AST key terms

A measure of a star's intrinsic brightness, defined as the apparent magnitude it would have if placed at a distance of 10 parsecs from the observer.

A logarithmic measure of the flux of light received from an object at Earth, where smaller numbers indicate brighter sources and a difference of five magnitudes equals a factor of 100.

The difference between an object's apparent and absolute magnitudes, which quantifies the dimming caused by distance and can be inverted to estimate how far away the object lies.

The apparent shift in an object's position against a background when viewed from different vantage points; for stars, the annual half-angle shift caused by Earth's orbit around the Sun.

The distance at which one astronomical unit subtends an angle of one arcsecond, equal to about 3.26 light-years or 3.09 times ten to the sixteenth meters.

The ordering of stars into types such as O, B, A, F, G, K and M according to the strength of absorption lines, which primarily reflects surface temperature rather than composition.

A plot of stellar luminosity against temperature or spectral type that reveals populations such as the main sequence, giants, supergiants and white dwarfs, and underlies theories of stellar evolution.

The band on the Hertzsprung-Russell diagram where stars fuse hydrogen into helium in their cores, the longest-lived phase of a star's life and the location of most stars.

The balance in a star between the inward pull of gravity and the outward pressure gradient of its gas and radiation, which keeps the star stable over long timescales.

The maximum mass, roughly 1.4 solar masses, that a white dwarf supported by electron degeneracy pressure can have before collapsing or exploding.

The dense, Earth-sized remnant left when a low-mass star sheds its outer layers, supported against gravity by electron degeneracy pressure and slowly cooling over billions of years.

A glowing shell of ionized gas ejected by a dying low-mass star and lit by the exposed hot core, which later becomes a white dwarf.

The merging of light atomic nuclei into heavier ones with a release of energy, which powers stars and proceeds mainly through the proton-proton chain and the CNO cycle in stellar cores.

The temperature of an ideal blackbody that would radiate the same total flux per unit surface area as a given star or planet.

The characteristic continuous spectrum emitted by an idealized perfect absorber, whose peak wavelength depends inversely on temperature according to Wien's law.

The change in observed wavelength of light caused by relative motion between source and observer, with motion toward the observer shifting light to shorter wavelengths.

The displacement of spectral features toward longer wavelengths, caused by motion away from the observer or by the expansion of the universe.

The observed relation that distant galaxies recede at speeds proportional to their distance, with the constant of proportionality called the Hubble constant.

The assumption that on sufficiently large scales the universe is homogeneous and isotropic, meaning it looks the same from every location and in every direction.

The nearly uniform microwave radiation filling the sky, released when the early universe cooled enough for neutral atoms to form, and now observed at about 2.7 kelvin.

The epoch in the early universe when free electrons and protons combined into neutral hydrogen, allowing photons to travel freely and producing the cosmic microwave background.

Unseen material inferred from its gravitational effects on galaxy rotation, cluster dynamics and lensing, which does not emit or absorb detectable light.

A graph of orbital speed versus distance from a galaxy's center; flat outer curves indicate substantial mass beyond the visible disk, commonly attributed to dark matter.

The bending of light by the gravity of intervening mass, which can magnify, distort or multiply images of background objects and reveals otherwise invisible mass.

The flattened, rotating component of a spiral galaxy containing stars, gas and dust, usually with spiral arms and a scale height much smaller than its radius.

The gas and dust filling the space between stars, spanning cold molecular clouds to hot ionized plasma and serving as the reservoir from which new stars form.

A cold, dense region of the interstellar medium rich in molecular hydrogen and dust, where gas can collapse under gravity to give birth to new stars.

The condition under which a cloud's self-gravity overcomes internal gas pressure, causing it to collapse; it occurs when the cloud mass exceeds a critical value for its temperature and density.

The absorption and scattering of light by dust grains along the line of sight, which dims and reddens distant stars and must be corrected in photometry.

The removal of one or more electrons from an atom or molecule, which in astronomy is driven by ultraviolet photons or collisions and controls the emission of nebulae and plasma.

Three empirical rules describing planetary motion: orbits are ellipses with the Sun at one focus, equal areas are swept in equal times, and orbital period squared scales with semimajor axis cubed.

A dimensionless number describing how much an orbit deviates from a circle, ranging from zero for circular orbits to values approaching one for highly elongated ellipses.

Half the longest diameter of an elliptical orbit, equal to the average of the perihelion and aphelion distances and used in Kepler's third law.

The component of an object's motion along the line of sight, measured from Doppler shifts of its spectral lines and used to detect exoplanets and binary companions.

Einstein's theory describing gravity as the curvature of spacetime produced by mass and energy, which predicts phenomena such as light bending and gravitational waves.

The ability of an instrument to distinguish closely spaced sources, limited by diffraction and improving with larger aperture and shorter wavelength.

A technique that measures and rapidly corrects wavefront distortions caused by atmospheric turbulence, sharpening images from ground-based telescopes toward their diffraction limit.

The blurring of astronomical images caused by turbulence in Earth's atmosphere, usually quoted as the angular diameter of a point source's image in arcseconds.