Outer Space Terms

This glossary provides definitions for common terms related to outer space, astronomy, and cosmology.

Astronomical Objects

Term	Definition	Example
Asteroid	A small, rocky body orbiting the Sun. Much smaller than a planet.
Black Hole	A region of spacetime where gravity is so strong that nothing, not even light, can escape.	Cygnus X-1
Comet	An icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases.	Halley's Comet
Galaxy	A large system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity.	Milky Way
Nebula	An interstellar cloud of dust, hydrogen, helium, and other ionized gases; often a star-forming region.	Orion Nebula
Planet	An astronomical body orbiting a star, which is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighboring region of planetesimals.	Earth

Orbit: The curved path of a celestial object or spacecraft around a star, planet, or moon.
Escape Velocity: The minimum speed needed for a free, non-propelled object to escape from the gravitational influence of a massive body.
Tides: The rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon, the Sun, and the rotation of the Earth.

Light-Year: A unit of astronomical distance equivalent to the distance that light travels in one year. This is approximately 9.46 trillion kilometers.
Redshift: The displacement of spectral lines toward longer wavelengths (the red end of the spectrum) in radiation from distant galaxies and celestial objects. This is interpreted as evidence that the universe is expanding.
Cosmic Microwave Background (CMB): The faint residual radiation from the Big Bang, detectable in every direction.