wave A pattern that repeats itself cyclically in time and space. Waves are characterized by the velocity with which they move, their frequency, and their wavelength.
wave period The amount of time required for a wave to repeat itself at a specific point in space.
wavelength The distance between successive crests of a wave.
weak force The nuclear force involved in radioactive decay. The weak force is characterized by the slow rate of certain nuclear reactions such as the decay of the neutron, which occur with a half-life of 11 min.
white light Visible light that contains approximately equal proportions of all colors.
Wolf-Rayet stars Wolf-Rayet stars are hot luminous stars that are rapidly losing mass in a wind. They represent a late stage of evolution for massive stars (initial mass greater than about 40 suns). The outer, hydrogen-rich, envelope of the star has been driven off by radiation pressure, exposing the hot helium core. In a few million years or less they will explode as a supernova. A Wolf-Rayet phase is also present in some central stars of planetary nebulae. In these stars, which have lower masses and will evolve into white dwarfs, the outer envelope has been expelled in the red giant phase, exposing the hot core. Such stars show many of the characteristics of standard Wolf-Rayet stars and are referred to as "Wolf-Rayet type" stars. [More Info: Field Guide]
XMM/Newton The European Space Agency's large X-ray observatory, launched on Dec 10, 1999, which is capable of sensitive x-ray spectroscopic observations. [More Info: Field Guide]
X-ray burster X-ray source that radiates thousands of times more energy than our Sun, in short bursts that last only a few seconds. A neutron star in a binary system accretes matter onto its surface until temperatures reach the level needed for hydrogen fusion to occur. The result is a sudden period of rapid nuclear burning and release of energy.
zero-age main sequence The region on the H-R diagram, as predicted by theoretical models, where stars are located at the onset of nuclear burning in their cores.
zeroth-order image A zeroth-order image obtained by HETG consists of photons that go "straight through" the gratings, i.e., are not redirected (dispersed) according to their energies. This is analogous to the case of an optical diffraction grating: if you look at a light bulb through such a grating, you see the bulb, and you also see spectra of the bulb off to each side of the bulb. The fact that you see the light bulb "straight through" the grating is due to the fact that the grating forms a "zeroth-order" image of the bulb at your eye.
zodiac The twelve constellations through which the Sun moves as it follows its path on the ecliptic. [More Info: Photo Album]