Glossary
52 key terms explained in plain language. Search, or pick a letter.
A
accretion disk
A fairly flat sheet of gas and dust circling a newborn star, a black hole or another massive object. Material in it can glow, even in X-rays.
Source: NASA Goddard (Imagine the Universe!) · Related articleaphelion
The point in an orbit where a planet or other body is farthest from the Sun.
Source: NASA Goddard (Imagine the Universe!)astronomical unit (AU)
The average distance from Earth to the Sun, given as 149,597,870 km. Distances inside the solar system are often stated in this unit.
Source: NASA Goddard (Imagine the Universe!)B
Big Bang
The widely accepted model in which the universe began in a hot, dense state and has been expanding ever since.
Source: NASA Goddard (Imagine the Universe!) · Related articlebinary star
Two stars that orbit around a shared center of mass.
Source: NASA Goddard (Imagine the Universe!)black hole
An object whose gravity is so strong that not even light can escape from it.
Source: NASA Goddard (Imagine the Universe!) · Related articleblueshift
A squeezing of light toward shorter wavelengths, seen in the spectral lines of an object that is approaching.
Source: NASA Goddard (Imagine the Universe!)C
cluster of galaxies
A system of galaxies with anywhere from a few to a few thousand member galaxies.
Source: NASA Goddard (Imagine the Universe!)corona
The uppermost layer of a star's atmosphere. The Sun's corona is far hotter than its visible surface.
Source: NASA Goddard (Imagine the Universe!)cosmic microwave background
A faint glow filling the universe in every direction, released 380,000 years after the Big Bang. It is often called the universe's baby picture.
Source: NASA Science · Related articleD
dark matter
Invisible mass known only from its gravity. Stars at the edges of spiral galaxies move too fast to be held by visible matter alone; about 27% of the universe is dark matter.
Source: NASA Science · Related articleDoppler effect
The apparent change in the wavelength of light or sound caused by motion of the source, the observer, or both.
Source: NASA Goddard (Imagine the Universe!)E
eccentricity
A number describing how stretched an orbit's ellipse is.
Source: NASA Goddard (Imagine the Universe!)eclipse
The passing of one celestial body in front of another, which cuts off some or all of the light from the second body.
Source: NASA Goddard (Imagine the Universe!)event horizon
The boundary around a black hole inside which nothing can escape, not even light. It is not a solid surface.
Source: NASA Science · Related articleG
galaxy
A huge system of gas and stars held together by gravity, ranging from a few thousand stars to trillions.
Source: NASA Science · Related articlegamma-ray burst
A burst of gamma rays from space that lasts from a fraction of a second to many minutes.
Source: NASA Goddard (Imagine the Universe!)globular cluster
A ball-shaped collection of stars that share a common origin.
Source: NASA Goddard (Imagine the Universe!)gravitational waves
Ripples in space-time produced by the motion of massive objects in the universe.
Source: NASA Goddard (Imagine the Universe!)H
Hubble's law
The relationship between how far away a galaxy is and how fast it is moving away through space.
Source: NASA Goddard (Imagine the Universe!) · Related articleI
interstellar medium
The gas and dust between the stars, which fills the plane of the Galaxy much as air fills a room.
Source: NASA Goddard (Imagine the Universe!) · Related articleK
Kepler's laws
Three rules for planetary motion: orbits are ellipses with the Sun at one focus; a planet sweeps out equal areas in equal times; the square of the orbital period is proportional to the cube of the orbit's average distance from the Sun.
Source: NASA Goddard (Imagine the Universe!)L
Lagrange point
A spot near two massive bodies, such as Earth and the Sun, where their pulls combine so a small object can stay in place relative to them. Webb orbits the Sun-Earth L2.
Source: NASA Goddard (Imagine the Universe!) · Related articlelight-year
A unit of length equal to the distance light travels in one year. It measures distances between stars and galaxies, not time.
Source: NASA Goddard (Imagine the Universe!) · Related articleluminosity
The rate at which a star or other object gives off energy, usually as light and other radiation.
Source: NASA Goddard (Imagine the Universe!)M
magnetosphere
The region of space around an object, such as Earth, that is controlled by that object's magnetic field.
Source: NASA Goddard (Imagine the Universe!)magnitude
A number on a scale that states how bright a star or other celestial body is. Lower numbers mean brighter objects.
Source: NASA Goddard (Imagine the Universe!) · Related articleN
nebula
A cloud of gas and dust in the space between stars.
Source: NASA Goddard (Imagine the Universe!) · Related articleneutron star
The collapsed core of a massive star left behind after a supernova explosion.
Source: NASA Goddard (Imagine the Universe!) · Related articlenova
A star that suddenly flares up, brightening for a time before fading again.
Source: NASA Goddard (Imagine the Universe!)nuclear fusion
A process in which small atomic nuclei join to make a larger one, releasing energy. It powers the Sun and other stars.
Source: NASA Goddard (Imagine the Universe!) · Related articleO
orbit
The path of one object as it moves around another object or point.
Source: NASA Goddard (Imagine the Universe!)P
parallax
The apparent shift in position of a nearby object against a more distant background when the viewpoint changes.
Source: NASA Goddard (Imagine the Universe!)parsec
The distance to an object that has a parallax of one arc second.
Source: NASA Goddard (Imagine the Universe!)perihelion
The point in an orbit where a planet or other body is closest to the Sun.
Source: NASA Goddard (Imagine the Universe!)planetary nebula
A shell of gas thrown off by a star like the Sun at the end of its life.
Source: NASA Goddard (Imagine the Universe!) · Related articleprotostar
A very dense core inside a molecular cloud where a star is in the process of forming.
Source: NASA Goddard (Imagine the Universe!) · Related articlepulsar
A rotating neutron star that sends out regular pulses of radiation.
Source: NASA Goddard (Imagine the Universe!)Q
quasar
An enormously bright, very distant object that gives off huge amounts of energy.
Source: NASA Goddard (Imagine the Universe!)R
red giant
A star with a cool surface and a diameter very large compared with the Sun.
Source: NASA Goddard (Imagine the Universe!) · Related articleredshift
A stretching of light toward longer wavelengths, seen in the spectral lines of an object. Distant, receding galaxies show it.
Source: NASA Goddard (Imagine the Universe!) · Related articleS
singularity
A term used for the center of a black hole, where the curvature of space-time is greatest.
Source: NASA Goddard (Imagine the Universe!) · Related articlesolar flare
A violent eruption of gas on the Sun's surface.
Source: NASA Goddard (Imagine the Universe!)solar mass
A unit of mass equal to the mass of the Sun, used to state the masses of stars and black holes.
Source: NASA Goddard (Imagine the Universe!) · Related articlesolstice
One of two days a year when Earth's tilt is most pronounced. The summer hemisphere has its longest day and the winter hemisphere its shortest.
Source: NASAspectrum
A plot showing how intense light is at different frequencies, which reveals what an object is made of.
Source: NASA Goddard (Imagine the Universe!)sunspot
A cooler, and therefore darker, region on the Sun where magnetic field loops rise out of the surface.
Source: NASA Goddard (Imagine the Universe!)supernova
The explosion at the end of a massive star's life, in which the star suddenly brightens a great deal and then slowly fades.
Source: NASA Goddard (Imagine the Universe!) · Related articleW
wavelength
The distance between neighbouring peaks of a repeating wave, such as a light wave.
Source: NASA Goddard (Imagine the Universe!)white dwarf
A star that has used up most or all of its nuclear fuel and collapsed to a very small size. The Sun is expected to end this way.
Source: NASA Goddard (Imagine the Universe!) · Related article