Light-year
A light-year is a unit of distance, not time. It is the distance light travels through a vacuum in one year, about 9.46 trillion kilometers.
ASTRONOMY GLOSSARY
The language of space, explained.
Thirty terms to help you read about the Universe with confidence.
Parsec
A parsec is a large astronomical distance equal to about 3.26 light-years. Its name comes from measuring a star’s annual parallax.
Astronomical Unit
An astronomical unit is the average distance between Earth and the Sun, about 149.6 million kilometers. It is useful for describing distances within the Solar System.
Exoplanet
An exoplanet is a planet orbiting a star other than the Sun. Astronomers can find one when it dims its star or causes a tiny gravitational wobble.
Habitable Zone
The habitable zone is the region around a star where conditions could allow liquid water on a planet’s surface. It guides the search for life but does not guarantee it.
Nebula
A nebula is a large cloud of gas and dust in space. It may reflect or emit light, and some nebulae are nurseries where new stars form.
Protostar
A protostar is a young star forming as a cloud of gas and dust collapses. It has not reached stable nuclear fusion yet, but gravity is already heating it.
Red Giant
A red giant is a late-life star that has expanded and cooled at its surface. The Sun will enter this stage far in the future.
White Dwarf
A white dwarf is the hot, dense core left after a star sheds its outer layers. It no longer sustains ordinary fusion and slowly cools for billions of years.
Neutron Star
A neutron star is an extremely dense remnant left by some supernovae. Its matter is compressed so intensely that a teaspoon could weigh billions of tons.
Supernova
A supernova is a powerful stellar explosion that releases enormous energy and elements into space. These events enrich galaxies with material for new stars and planets.
Pulsar
A pulsar is a rapidly spinning neutron star that emits narrow beams of radiation. When a beam sweeps past Earth, telescopes detect remarkably regular pulses.
Galaxy
A galaxy is a large gravitationally bound system of stars, gas, dust and dark matter. Galaxies can be spiral, elliptical or irregular.
Milky Way
The Milky Way is the spiral galaxy that contains our Solar System. It holds hundreds of billions of stars and a supermassive black hole at its center.
Dark Matter
Dark matter does not emit light, so it cannot be seen directly. Its presence is inferred from the gravitational effects it has on stars, galaxies and light.
Dark Energy
Dark energy is the name given to the unknown influence linked with the accelerating expansion of the Universe. Its nature remains unresolved, but its effect appears on the largest scales.
Black Hole
A black hole is a region of space with gravity so strong that even light cannot escape. It can form after a massive star collapses or compact objects merge.
Event Horizon
The event horizon is the boundary around a black hole beyond which no signal can return outward. It is not a solid surface, but a boundary in spacetime geometry.
Singularity
A singularity is a theoretical region where general relativity produces infinite values. It marks the limits of current physics rather than necessarily describing a literal physical object.
Accretion Disk
An accretion disk is a hot, rotating disk of gas and dust gradually falling onto a compact object. Friction heats the material, allowing it to shine brightly.
Gravitational Lensing
Gravitational lensing bends and magnifies light from a distant object when a massive body lies between it and the observer. It helps scientists study distant galaxies and dark matter.
Big Bang
The Big Bang model says the Universe began about 13.8 billion years ago in an extremely hot, dense state. It describes expanding space, not an explosion into empty space.
Cosmic Microwave Background
The cosmic microwave background is faint heat left from the early Universe. It fills space and lets scientists measure important properties of cosmic history.
Redshift
Redshift is a shift of light toward longer, redder wavelengths. It can indicate a source moving away, motion in gravity, or the expansion of the Universe itself.
Asteroid
An asteroid is a small rocky or metallic body orbiting the Sun. Most asteroids occupy the main belt between the orbits of Mars and Jupiter.
Comet
A comet is a small body made of ice, dust and rock. Near the Sun it heats up, forming a coma and often a long tail pointing away from the Sun.
Kuiper Belt
The Kuiper Belt is a cold region beyond Neptune filled with icy small bodies. Some comets originate there, and dwarf planets such as Pluto reside in that region.
Oort Cloud
The Oort Cloud is a hypothetical distant spherical shell of icy bodies around the Solar System. Long-period comets may arrive from this remote reservoir.
Heliosphere
The heliosphere is the huge region around the Sun shaped by the solar wind and magnetic field. It partly shields the planets from interstellar space and energetic particles.
Spectrum
A spectrum is light separated by wavelength. Astronomers use it to determine the composition, temperature, motion and other properties of distant objects.