| A | B |
|---|
| apparent magnitude | the brightness of an object as it appears to an observer on Earth |
| binary star system | a pair of stars that orbit their common center of mass |
| absolute magnitude | the brightness of a star measured by an observer who is a standard 32.6 light-years away |
| black hole | a massive celestial object with gravity so strong that not even light can escape |
| pulsar | a spinning neutron star that gives off pulses of radiation at regular intervals |
| star cluster | groups of stars that have a common origin and are held together by mutual gravitational attraction |
| galaxy | large systems of stars and their solar systems, gas, and dust held together by gravity |
| supernova | a violent explosion of a star |
| Doppler effect | the apparent shift in frequency of waves emitted by a moving source |
| parallax | the apparent shift in an object’s direction when viewed from two geographically distant locations |
| spiral galaxies: | have a reddish central bulge of gases, dust, and older, cooler stars with spiral arms of hot blue stars revolving around the bulge; |
| elliptical galaxies | have very bright centers and very little gas or dust, contain only old stars |
| irregular galaxies: | have no definite shape, contain very little mass, and can contain lots of gas and new stars |
