Space & Astronomy Glossary

A growing reference of 100+ space and astronomy terms, each explained in plain English with at least one concrete example. Use Ctrl+F to find any term quickly.

Aphelion

The point in an orbit where an object is farthest from the Sun. Earth reaches aphelion in early July, about 152 million km from the Sun.

Apogee

The point in an orbit around Earth where a satellite is farthest from Earth's center. (Compare: perigee.)

Asteroid

A rocky, airless object orbiting the Sun, mostly between Mars and Jupiter in the asteroid belt. Ceres is the largest, at 940 km across.

Asteroid belt

The region between Mars and Jupiter (2.2–3.2 AU) containing millions of asteroids. Total mass is less than 4% of Earth's Moon.

Astronomical unit (AU)

The average distance from Earth to the Sun: 149,597,870.7 km. Used as a convenient distance unit within the solar system.

Aurora

Light displays in polar skies caused by charged particles from the Sun colliding with Earth's atmosphere, guided by Earth's magnetic field.

Big Bang

The cosmological model in which the universe expanded from an extremely hot, dense state about 13.8 billion years ago.

Black hole

A region of spacetime with gravity so strong that nothing — not even light — can escape from inside its event horizon.

Blueshift

The compression of light wavelengths into the blue end of the spectrum when a source is moving toward the observer (the optical Doppler effect).

Brown dwarf

An object too massive to be a planet but too small to sustain hydrogen fusion in its core. Failed stars.

Comet

A ball of dust and ice that develops a glowing coma and tail when it nears the Sun. Halley's Comet returns every 76 years.

Constellation

A pattern of stars in Earth's sky. The IAU recognizes 88 official constellations covering the entire celestial sphere.

Corona

The outermost layer of the Sun's atmosphere, visible as a halo during a total solar eclipse. Strangely, it's much hotter than the surface (millions of degrees vs. 5,500°C).

Cosmic microwave background (CMB)

The faint afterglow of the Big Bang — the oldest light in the universe, dating to 380,000 years after the universe began.

Cosmic ray

A high-energy particle (usually a proton) traveling through space at nearly the speed of light. Origin: supernovae and other extreme events.

Cosmological constant

Einstein's term for a constant in his field equations representing dark energy — the force driving accelerating expansion of the universe.

Crater

A bowl-shaped depression on a planet or moon, usually formed by an impact. The Moon has hundreds of thousands of visible craters.

Dark energy

A mysterious form of energy thought to make up about 68% of the universe and to be driving its accelerating expansion.

Dark matter

Invisible matter that makes up about 27% of the universe; detected only through its gravitational effects on visible matter.

Deep Space Network (DSN)

NASA's worldwide network of giant radio antennas (in California, Spain, and Australia) used to communicate with interplanetary spacecraft.

Doppler effect

The change in observed frequency or wavelength when a source moves relative to the observer. Used in astronomy to measure cosmic motion.

Dwarf planet

A solar-system body that orbits the Sun, is round, but hasn't 'cleared its orbital neighborhood.' Examples: Pluto, Eris, Ceres.

Eccentricity

A number from 0 to 1 describing how non-circular an orbit is. 0 = perfect circle, near 1 = very elongated ellipse.

Eclipse

When one celestial body passes through the shadow of another. Solar = Moon between Sun and Earth; Lunar = Earth between Sun and Moon.

Ecliptic

The plane of Earth's orbit around the Sun. Most planets orbit close to this plane, which is why they appear to follow a similar path across the sky.

Electromagnetic spectrum

The full range of light: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. Each carries different astronomical information.

Ellipse

A flattened circle; the shape of every planetary orbit, as proven by Kepler's First Law.

Escape velocity

The minimum speed needed to break free of a body's gravity without further propulsion. Earth: 11.2 km/s. Moon: 2.4 km/s.

Event horizon

The boundary around a black hole beyond which no information can escape to outside observers.

Exoplanet

A planet orbiting a star other than the Sun. Over 5,500 confirmed as of 2026.

Gamma-ray burst

The most energetic explosions known, produced by collapsing massive stars or neutron-star mergers. Visible across the entire universe.

Gas giant

A planet composed mainly of hydrogen and helium with no solid surface. Jupiter and Saturn are gas giants.

Geostationary orbit

An orbit 35,786 km above Earth's equator where a satellite circles Earth in exactly 24 hours, appearing to hover over one spot.

Goldilocks zone

Informal name for the habitable zone — the orbital region around a star where liquid water could exist on a planet's surface.

Gravitational lens

When a massive object bends the light of more distant objects, magnifying or distorting them — predicted by Einstein's general relativity.

Gravity assist

A maneuver where a spacecraft uses a planet's gravity and orbital motion to gain (or lose) speed. Voyager 1 used Jupiter to slingshot to Saturn.

Habitable zone

The orbital region around a star where temperatures could allow liquid water on a rocky planet's surface.

Heliopause

The boundary where the Sun's solar wind gives way to interstellar space. Voyager 1 crossed it in 2012, Voyager 2 in 2018.

Heliosheath

The turbulent outer layer of the heliosphere, between the termination shock and the heliopause.

Heliosphere

The bubble of solar wind surrounding the entire solar system.

Hubble constant

The current rate of cosmic expansion: about 70 km/s per megaparsec. The exact value is still debated (the 'Hubble tension').

Inferior conjunction

When Mercury or Venus passes between Earth and the Sun, appearing as a thin crescent or transit.

Infrared

Light with wavelengths longer than visible red. The James Webb Space Telescope sees in infrared, which can pierce dust clouds.

Interstellar medium

The thin gas and dust between stars in a galaxy. About one atom per cubic centimeter on average.

Interstellar space

The space between stars, beyond the influence of any single star's solar wind.

Ion drive

A propulsion system that uses electric fields to accelerate ions, providing very low thrust over very long durations. Highly fuel-efficient.

Kuiper Belt

A region beyond Neptune (30–55 AU) containing icy bodies including Pluto, Eris, and Haumea.

Lagrange point

Five gravitational equilibrium points in any two-body orbital system. The James Webb Telescope sits at the Sun-Earth L2 point, 1.5 million km from Earth.

Light-year

The distance light travels in one Earth year — about 9.461 trillion km. A unit of distance, not time.

Local Group

The cluster of about 80 galaxies (including the Milky Way and Andromeda) bound by mutual gravity, spanning roughly 10 million light-years.

Magnetar

A neutron star with an extreme magnetic field — trillions of times stronger than Earth's. Source of fast radio bursts.

Magnetosphere

The region around a planet dominated by its magnetic field. Earth's magnetosphere protects us from the solar wind.

Main sequence

The stable, hydrogen-fusing phase of a star's life. The Sun is a main-sequence star and will remain so for another 5 billion years.

Meteor

The streak of light from a meteoroid burning up in Earth's atmosphere — a 'shooting star.'

Meteorite

A piece of rock from space that survives passage through Earth's atmosphere and lands on the surface.

Meteoroid

A small piece of rock or metal in space, smaller than an asteroid.

Milky Way

Our home galaxy: a barred spiral with 100–400 billion stars, about 100,000 light-years across.

Nebula

A cloud of gas and dust in space. Some are stellar nurseries (e.g., the Orion Nebula); others are the remnants of dead stars.

Neutron star

The collapsed core of a massive star — incredibly dense (a teaspoon weighs a billion tons), only about 20 km across.

Nova

A sudden brightening of a white dwarf as it accretes matter from a companion star and undergoes a thermonuclear flash. Different from a supernova.

Oort Cloud

A hypothetical spherical shell of icy bodies surrounding the solar system at 2,000 to 100,000 AU. Source of long-period comets.

Opposition

When a planet is directly opposite the Sun in Earth's sky — fully illuminated and at its closest approach. Best time to observe outer planets.

Orbit

The curved path of an object around a more massive body, governed by gravity. Most orbits are elliptical.

Orbital period

The time an object takes to complete one orbit. Earth: 1 year. Neptune: 165 years. Voyager 1: not orbiting — escape trajectory.

Parallax

The apparent shift of a nearby object against distant background when viewed from two positions. Used to measure stellar distances.

Parsec

A unit of distance equal to 3.2616 light-years. Defined by stellar parallax: the distance at which 1 AU subtends an angle of 1 arcsecond.

Perihelion

The point in an orbit where an object is closest to the Sun. (Compare: aphelion.)

Photon

A particle of light. The quantum of electromagnetic radiation.

Planet

By IAU definition: a body that orbits the Sun, is massive enough to be round, and has cleared its orbital neighborhood. Eight planets in our solar system.

Planetary nebula

The glowing shell of gas ejected by a low-mass dying star (like the Sun in 5 billion years). Nothing to do with planets — historical misnomer.

Plasma

The fourth state of matter — ionized gas. Stars are made of plasma; so is most ordinary matter in the universe.

Pulsar

A rapidly rotating neutron star whose radio beam sweeps across Earth like a lighthouse, producing regular pulses.

Quasar

Extremely luminous active galactic nuclei powered by supermassive black holes accreting matter. The most powerful objects in the universe.

Radial velocity

The component of a star's motion along the line of sight, measurable via Doppler shift. Used to detect exoplanets.

Redshift

The stretching of light wavelengths into the red when a source moves away from the observer. Used to measure cosmic expansion.

Regolith

The layer of loose, dusty material on the surface of the Moon, Mars, asteroids, and other airless worlds. Apollo astronauts walked on lunar regolith.

Retrograde motion

Apparent backward motion of a planet against the background stars, caused by Earth overtaking it (or vice versa) in orbit.

Roche limit

The minimum distance at which a moon can orbit without being torn apart by tidal forces. Saturn's rings are inside Saturn's Roche limit.

RTG

Radioisotope Thermoelectric Generator — a nuclear power source used by deep-space probes (Voyager, Cassini, Curiosity, Perseverance, New Horizons).

Schwarzschild radius

The radius of the event horizon of a non-rotating black hole. The Sun's would be just 3 km.

Singularity

The infinitely dense point at the center of a black hole, where general relativity breaks down.

Solar flare

A sudden burst of energy from the Sun's surface, releasing as much energy as billions of nuclear bombs. Can disrupt Earth's communications.

Solar wind

A stream of charged particles continuously flowing from the Sun at 400–800 km/s.

Spacetime

The four-dimensional fabric of the universe (3 space + 1 time) that gravity warps, according to Einstein's general relativity.

Spectroscopy

The technique of splitting light into its component wavelengths to determine the chemistry, temperature, and motion of a source.

Standard candle

An object of known luminosity, like a Type Ia supernova or a Cepheid variable, used to measure cosmic distances.

Stellar mass

The mass of a star, usually expressed in units of the Sun's mass (M☉). Stars range from about 0.08 to 200+ solar masses.

Sunspot

A cooler, darker region on the Sun's surface caused by intense magnetic activity. Sunspots come and go on an 11-year cycle.

Supernova

The catastrophic explosion of a massive star at the end of its life. A single supernova can briefly outshine an entire galaxy.

Synodic period

The time between two successive identical alignments of an object with the Sun, as seen from Earth. The Moon's synodic period is 29.5 days (one full lunar cycle).

Telescope

An instrument that gathers and focuses light (or other electromagnetic radiation) to observe distant objects.

Termination shock

The boundary in the solar system where the solar wind slows abruptly from supersonic to subsonic speed. Voyager 1 crossed it in 2004.

Terminator

The boundary line between the lit and dark hemispheres of a planet or moon — the line of sunrise/sunset.

Tidal force

The differential gravitational pull on different parts of an object, caused by gravity weakening with distance. Source of Earth's ocean tides and Io's volcanism.

Tidal locking

When an object's rotation period matches its orbital period, so it always shows the same face to its primary. Earth's Moon is tidally locked to Earth.

Transit

When a planet passes in front of its star (or another body), causing a small dip in the star's brightness. Primary method for detecting exoplanets.

Universe

All of space, time, matter, and energy. Currently 13.8 billion years old, and expanding at an accelerating rate.

Variable star

A star whose brightness changes over time. Cepheid variables are critical 'standard candles' for measuring cosmic distances.

Visible light

Electromagnetic radiation with wavelengths from 380 to 700 nanometers — the slim band human eyes can detect.

Voyager Golden Record

A phonograph record carried aboard Voyager 1 and 2 containing sounds, music, and images from Earth as a message to any extraterrestrial intelligence.

White dwarf

The dense, cooling remnant left when a low-mass star like the Sun exhausts its fuel. About Earth-sized but as massive as the Sun.

Wormhole

A theoretical 'tunnel' through spacetime connecting two distant points. Allowed by general relativity but never observed; would require exotic matter to remain stable.

X-ray astronomy

The study of cosmic X-ray sources (black holes, neutron stars, hot gas in galaxy clusters) using space telescopes — X-rays don't penetrate Earth's atmosphere.

Year

The time a planet takes to orbit its star. Earth's year: 365.25 days. Neptune's year: 165 Earth years.

Zenith

The point on the celestial sphere directly overhead from the observer.

Zodiacal light

A faint, diffuse glow in the night sky caused by sunlight scattering off interplanetary dust along the ecliptic.