meteoroid any one of the small rocks that travel throughout space and are officially designated as meteors when they enter Earth's atmosphere and flare across the sky.
meteor shower a raining down of a mass of meteors to Earth, caused when Earth passes through a cloud of debris from an old comet. Many meteor showers occur annually because the Earth passes through the same debris clouds on the same day or week each year.
Milky Way the galaxy of 100 billion stars within which the Earth and Sun are located. The faintly luminous band or river of stars that crosses the night sky in summer, which most people call the Milky Way, is actually only one arm of the galaxy, which is a spiral. The constellation of Sagittarius serves as a beacon for the galaxy's center.
minor planet see asteroid.
northern lights see aurora.
nova an erupting star that temporarily brightens.
occultation the obscuring of a small or distant celestial body by a larger or closer body, such as the Moon passing in front of a star or planet.
old referring to a moon or planetary surface with numerous craters, an indication of great age.
Oort Cloud a vast belt thought to contain a trillion or more comets, located in the outermost reaches of the solar system, from one light-year out from the Sun to approximately halfway to the nearest star, Proxima Centauri.
open universe a theoretical model in which the universe never ends but expands infinitely in every direction.
opposition orbital position of a superior planet when it is on the opposite side of the Earth from the Sun.
orbit the path followed through space by a celestial body.
parallel universe a hypothetical, unseen universe that is postulated to exist in another dimension and that may have played a role in the creation of our own universe.
parsec astronomical unit of measurement equaling 3.26 light-years.
patera a shallow crater.
perigee point in the orbit of the Moon closest to Earth.
perihelion point of a body's orbit nearest the Sun.
planetesimal any one of the millions of small orbiting bodies that may have come together to form the planets.
proper motion the motion of the stars not related to the Earth's rotation.
protogalaxy a "baby" galaxy in its earliest stages of formation.
Proxima Centauri the nearest star to the Sun, located 4.22 light-years away; it requires a telescope to be seen.
pulsar a neutron star that emits X-rays or other types of radiation.
quasars quasi-stellar radio sources, the oldest and most remote visible objects in the universe, and among the most mysterious. They are believed to be active galactic nuclei powered by supermassive black holes. Also known as quasi-stellar objects.
radiant point where meteor showers appear to originate in the sky.
radio astronomy the study of radio frequency radiation from space through radio telescopes.
radio telescope a giant, dish-shaped telescope that gathers faint radio signals from space.
red giant a cool, red star many times larger than our sun.
redshift the shift of an emitted light's spectrum to a lower frequency, specifically toward the red end, when the light's source—a galaxy or other celestial body—moves away from an observer. The farther away an object is, generally, the larger the redshift exhibited. Astronomers use the light spectrum emitted by a celestial body to determine if it is moving away or toward us.
relativity, general theory of Albert Einstein's theory that matter causes space to curve. First published in 1916, this theory expands on his theory of special relativity, developed in 1905. In the general theory, Einstein postulated that large masses attract smaller masses, not because of some invisible force waves but because large masses "warp" space, just as a heavy ball placed in the middle of a rubber sheet warps the space around it and will cause other balls to roll toward it.
relativity, theory of special Albert Einstein's widely accepted and largely proven theory describing the laws of particles moving at close to the speed of light, which are different from those governing particles moving at slower speeds. Although the speed of light is always the same, no matter if an observer is in motion or standing still, Einstein postulated that moving clocks tick more slowly than stationary ones because time is literally slowed down. This effect is especially pronounced when approaching speeds near that of light.
retrograde an orbit of a satellite that is or appears opposite that of the Earth's rotation. Also, an orbit of a celestial object that is opposite of Earth's orbit around the Sun.
satellite an object or moon in orbit around a planet.
scintillation the apparent "twinkling" of the stars, which is caused by refraction of light passing through Earth's atmosphere, especially close to the horizon.
SETI Search for Extraterrestrial Intelligence; a scientific program dedicated to finding intelligent life in outer space through the gathering and study of radio waves.
Seyfert galaxy a type of spiral galaxy with an intensely bright nucleus and thought to contain a black hole.
shooting star a meteor.
sidereal as measured with respect to the stars.
singularity the center of a black hole, where matter is believed to be immensely compacted and dense and the force of gravity infinitely powerful.
solar wind a stream of gas continually blown out from the Sun at hundreds of kilometers per second.
solstice position of the Sun when farthest north (summer solstice) or farthest south (winter solstice) in the sky.
southern lights see aurora.
spacetime the four-dimensional continuum composed of the three dimensions of space with time, through which an event can be precisely plotted.
spectroscope an instrument astronomers use to separate light into its component colors, creating a spectrum that can be analyzed to determine what elements a celestial body is made of.
spiral galaxy a galaxy in the configuration of a spiral or pinwheel, with the oldest stars clustered in a sphere in the center and the youngest stars forming the outer arms.
star a fiery sphere of gas; a sun.
starburst galaxy a galaxy in which new stars are rapidly forming.
superior planets the planets Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto, whose orbits are farther from the Sun than the Earth's.
supermassive black hole a black hole that resides at the center of a galaxy and may contain the mass of millions of consumed stars.
sunspot cooler area visible as a dark spot in the surface of the Sun.
supernova an exploding star that increases its brightness thousands of times, dimming only after a period of months.
supernova remnant a nebula surrounding the site of an earlier supernova explosion.
symbiotic stars two stars so close together they exchange gases and mass.
terminator the line that delineates the dark side of a planet or moon from the lighted side.
transit method a method of discovering new planets by observing if a star dims temporarily, which sometimes means a planet or another star has passed in front of it.