Astrophysics Glossary

A rotating disk of gas, dust, and other material that forms around a massive object such as a black hole or protostar, as matter spirals inward due to gravity and angular momentum conservation.

Ordinary matter composed of protons, neutrons, and electrons, which makes up atoms, molecules, stars, and planets. It constitutes only about 5% of the total energy content of the universe.

A region of spacetime where gravity is so intense that nothing, including light, can escape from within its event horizon. Black holes can be stellar-mass, intermediate-mass, or supermassive.

The decrease in wavelength of light from an object moving toward the observer, shifting the light toward the blue end of the electromagnetic spectrum.

The thermal radiation filling the observable universe almost uniformly, representing the oldest light in the universe emitted approximately 380,000 years after the Big Bang.

The unknown form of energy that permeates all of space and drives the accelerating expansion of the universe, comprising roughly 68% of its total energy density.

Non-luminous matter that interacts gravitationally but not electromagnetically, inferred from its effects on galaxy rotation curves, gravitational lensing, and large-scale structure formation.

The change in observed frequency or wavelength of a wave due to relative motion between the source and the observer. In astrophysics, it manifests as redshift or blueshift of spectral lines.

The boundary around a black hole inside which the escape velocity exceeds the speed of light, making it a point of no return for matter and radiation.

A planet that orbits a star outside our solar system. Thousands of exoplanets have been discovered using methods such as the transit method and radial velocity measurements.

The most energetic electromagnetic events in the universe, consisting of intense bursts of gamma rays lasting from milliseconds to minutes, associated with supernovae, neutron star mergers, or collapsars.

A ripple in the curvature of spacetime that propagates outward from its source at the speed of light, generated by the acceleration of massive objects such as merging compact binaries.

The range of orbital distances around a star within which a planet's surface could support liquid water, a key condition for life as we know it. Also called the Goldilocks zone.

The current rate of expansion of the universe, denoted $H_0$, with a value of approximately 67 to 73 km/s/Mpc depending on the measurement method. It relates a galaxy's recession velocity to its distance.

The total amount of energy emitted by a star or other astronomical object per unit time, typically measured in watts or in units of solar luminosity.

The continuous band on the Hertzsprung-Russell diagram where stars spend the majority of their lives fusing hydrogen into helium. A star's position on the main sequence is determined primarily by its mass.

A large cloud of gas and dust in interstellar space, which can be a region of star formation (emission nebula) or the remnant of a dying star (planetary nebula or supernova remnant).

An extremely dense stellar remnant composed almost entirely of neutrons, formed when a massive star's core collapses during a supernova. Neutron stars have typical masses of 1.4 to 2 solar masses compressed into a sphere about 20 km in diameter.

A unit of distance equal to approximately 3.26 light-years, defined as the distance at which one astronomical unit subtends an angle of one arcsecond of parallax.

An extremely luminous active galactic nucleus powered by a supermassive black hole accreting matter at a high rate. Quasars can outshine their entire host galaxy and are visible across billions of light-years.

The increase in wavelength of electromagnetic radiation from an object, caused by the object's recession from the observer or the expansion of space. Cosmological redshift is the primary tool for measuring distances to remote galaxies.

A point of theoretically infinite density and zero volume at the center of a black hole, where the known laws of physics break down. Resolving the singularity is a major goal of quantum gravity research.

The study of the interaction between matter and electromagnetic radiation, used in astrophysics to determine the chemical composition, temperature, density, and velocity of celestial objects by analyzing their spectra.

A catastrophic stellar explosion that can briefly outshine an entire galaxy, occurring either through core collapse of a massive star or thermonuclear detonation of a white dwarf that exceeds the Chandrasekhar limit.

The compact, dense remnant of a low- to intermediate-mass star that has shed its outer layers, supported against further collapse by electron degeneracy pressure. White dwarfs slowly cool and fade over billions of years.