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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.


Short for extrasolar planet, a planet outside our solar system


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.

Hubble's law

The observed relationship that the velocity of recession of a galaxy is proportional to its distance.


A fragment of extraterrestrial material that falls to Earth.

Nebula hypothesis

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.

Proton-proton chain

A chain of nuclear reactions in which hydrogen nuclei (or protons) fuse together to form helium nuclei, releasing energy.


A stellar explosion marking the death of a massive star.

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