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
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