AstroGlossary - G
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galaxy
A vast "island" of stars, gas, and dust, which populates the universe by the billions. Galactic size and structure range from subtle ellipticals to grand spirals with the mass of at least 100 billion stars. Instead of randomly scattered throughout the universe, galaxies clump together in web-like structures. Despite its great luminosity, most of a galaxy's mass consists of dark matter, which produces no detectable energy. Galilei, Galileo
An Italian scientist who lived from 1564 to 1642. Among many other interests, Galileo studied astronomy and constructed the first telescope for astronomy. He used the telescope to discover sunspots, craters on the Moon, and the four largest moons of Jupiter. Galileo supported the Copernican system for the Solar System, postulated by Nicholas Copernicus in 1543. As a result, he was brought up before the Catholic Inquisition, where he was forced to recant. gamma-ray radiation
The most energetic wavelength, or frequency, of electromagnetic radiation. Gamma-rays have wavelengths smaller than 10^-11 meters and frequencies greater than 10^20 Hz. They can be quite harmful to life because they are strong enough to ionize atoms and thus destroy cells. Fortunately, Earth's atmosphere shields us from all astronomical gamma-ray radiation. globular cluster
A ball of a few hundred thousand stars that resides in the halo of our galaxy and other galaxies. These clusters contain stars that evolve separately from galactic stars; thus, globular clusters hold the original recipe of the galaxy. About 250 globular clusters form a spherical halo around our galaxy. M13, a bright globular cluster in Hercules, is a classic example easily seen with binoculars or a small telescope. gravity
The most feeble of the four fundamental forces in the universe that affect all matter. The magnitude of gravitational attraction depends directly on mass and inversely on distance squared. For instance, the gravitational attraction between you and Earth is much greater than that between you and the Sun, even though the Sun is 333,000 times more massive than Earth. The distance separating you from the Sun weakens the mutual gravitational attraction, so as you stand on Earth's surface, Earth's gravitational pull on you is 1,650 times greater than the Sun's. gravity wave
Ripples in space-time created by the stretching and squeezing effects of massive objects, notably supernovae, binary black holes, and binary neutron stars. Gravity waves are transverse waves, meaning that they stretch and squeeze space-time in directions perpendicular to the wave's motion. Einstein's Theory of General Relativity predicts the existence of gravity waves, but they have not yet been detected. See also: Great Red Spot
A raging red swirling storm located in Jupiter's upper atmosphere. The storm has been observed since at least the late 1800s, and perhaps as far back as the 1600s. It appears to rotate counterclokwise once every six days. Its size varies from about 1 to 2 times the diameter of Earth, while its color varies from pale pink to vivid red. greatest elongation
The maximum separation between Mercury or Venus and the Sun in the sky. Mercury and Venus peak as "evening stars" at their greatest eastern elongations, and as "morning stars" during their greatest western elongation. Geometrically, the greatest elongation of Venus occurs when a line tangent to the orbit of Venus intersects Venus and the Earth, and the greatest elongation of Mercury occurs when a line tangent to the orbit of Mercury intersects Mercury and the Earth.
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