(definitions from the Yahoo! Kids Dictionary at: http://kids.yahoo.com/reference/dictionary/english)
galaxy: A giant collection of stars, gas, dust, planets and sometimes even black holes, held together by gravity.
gravity: The natural force of attraction between any two objects that have mass.
binary star: A stellar system consisting of two stars orbiting about a common center of mass and often appearing as a single visual or telescopic object.
mass: A property of matter equal to the measure of an object’s resistance to changes in either the speed or direction of its motion. The mass of an object is not dependent on gravity and therefore is different from but proportional to its weight.
Milky Way: The galaxy containing the solar system we live in, visible as a broad band of faint light in the night sky.
black hole: An astronomical object that contains a lot of mass in a small volume. Black holes have so much gravity that not even light can escape from them.
physicist: A scientist who studies the way that objects move and interact with each other. A physicist studies things like energy, forces and matter.