G

Galaxy - A type of deep-sky object, galaxies are huge "island universes" comprised of billions of stars. Each galaxy is home to stars, planets, and clouds of gas and dust. Between galaxies are millions of light-years of empty space, so galaxies are the basic unit of the universe. Most of the deep-sky objects we see through telescopes--star clusters and nebulae--are part of our own Milky Way galaxy. In general, the only things we see outside of our galaxy are other galaxies. The nearest galaxy outside the Milky Way is nearly 3 million light-years away. Galaxies are divided into three basic types: spiral, elliptical, and irregular. Elliptical galaxies show little detail, looking mostly like indistinct balls of fuzz. Spiral and irregular galaxies can show much structure. Examples of galaxies include the Great Andromeda Galaxy and Messier objects M81 and M82.

German Equatorial Mount - Abbreviated GEM, these mounts use counterweights to balance the telescope around a polar axis, which is aligned with Earth's rotation axis, allowing automatic tracking of the night sky. GEMs tend to be more stable than fork mounts (of similar size) and high-end models are excellent for long-exposure astrophotography. GEMs also allow easy interchangeability of different telescopes for observers with multiple instruments.

Globular Star Cluster - A type of deep-sky object, a globular cluster is a dense grouping of hundreds of thousands to millions of stars. Larger telescopes (6"+) can resolve the individual stars in many globular clusters. A globular cluster is distinct in appearance from an open cluster in that the stars are tightly bound, whereas they are scattered and less numerous in an open cluster. They are also different in that open clusters tend to be groups of young stars (millions of years old) while globular clusters are comprised of much older stars (billions of years old). Examples of globular clusters include the Messier objects M3 and M13.

Goto Telescope - A goto telescope is a computerized telescope that has high-speed motors to automatically move itself to point to a selected object.

GPS - Global Positioning System. GPS uses an array of satellites to accurately determine locations on Earth. Most goto telescopes use the observer's location to help find initial alignment stars, which is a helpful feature, but GPS is not a necessary component of a goto telescope, so many goto telescopes do not have (or need) GPS.