During secondary eclipse the planet passes behind its star. The lost system light measures the planet-star flux ratio from reflected starlight and planetary thermal emission.
Geometric albedo compares a body's full-phase brightness with that of a flat, perfectly diffusing reference disc of the same cross-section.
Bond albedo is the fraction of total incident power reflected in all directions and over all wavelengths.
A Lambertian surface has direction-independent radiance. A perfectly reflecting Lambertian patch receiving normal flux emits specific intensity into its outward hemisphere.
An exoplanet transmission spectrum measures the wavelength-dependent area blocked as starlight passes tangentially through the atmosphere during transit.
For an isothermal ideal-gas atmosphere, . A strong transmission feature spanning scale heights has approximate transit-depth amplitude .
If extinction scales as , an isothermal hydrostatic atmosphere has . The measured continuum slope can therefore estimate atmospheric temperature when gravity and mean molecular mass are known.
An exoplanet emission spectrum is the wavelength-dependent thermal radiation emerging from the planet. Vertical temperature gradients turn opacity differences into absorption or emission features.
Atmospheric escape removes gas from a planet through thermal motion, hydrodynamic outflow, photochemistry, and interactions with stellar radiation and plasma.
For particles of mass at radius and temperature , the Jeans escape parameter isLarge gives weak tail escape, while order-unity indicates hydrodynamic blow-off.
Hydrodynamic atmospheric escape is a collisional transonic outflow in which bulk gas carries multiple species away. It replaces the hydrostatic Jeans-tail description when gravity is weak compared with thermal energy.
Exoplanet habitability concerns whether planetary conditions could sustain liquid water and life, while recognizing that observability and habitability are distinct questions.
For transmission through scale heights, a useful estimate isSmall host stars, low gravity, high temperature, and low mean molecular mass make atmospheric features larger.
A bulk-composition ternary diagram represents the mass fractions of three planetary components at a point whose barycentric coordinates sum to one.
Specific intensity is power per projected area, solid angle, and frequency interval. In vacuum it is conserved along a ray.
In local thermodynamic equilibrium, matter populations are thermal at the local temperature and the source function is the Planck function, .
When the source function varies approximately linearly with optical depth, the emergent intensity obeys .
Planetary science studies planets, moons, small bodies, planetary systems, and the processes that form and evolve them.
Planetary system dynamics studies the orbital evolution of planets, planetesimals, and dust under gravity, radiation, collisions, and dissipative forces.
A planetesimal is a solid body in a young or evolved planetary system whose collisions and gravitational interactions help build or erode larger bodies.
For a small body with orbital elements relative to a planet on a circular orbit of semi-major axis , the Tisserand parameter is approximatelyIt is derived from the Jacobi constant and is approximately conserved across separated weak encounters in the circular restricted three-body problem.
A debris disk is an optically thin circumstellar population of dust and larger solid bodies maintained mainly by collisions.
The fractional luminosity is ; for blackbody grains in an optically thin belt at radius , it equals their total geometric cross-section divided by .
A collisional cascade transfers mass from large bodies to progressively smaller fragments. A steady cascade has a size distribution whose mass flux through logarithmic size bins is independent of size.
The catastrophic disruption threshold is the impact energy per unit target mass required both to shatter a body and to disperse enough fragments against self-gravity.
A rubblising collision shatters a body but does not supply enough energy to disperse its fragments against self-gravity, leaving a gravitationally bound rubble pile.
Interplanetary dust consists of small solid particles orbiting within a planetary system and responding to gravity, radiation, plasma, and collisions.
The radiation-pressure coefficient is the ratio of outward stellar radiation pressure to inward stellar gravity on a dust grain. The grain therefore feels effective gravitational parameter .
Poynting–Robertson drag is the tangential component of stellar radiation force caused by aberration in an orbiting dust grain's frame. It removes orbital energy and angular momentum and drives the grain inward.
During inward migration, dust can be captured into an exterior mean-motion resonance with a planet. A phase-shifted conjunction then lets planetary torque replace angular momentum lost to Poynting–Robertson drag.
Stellar astrophysics studies the structure, formation, energy production, observable properties, and evolution of stars.
Stellar structure follows from mass conservation, hydrostatic equilibrium, energy generation, and energy transport together with an equation of state and opacity law.
A stellar polytrope obeys for constant and polytropic index . Its dimensionless density profile satisfies the Lane-Emden equation.
Homologous stellar models have the same dimensionless radial profiles after mass, radius, density, pressure, and temperature are scaled by characteristic values.
In a radiative stellar region, photons carry luminosity down the temperature gradient. Dimensional scaling of the radiative-diffusion equation gives .
The mass-luminosity relation connects a star's mass to its luminosity. Its slope depends on opacity, nuclear burning, pressure support, composition, and evolutionary state.
Balancing outward radiative acceleration against gravity givesIt is an approximate upper luminosity for a hydrostatic star of opacity .
Stellar nuclear fusion converts light nuclei into more tightly bound nuclei and supplies most main-sequence stellar luminosity.
The proton–proton chain converts hydrogen to helium and dominates hydrogen burning in stars near and below the Sun's mass. Near , its specific energy generation is roughly proportional to .
The CNO cycle catalyses hydrogen fusion through carbon, nitrogen, and oxygen nuclei. Near , its specific energy generation is approximately proportional to and is therefore strongly concentrated near a star's centre.
The zero-age main sequence is the locus where stars begin stable core hydrogen burning with nearly their initial composition.
Stellar evolution is the change of a star's structure and composition as nuclear reactions, energy transport, mass loss, and interactions proceed.
A Roche lobe is the region around one component of a circular binary inside the critical effective-potential surface through the inner Lagrange point.
Conservative binary mass transfer moves matter between the components while preserving total binary mass and orbital angular momentum.
Binary mass transfer is dynamically stable when the donor's radius decreases relative to its Roche-lobe radius after a small mass loss, reducing overflow instead of amplifying it.
A common-envelope phase occurs when unstable binary interaction engulfs both stellar cores in one envelope. Orbital energy and angular momentum may eject the envelope, leaving a compact binary, or the cores may merge.
An astrophysical disk is a flattened rotating distribution of gas, dust, stars, or other matter whose orbital motion strongly influences its structure and evolution.
A thin disk has vertical semi-thickness much smaller than cylindrical radius . Vertical gravity is then approximately linear in height and orbital dynamics is nearly planar.
An accretion disk transports mass inward and angular momentum outward through stresses, while dissipating orbital energy as heat.
Viscous disk evolution combines mass conservation with radial transport of angular momentum by stress. In a steady Keplerian disk, the sum of advective and stress-carried angular-momentum fluxes is independent of radius.
A decretion disk is fed near its inner edge and transports mass outward. Rapidly rotating Be stars can sustain gaseous decretion disks whose outer material is removed by a companion, radiation, or a wind.
A Be star is a rapidly rotating B-type star showing Balmer emission from a circumstellar gaseous decretion disk.
The alpha-disk prescription writes turbulent kinematic viscosity as , with dimensionless . It reflects eddies no larger than the disk thickness and no faster than the sound speed.
The Shakura--Sunyaev thin disk is a geometrically thin, optically thick, radiatively efficient accretion-disk model whose turbulent stress is parametrized by the alpha prescription.
The magnetorotational instability destabilizes a magnetized differentially rotating flow when angular velocity decreases outward. Magnetic tension couples neighboring fluid elements, allowing the inner one to lose angular momentum and fall inward while the outer one gains angular momentum and moves outward.
For a vertical magnetic field in a Keplerian disk, axisymmetric vertical MRI modes are unstable when . The shortest unstable wavelength is .
A local thermal equilibrium is stable when a small temperature increase raises cooling faster than heating at fixed surface density. The standard total-pressure alpha prescription makes a radiation-pressure-dominated, Thomson-opacity disk thermally unstable.
Disk self-gravity can overcome pressure and rotational support, producing growing density disturbances, spiral structure, or fragmentation.
For a razor-thin isothermal gas disk,Axisymmetric disturbances are stable when : pressure stabilizes short wavelengths, epicyclic motion stabilizes long wavelengths, and self-gravity drives intermediate wavelengths.
The shearing sheet is a local Cartesian approximation to a differentially rotating disk. It retains Coriolis force, linearized tidal gravity, and a background linear shear.
Thermal relaxation drives the local sound speed toward an equilibrium value on a cooling time . Fast relaxation gives an isothermal response, while slow relaxation gives an adiabatic response.
A galaxy is a gravitationally bound system of stars, interstellar gas and dust, stellar remnants, and dark matter.
The interstellar medium is the gas, dust, magnetic field, and energetic-particle population between stars in a galaxy.
A molecular cloud is a cold dense region of the interstellar medium in which hydrogen is predominantly molecular and stars can form.
Rotational carbon-monoxide line emission traces cold molecular clouds because abundant molecular hydrogen is difficult to observe directly. A conversion factor relates integrated CO intensity to molecular-hydrogen column density.
Interstellar dust absorbs and scatters short-wavelength starlight and reradiates the absorbed energy thermally in the infrared.
The neutral-hydrogen 21-centimeter line is the ground-state hyperfine transition of atomic hydrogen. Its brightness and Doppler shift map atomic-gas column density and line-of-sight velocity.
A cosmic ray is a high-energy charged particle propagating through space. Galactic cosmic-ray electrons emit synchrotron and inverse-Compton radiation, while cosmic-ray nuclei produce gamma rays through collisions with gas.
Galactic multiwavelength astronomy combines spectral bands that respond differently to stars, gas, dust, magnetic fields, temperature, and energetic particles.
Galactic synchrotron emission is nonthermal radio radiation from relativistic electrons spiralling in the Galactic magnetic field. Its broad latitude distribution reflects cosmic-ray transport and the magnetic halo.
Diffuse Galactic gamma rays arise mainly from neutral-pion decay after cosmic-ray nuclear collisions, electron bremsstrahlung in gas, and inverse-Compton scattering from radiation fields.
The Galactic scale height measures the vertical thickness of a tracer. Gravity, velocity dispersion, thermal pressure, magnetic and cosmic-ray support, particle transport, and source lifetime determine its value.
The Galactic bar is the elongated central stellar structure of the Milky Way. Extinction-corrected near-infrared standard candles and gas position--velocity maps constrain its distance, orientation, and three-dimensional shape.
The star formation rate is the mass converted into stars per unit time. Infrared luminosity traces dust-reprocessed light from young stars, while molecular-gas tracers locate their cold fuel.
A galaxy rotation curve gives circular speed as a function of radius. Its decomposition into stellar, gaseous, and dark-matter contributions is generally nonunique.
The maximum disk hypothesis assigns the largest stellar mass-to-light ratio compatible with a measured rotation curve, conventionally making the disk supply about of the circular speed near .
The disk--halo degeneracy is the ability of different stellar mass-to-light ratios and dark-halo profiles to reproduce nearly the same total galaxy rotation curve.
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Astrophysics is a branch of astronomy that focuses on understanding the physical properties and underlying phenomena of celestial objects and the universe as a whole. It combines principles from physics and astronomy to study a wide range of topics, including the formation, evolution, and behavior of stars, galaxies, black holes, nebulae, and the overall structure of space-time.
