ZEEMAN EFFECT - Splitting of atomic energy levels into a larger number of levels when magnetic fields are present; the resulting spectral lines are also split. The pattern and amount of splitting indicate the strength of the magnetic field. Splitting is associated with the orbital angular momentum quantum number, l, of the atomic level, which can take non-negative integer values. The number of split levels in a magnetic field is 2 x l + 1. The spectroscopic notation used of chemists uses letters for l: s = 0, p = 1, d = 3, etc.
ZENITH - Point directly overhead, as determined by the indefinite upward extension of a plumb line.
ZERO-AGE MAIN SEQUENCE (ZAMS) - Region on the Hertzsprung-Russell diagram, as predicted by theoretical models, where stars are located at the onset of nuclear hydrogen burning in their cores. These stars are new additions to the main sequence, a place where they will spend about 90% of their lives and maintain remarkably stable luminosities. After this time, they evolve off the ZAMS to the upper right of the main sequence to become red giant or supergiant stars.
ZIRCON - Orthosilicate mineral, Zr(SiO4), observed in all terrestrial rocks type and in ordinary chondrites, eucrites, mesosiderites, and lunar rocks.
ZODIAC - Twelve constellations along the ecliptic through which the sun, the moon and the planets move: Ares (Ram), Taurus (Bull), Gemini (Twins), Cancer (Crab), Leo (Lion), Virgo (Virgin), Libra (Scales), Scorpio (Scorpion), (Sagittarius) Archer, (Capricorn) Sea Goat, (Aquarius) Water Carrier, and (Pisces) Fishes.
ZODIACAL LIGHT - Light resulting from scattering of sunlight off small (1-300 µm) interplanetary dust particles lying in the plane of the ecliptic. This dust originates through the sublimation of cometary nuclei and collisions between asteroids, and forms a band of dust that completely circles the Sun and stretches out to the orbit of Jupiter.
ZONE OF AVOIDANCE (ZoA) - Region of sky that is obscured at optical wavelengths by our own galaxy, the Milky Way. It covers roughly 20% of the sky and is centered along the galactic equator, the ZoA is irregular in shape, varying in height with the widest point located towards the center of our Galaxy in the constellation Sagittarius. Extragalactic objects (red dots in figure) do exist in this region of sky, but the extinction through the Galactic plane is very high (Galactic dust and dense gas absorb light from background sources). The high concentration of stars also makes faint extragalactic objects difficult to detect within the ZoA. To penetrate the ZoA, we must observe this region at wavelengths that are not as affected by dust (such as infrared, radio, and X-rays).
ZZ CETI STARS - Class of variable star characterized by modest luminosity variations (from 0.001 to 0.2 magnitudes or 0.1% to 20% of their average luminosities) and periods of 30 to 1,200 seconds. Their location on the Hertzsprung-Russell diagram identifies them as white dwarf stars that have entered the instability strip as they evolve along the white dwarf track. ZZ ceti stars are also known as DAV stars. ZZ Ceti stars evolve from intermediate mass stars (6 to 8 Msun) that have managed to retain hydrogen in their outer envelopes. This type of white dwarf is classified as a DA white dwarf, as opposed to DB white dwarfs that have helium, but no hydrogen in their outer envelopes (hence the alternative name DAV stars). The surface temperatures of ZZ Ceti stars are ~12,000 K. The pulsations responsible for their variability are the result of the ionization and then recombination of hydrogen in the outer envelope.