Imagine Home  |   Teachers' Corner   |   Current page Event Horizon (also known as Schwarzschild Radius) - the virtual surface around a black hole (often considered as the surface of the black hole) within which gravitational forces prevent anything, including light, from escaping Expansion Velocity - the outward material velocity away from the central point of an explosion, such as a supernova General Relativity - the geometric theory of gravitation developed by Albert Einstein, incorporating and extending the theory of special relativity to accelerated frames of reference and introducing the principle that gravitational and inertial forces are equivalent Gravitational Energy - energy that can be released by the gravitational collapse of a system Hertzsprung-Russell Diagram - a plot of absolute magnitude versus spectral type (or temperature) for a group of stars Isotope - any of two or more forms of the same element, whose atoms all have the same number of protons but different numbers of neutrons Kinetic Energy - energy associated with motion; the kinetic energy of an object is equal to one-half the product of its mass and the square of its velocity Light-Year - the distance light travels in one Earth year, equal to 9.46 x 1012 km Luminosity - the rate of radiation of electromagnetic energy into space by a star or other object Main-Sequence - diagonal region of the Hertzsprung-Russell diagram in which most stars are located; generally these are stable stars during the bulk of their lives Neutron - a subatomic particle with no electrical charge; one of the constituents of the atomic nucleus Neutron Star - a star of extremely high density composed almost entirely of neutrons Nuclear Reaction - a reaction, as in fission, fusion, or radioactive decay, that alters the energy, composition, or structure of an atomic nucleus Parsec - unit of distance often used by astronomers, equal to 3.2616 light-years (a kiloparsec is equal to 1,000 parsecs) Photon - a unit of electromagnetic energy associated with a specific wavelength or frequency Planetary Nebula - a shell of gas ejected from, and expanding away from, a star that is nearing the end of its life Download a pdf version.
 Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center. The Imagine Team Acting Project Leader: Dr. Barbara Mattson All material on this site has been created and updated between 1997-2012.