MENU  HOME PAGE  HISTORY OF COSMOLOGY  ASTRONOMY  ROCKETRY  SATELLITES & ORBITS  SPACE CRAFTS  SOLAR SYSTEM  STARS  GALAXIES  THE UNIVERSE  EXTRATERRESTRIAL LIFE  MODERN DEVELOPMENTS  STARGAZING  REFERENCES & LINKS  INTERACTIVE  ABOUT US  OTHER  WORMHOLES  ASTRONOMICAL NAMES  CUSTOMIZE  SEARCH  SITEMAP  GLOSSARY  DIFFERENT LANGUAGES

Glossary v4   A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z   X X-Ray A relatively high-energy photon with a very short wavelength.   back to top X-Ray Stars Stars (other than the Sun) that emit observable amounts of radiation at x-ray frequencies.   back to top Y Year One complete orbit of a planet around a "Sun."   back to top Yellow Dwarf An ordinary star such as the sun at a comparatively stable and long-lived stage of evolution.   back to top Young When used to describe a planetary surface "young" means that the visible features are of relatively recent origin, i.e. that older features have been destroyed (e.g. by erosion or lava flows). Young surfaces exhibit few impact craters and are typically varied and complex. In contrast an "old" surface is one that has changed relatively little over geologic time. The surfaces of Earth and Io are young; the surfaces of Mercury and Callisto are old.   back to top Z Z The ratio of the observed change in wavelength of light emitted by a moving object to the rest wavelength of the emitted light. See Doppler Effect. This ratio is related to the velocity of the object. In general, with v = velocity of the object, c is the speed of light, lambda is the rest wavelength, and delta-lambda is the observed change in the wavelength, z is given by z = (delta-lambda)/lamda = (sqrt(1+v/c) / sqrt(1-v/c)) - 1. If the velocity of the object is small compared to the speed of light, then z = (delta-lambda)/lamda = v/c Objects at the furthest reaches of the known universe have values of z = 5 or slightly greater.   back to top Zeeman Effect A splitting or broadening of spectral lines due to magnetic fields.   back to top Zenith 1: The point on the celestial sphere that is directly above the observer. 2: The highest point above the observer's horizon attained by a celestial body.   back to top Zero-Age Main Sequence Main sequence of the H-R diagram for a system of stars that have completed their contraction from interstellar matter and are now deriving all their energy from nuclear reactions, but whose chemical composition has not yet been altered by nuclear reactions. back to top Zodiac A ring of constellations that lie along the ecliptic: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Ophiuchus, Sagittarius, Capricornus, Aquarius, and Pisces in order east of the vernal equinox. Precession has shifted the position of the Vernal equinox from its position in Aries 2,000 years ago. Presently, the Vernal equinox is located 8 degrees SE of the Circlet of Pisces. In addition, the Sun intersects 13 not 12 constellations along the ecliptic. All these factors combined set horoscope dates off by 1 month, as quoted in daily newspapers. Newspaper horoscopes refer to the sky as it appeared more than 2000 years ago. For example, the Sun intersects Pisces, not Aries, between the dates of March 21 - April 19, 1996. The Sun passes through Aries on April 19 - May 13, 1996.   back to top Zodiacal Light A faint glow from light scattered off of interplanetary dust along the plane of the ecliptic.     back to top Zone of Avoidance A region near the Milky Way where obscuration by interstellar dust is so heavy that few or no exterior galaxies can be seen.   back to top