Big Bang Theory
The theory that the universe began to expand after an explosion of concentrated matter and energy.
Cosmic microwave background (CMB) radiation
Radiation left over from the big bang, detected in the microwave portion of the spectrum with a temperature of only 2.7 K.
A model of the universe in which a stationary Earth is at the center and all other celestial bodies revolve around it.
A model of the universe in which a stationary Sun is at the center and the planets revolve around it.
Hertzsprung-Russell (H-R) diagram
A plot of stars according to their luminosity and surface temperature.
The observed relationship that the velocity of recession of a galaxy is proportional to its distance.
A model that explains the formation of the solar system from a large cloud of gas and dust floating in space 4.56 billion years ago.
A chain of nuclear reactions in which hydrogen nuclei (or protons) fuse together to form helium nuclei, releasing energy.