Galactic clusters;Collection of galaxies that are held together by gravitational forces.
Galaxy;Collection of stars, gas and dust (the Milky Way is a galaxy)
Galileo;Italian astronomer and physicist, first to develop an optical telescope around 1600
Gamma radiation;High-energy electromagnetic radiation with extremely short wavelengths.
Gaseous giants;The planets of Jupiter, Saturn, Uranus and Neptune.
Geocentric world-view;Ancient theory where the Earth is the center of the universe.
GHRS;Goddard high-resolution spectrograph: Instrument for the recording of spectra of objects.
Giant;Very large star with enormous luminosity.
Giotto;European space probe launched for the study of Halley's comet.
Globular cluster;Spherical group of stars that are held together by gravitational forces.
Gravitation;Force affecting objects at the surface of planets or other object, pulling its mass towards the center of this body.
Gravitational force;(Gravitation): force of attraction between objects.
Gravitational lens;Refraction of light by an extremely massive object.
Greenhouse effect;The water vapor and CO2 in the atmosphere of the Earth allow the short-wave radiation from the Sun to pass with a relatively small decrease in intensity.
H II region;Gas cloud of ionized hydrogen.
Halo;Circle of colored light appearing around the Moon, the Sun or another planet.
Heliocentric world-view;Theory in which the Sun is at the center of the solar system.
Hertzsprung-Russell diagram;Diagram comparing the magnitudes and spectral classes of stars.
Horizon;Intersection between a horizontal plane and the celestial sphere.
HSP;High speed photometer: measures the fluctuations of luminosity of an object over time.
HST;Abbreviation of Hubble space telescope: telescope orbiting the Earth, named after the astronomer Edwin Hubble.
Hubble;Hubble, Edwin Powell 1889-1953, American astronomer.
Hubble space telescope;HST: telescope orbiting the Earth, named after the astronomer Edwin Hubble.
Hubble's law;Law of the expansion of the universe.
Hydrogen;Lightest element and first element in the periodic table.  It is also the most common substance in the universe and forms the main component of stars.