| A | B |
|---|
| absolute magnitude | the apparent brightness of a star if it were viewed from a distance of 32.6 light-years; used to compare the true brightness of stars |
| apparent magnitude | the brightness of a star when viewed from Earth |
| big bang theory | the theory that proposes that the universe originated as a single mass, which subsequently exploded |
| binary star | one of two stars revolving around a common center of mass under their mutual gravitational attraction |
| black hole | a massive star that has collapsed to such a small volume that its gravity prevents the escape of everything, including light |
| cepheid variable | a star whose brightness varies periodically because it expands and contracts; a type of pulsating star |
| constellation | an apparent group of stars originally named for mythical characters; The sky is presently divided into 88 constellations. |
| galaxy | a group of stars, dust, and gases held together by gravity |
| H-R diagram | a plot of stars according to their absolute magnitudes and temperatures |
| Hubble’s law | a law that states that the galaxies are retreating from the Milky Way at a speed that is proportional to their distance |
| light-year | the distance light travels in a year, about 9.5 trillion kilometers |
| main-sequence star | a star that falls into the main sequence category on the H-R diagram; This category contains the majority of stars and runs diagonally from the upper left to the lower right on the H-R diagram. |
| neutron star | a star of extremely high density composed entirely of neutrons |
| nova | a star that explosively increases in brightness |
| protostar | a collapsing cloud of gas and dust destined to become a star; a developing star not yet hot enough to engage in nuclear fusion |
| pulsar | a variable radio source of small size that emits radio pulses in very regular periods |
| red giant | a large, cool star of high luminosity; a star occupying the upper-right portion of the H-R diagram |
| supergiant | a very large, very bright red giant star |
| supernova | an exploding star that increases in brightness many thousands of times |
| white dwarf | a star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size, believed to be near its final stage of evolution |
