Key Concepts in Stellar Astronomy

apparent brightness

a measure of the amount of light received by Earth from a star or other object—that is, how bright an object appears in the sky, as contrasted with its luminosity

brown dwarf

an object intermediate in size between a planet and a star; the approximate mass range is from about 1/100 of the mass of the Sun up to the lower mass limit for self-sustaining nuclear reactions, which is about 0.075 the mass of the Sun; brown dwarfs are capable of deuterium fusion, but not hydrogen fusion

color index

difference between the magnitudes of a star or other object measured in light of two different spectral regions—for example, blue minus visual (B-V) magnitudes

giant

a star of exaggerated size with a large, extended photosphere

luminosity

the rate at which a star or other object emits electromagnetic energy into space; the total power output of an object

magnitude

an older system of measuring the amount of light we receive from a star or other luminous object; the larger the magnitude, the less radiation we receive from the object

proper motion

the angular change per year in the direction of a star as seen from the Sun

radial velocity

motion toward or away from the observer; the component of relative velocity that lies in the line of sight

space velocity

the total (three-dimensional) speed and direction with which an object is moving through space relative to the Sun

spectral class

(or spectral type) the classification of stars according to their temperatures using the characteristics of their spectra; the types are O, B, A, F, G, K, and M with L, T, and Y added recently for cooler star-like objects that recent survey have revealed