The vocabulary of the universe
Astronomy has its own language. These are the terms you will meet in every textbook, telescope log and lecture — defined plainly, without dumbing them down.
43 terms
Cosmology
1Redshift
The stretching of light toward longer (redder) wavelengths as a source moves away. It is the main evidence that the universe is expanding.
Earth & Sky
5Aurora
Light emitted when charged particles from the solar wind excite gases in Earth's upper atmosphere, guided toward the poles by the magnetic field.
Eclipse
An event where one body passes into the shadow of another. A solar eclipse occurs at new Moon; a lunar eclipse occurs at full Moon.
Occultation
When a nearer body passes in front of a more distant one and hides it, such as the Moon occulting a star or planet.
Solar wind
A continuous stream of charged particles flowing outward from the Sun's corona at hundreds of kilometres per second.
Transit
When a smaller body passes in front of a larger one, such as Mercury or Venus crossing the Sun, or an exoplanet crossing its star.
Measurement
9Absolute magnitude
How bright a star would appear from a standard distance of 10 parsecs (32.6 light-years). It describes true intrinsic brightness, unlike apparent magnitude.
Albedo
The fraction of light a surface reflects. Venus has a high albedo (about 0.7) because of its clouds; the Moon's average albedo is only about 0.12.
Apparent magnitude
A measure of how bright an object looks from Earth. The scale is reversed: smaller numbers mean brighter objects, and the brightest stars are near magnitude 0 or negative.
Astronomical unit (AU)
The average distance from Earth to the Sun: 149.6 million km. It is the standard unit for distances within the solar system.
Cepheid variable
A pulsating yellow supergiant whose period is tightly linked to its luminosity. That relationship lets astronomers measure distances across the galaxy.
Light-year
The distance light travels in one Julian year — about 9.46 trillion km. It measures distance, not time.
Magnitude
The brightness scale used in astronomy. Each step of 1 magnitude is a factor of about 2.512 in brightness.
Parallax
The apparent shift of a nearby object against a distant background as the observer moves. Stellar parallax is the basis of the parsec.
Parsec
The distance at which 1 AU subtends one arcsecond — about 3.26 light-years. It is the unit most used for interstellar distances.
Moon
2Gibbous
A phase in which more than half, but not all, of the Moon's visible disc is illuminated.
Terminator
The line separating day and night on a planet or moon. Shadows are longest there, making it ideal for observing surface relief.
Orbits
6Aphelion
The point in an orbit farthest from the Sun. Earth reaches aphelion in early July, about 152 million km from the Sun.
Conjunction
When two celestial bodies appear close together in the sky from Earth's perspective, such as a planet passing near the Moon.
Opposition
When a planet lies opposite the Sun in our sky — closest and brightest for observation. Mars is at opposition roughly every 26 months.
Perihelion
The point in an orbit closest to the Sun. Earth reaches perihelion in early January, about 147 million km from the Sun.
Retrograde motion
Apparent westward motion of a planet against the stars, caused by Earth overtaking it on the inside of its orbit. Venus and Uranus also rotate retrograde.
Sidereal day
The time for Earth to rotate once relative to the stars: 23 hours 56 minutes. A solar day is four minutes longer because Earth also moves along its orbit.
Physics
2Escape velocity
The speed needed to leave a body's gravitational field permanently. It is 11.2 km/s for Earth and about 618 km/s at the Sun's surface.
Roche limit
The distance from a body within which tidal forces overcome self-gravity and break a smaller object apart — how Saturn's rings stay as rings.
Planetary science
3Dwarf planet
A round body orbiting the Sun that has not cleared other objects from its orbital neighbourhood. Pluto, Eris, Ceres, Haumea and Makemake are recognised examples.
Exoplanet
A planet orbiting a star other than the Sun. Most are detected by the dips in starlight they cause (transits) or by the wobble they induce (radial velocity).
Kuiper Belt
A ring of icy bodies beyond Neptune, roughly 30–50 AU from the Sun, containing Pluto and many short-period comets.
Stellar evolution
15Black hole
A region where gravity is so strong that nothing, not even light, can escape. Stellar-mass black holes form when the core of a massive star collapses.
Brown dwarf
An object between about 13 and 80 Jupiter masses that never sustained stable hydrogen fusion. It glows faintly from leftover heat and slow deuterium burning.
Event horizon
The boundary of a black hole beyond which nothing can escape, because the escape velocity exceeds the speed of light.
Hertzsprung–Russell diagram
A plot of stellar luminosity against temperature. Stars cluster along the main sequence, with giants and white dwarfs in separate regions.
Luminosity
The total energy a star radiates per second. The Sun's luminosity, 3.828 × 10²⁶ watts, is the standard unit (L☉).
Main sequence
The stage in which a star fuses hydrogen into helium in its core, with fusion pressure balancing gravity. About 90% of a star's life is spent here.
Nebula
A cloud of interstellar gas and dust. Emission nebulae glow because nearby hot stars ionise their gas; dark nebulae block the light behind them.
Neutron star
The collapsed core of a massive star, typically about 20 km across and 1.4–2 solar masses. Rapidly rotating, magnetised examples appear as pulsars.
Nucleosynthesis
The creation of new atomic nuclei. Light elements formed in the early universe; most heavier elements are forged inside stars and released by supernovae.
Planetary nebula
The expanding shell of ionised gas shed by a low-mass star late in its life. Despite the name, it has nothing to do with planets.
Protostar
A collapsing cloud core that has not yet begun hydrogen fusion. It shines from gravitational energy and is often wrapped in a disc of gas and dust.
Pulsar
A rapidly rotating neutron star whose magnetic poles sweep beams of radiation toward Earth, producing regular pulses.
Spectral classification
The temperature sequence of stars — O, B, A, F, G, K, M from hottest to coolest — read from absorption lines in their spectra. The Sun is type G.
Supernova
The explosive death of a star. A Type II supernova ends a massive star's life; a Type Ia comes from a white dwarf exceeding its mass limit.
White dwarf
The dense remnant core of a low-mass star, about Earth-sized and supported by electron degeneracy pressure. It cannot exceed about 1.4 solar masses.