Glossary

Absolute Magnitude - apparent magnitude a star would have if placed at a distance of 10 parsecs from Earth

Accretion - gradual accumulation of mass

Accretion Disk - a disk of material falling in toward a massive object such as a neutron star or black hole (the disk shape is the result of conservation of angular momentum)

Active Galactic Nuclei - galaxies whose central regions are emitting enormous amounts of electromagnetic radiation

Apparent Magnitude - a measure of observed light flux received from an object at the Earth

Arc Minutes - a unit of measurement used for very small angles; there are 60 arc minutes in one degree

Arc Seconds - a unit of measurement used for very small angles; there are 60 arc seconds in one arc minute

Black Dwarf - the presumed final state of evolution of a low mass star in which no radiation is emitted

Black Hole - region in space where the escape velocity is equal to, or greater than, the speed of light. Thus, nothing (including radiation) can escape from it

Electrical Repulsion - the force which acts between particles of like electrical charge to repel them from each other

Electromagnetic Radiation - radiation consisting of periodically varying electric and magnetic fields that vibrate perpendicular to each other and travel through space at the speed of light

Electromagnetic Spectrum - the full range of electromagnetic radiation spread out by wavelength, it consists of gamma-rays, X-rays, ultraviolet rays, optical light, infrared radiation, microwaves, and radio waves

Electron - a negatively charged subatomic particle that normally moves about the nucleus of an atom

Escape Velocity - minimum velocity an object must achieve to break free from the gravity of another body (in physics, it is achieved when the objectÆs kinetic energy is equal to its gravitational potential energy)