The following glossary includes terms that are used in Student Text, Epoch Five in our Evolving Universe.

Nuclear decay ('nü-klE-&r di-'kA)
Nuclear decay is the spontaneous change of an unstable nucleus into a nucleus with a different composition.

Neutrino (nü-'trE-(")nO)
A neutrino is an electrically neutral particle with very small mass that is produced in some nuclear decay changes.

Positron ('pä-z&-"trän)
A positron is a particle with the same mass as an ordinary electron but has a positive charge that is exactly equal to the negative charge of an electron.

Matter-antimatter annihilation ('ma-t&r-'an-ti-"ma-t&r &-"nI-&-'lA-sh&n)
This process occurs when a particle of matter and its anti-particle collide. During the process, both of these particles disappear as other, lighter particles are formed.

Photon ('fO-"tän)
A photon is the elementary quantity of radiant energy. We sometimes refer to photons as “packets” of light.