Galaxy: A large aggregation of stars and clouds of gas and dust. Galaxies may be elliptical, spiral or irregular in shape and contain from a few million to a several trillion stars.
Galilean Moon: One of the four largest natural satellites of the planet Jupiter that were discovered in 1610 by Italian astronomer Galileo Galilei.
Gas Planet (Gas Giant): A large planet that, like Jupiter or Saturn, consists predominantly of hydrogen and helium.
Globular Cluster: A near spherical cluster of between 10,000 and more than 1 million stars.
Gravity: The attractive force that acts between material bodies, particles and photons.