Descriptionary

galaxy cluster a grouping of galaxies held in close association by the strength of their collective gravity. A super cluster may hold as many as tens of thou­sands of galaxies across 100 million light-years of space.

geocentric relating to the Earth as a center; relative to the Earth.

geosynchronous orbit an orbit in which a satellite or other object keeps pace with the Earth's rotation. Also known as a geostationary orbit.

globular cluster a spherelike cluster of old stars, sometimes numbering in the hundreds of thousands.

gravitational clustering the natural tendency for a large mass, such as a galaxy, to attract other masses, including stars and galaxies, and to grow ever larger over time.

gravitational lens a massive object in space, such as a galaxy, that distorts, bends, or magnifies the light from objects behind it.

gravitational wave as predicted by Albert Einstein's general theory of relativity, a hypothetical wave, oscilla­tion, or disturbance originating in a black hole or other source and thought to travel unimpeded across space.

Great Red Spot a massive perpetual storm on the surface of Jupiter.

Hubble constant the ratio of a galaxy's velocity in traveling away from the Earth divided by its distance from the Earth.

Hubble's law a law that states that due to the expanding universe, the velocity of a galaxy moving away from Earth is directly proportional to its dis­tance from Earth.

hydrogen the Sun's primary gas and the most com­mon element throughout space.

inferior planets the planets Venus and Mercury, whose orbits are closer to the Sun than Earth's.

interferometer the combination of two or more optical telescopes to produce sharper focus. Also, two or more radio telescopes combined to magnify radio signals.

interstellar dust dust particles between the stars.

interstellar space the vast regions of empty space between the stars.

inverse square law a law of physics that holds that gravity decreases with the square of the distance between two masses. Doubling the distance between two masses, for example, would reduce gravity by three-quarters. The same law applies to the magni­tude or brightness of stars. Doubling a star's distance reduces brightness by three-quarters.

irregular galaxy a galaxy without an organized form, such as a spiral or globe.

island universe see galaxy.

Kuiper belt a disk-shaped swarm of 200 million comets and comet fragments located from just beyond the orbit of Neptune and extending past Pluto.

light-year an astronomical unit of measurement, specifically the distance light travels in a year, approx­imately 5,880,000,000,000 miles.

Local Group, the a cluster of more than 30 galax­ies, including Andromeda, the Milky Way, and the Magellanic Clouds.

Magellanic Clouds two irregular galaxies that can be seen with the naked eye in the southern sky.

magnetosphere the magnetic field that surrounds a planet.

magnetotail the outer portion of a planet's mag­netosphere, which is pushed away from the Sun by the solar wind. It is so named because it forms a cometlike tail that extends away from a planet's night side.

magnitude a scale for measuring the apparent brightness of celestial bodies, the brightest being neg­ative, zero, or first magnitude, the dimmest visible to the naked eye being sixth magnitude.

meridian the great circle passing through the sky's zenith and touching the north and south horizons.

meteor a rock or metal fragment entering Earth's atmosphere and burning up. Popularly known as a shooting star.

meteorite a meteor that is not completely burned away by the atmosphere and strikes the Earth.

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.