ZAMS This is the position of a zero-age star when it arrives on the main sequence. It is used to infer the age of cluster stars.

X-ray radiation is part of the electromagnetic spectrum between the UV and gamma-ray components. As X-rays are blocked by our atmosphere X-ray astronomy is only possible from space such as with the Chandra telescope. X-ray emissions are associated with high-energy astrophysical events such as accretion onto neutron stars and black holes.

White dwarfs (luminosity class wd) are the hot, compact, collapsed remains of stars that have exhausted their core fusion. They are about the size of the Earth but with the mass of the Sun hence are extremely dense. Electron degeneracy pressure prevents further collapse.

A binary system in which component star can be visually resolved through a telescope. Examples include α Cen and β Cru.

UV Part of the electromagnetic spectrum between the visible and X-ray regions. Ultraviolet radiation is absorbed in our atmosphere so UV telescopes such as FUSE need to be placed in space.

A variable star is simply one whose brightness (or other physical property such as radius or spectral type) changes over time. Intrinsic variables are those in which the change in brightness is due to some change within the star itself such as in pulsating stars like the Cepheids. Extrinsic variables are those in which the light output changes due [..]

Nearby celestial objects exhibiti a perioidic shift in position relative to background stars due to the earth's motion around the Sun. By measuring this stellar parallax and applying trigonometry the distance to the nearby star can be determined. This method is used to define the parsec.

(see effective temperature)

SN The explosion of a star at the end of its life . Not all stars explode, only those about ten to twenty times more massive than our Sun. A supernova creates the elements that are heavier than iron (such as gold, silver and most of the other metals).

These are the largest, brightest stars known. Of luminosity classes I, they are found along the top of the HR diagram and have evolved from high-mass main sequence stars (> about 11 solar masses). They are short-lived and will eventually explode as a supernova. Examples include Rigel, Deneb and Betelgeuse.