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Flash Stuff:

 

Solar System

 

X (2)

 

X-Ray

A relatively high-energy photon with a very short wavelength.

 

X-Ray Stars
Stars (other than the Sun) that emit observable amounts of radiation at x-ray frequencies.

 

 

Y (3)

 

Year
One complete orbit of a planet around a "Sun."

 

Yellow Dwarf

An ordinary star such as the sun at a comparatively stable and long-lived stage of evolution.

 

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.

 

 

Z (6)

 

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.

 

Zeeman Effect
A splitting or broadening of spectral lines due to magnetic fields.

 

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.

 

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.

 

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.

 

Zodiacal Light

A faint glow from light scattered off of interplanetary dust along the plane of the ecliptic.

 

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.