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VACUUM A vacuum is a space with no or very little gas pressure.

VAN ALLEN, JAMES A. James A. Van Allen was an American physicist who discovered doughnut-shaped belts of radiation that circle the Earth (the van Allen Belts).

VAN ALLEN BELTS The Van Allen radiation belts are two doughnut-shaped belts of ionized gas (plasma) that circle the Earth. The belts are are caused by the interaction between the Earth's magnetic field and the solar wind (stream of ions, electrically charged particles, that are given emitted by the sun). The particles (plasma) is trapped by the Earth's magnetosphere. When stray particles from these belts collide with air, they emit light, causing an aurora.

Some Variable Stars Type of Variable Magnitude Range Period (days) Eta Auilae Cepheid 3.5-4.4 7.2 R Carinae Mira 3.9-10.5 308.7 R Centauri Mira 5.3-11.8 546.2 Delta Cephei Cepheid 3.5-4.4 5.4 Mira (Omicron Ceti) Mira 3.4-9.3 332.0 Zeta Geminorum Cepheid 3.7-4.2 10.2 Delta Librae Eclipsing binary 4.9-5.9 2.3 Algol (Beta Persei) Eclipsing binary 2.1-3.4 2.9 VARIABLE STAR A variable star is one whose brightness changes regularly. They can have periods ranging from minutes to years. The apparent changes in brightness are caused by different phenomena; some change in size, some eject material, and others are in pairs that periodically obscure and enhance each other.

VARUNA Varuna is a large Kuiper object - a body in our Solar System that orbits far from the Sun. Varuna (2000 WR106) is a rocky body about 900 km in diameter. It is roughly 43 AU from the Sun (assuming a circular orbit). One orbit around the Sun takes about 285 years. It has an apparent magnitude of 20. This Transneptunian object (TNO) was discovered from the Kitt Peak Observatory by Robert S. McMillan on November 28, 2000. Varuna was named for the oldest of the vedic (Hindu) deities, the maker and upholder of heaven and the earth.

VASTITAS A vastitas is an extensive plain.

VECTOR A vector is a number (a magnitude) plus a direction (compare with scalar). A vector can be represented by an arrow whose length represents the magnitude and the direction represents the direction. For example, velocity is a vector; velocity tells you how fast something is traveling, and its direction.

VEGA Vega (Alpha Lyrae) is a very bright star in the constellation Lyra. It is also known as the Harp Star and Fidis. Vega is the 5th brightest star in the sky and is pale blue. It is about 25 light years from Earth. Its spectral type is A0Va. A disk of dust surrounds Vega, from which planets might form. Vega, together with Deneb and Altair form the Summer Triangle.

VELOCITY Velocity is both the speed and the direction that a body is moving. It has more information than speed alone. Velocity is a vector

VELOCITY DISPERSION Velocity dispersion is the range or average of the different velocities found a system with random internal motions, like a galaxy or a globular cluster.

VENUS Venus is the second planet from the sun.

VERNAL EQUINOX Equinoxes are days in which day and night are of equal duration. The two yearly equinoxes occur when the Sun crosses the celestial equator. The vernal (spring) equinox occurs on March 21; the autumnal equinox occurs on September 21.

VERONIKA (612) Veronika is an asteroid. Veronika (1906 VN) was discovered on October 8, 1906 by August Kopff (who also discovered many other asteroids) in Heidelberg, Germany.

VIKING NASA had two Viking missions, Viking 1 and Viking 2, ( both were launched 1975 and equipped with an orbiter and a lander). They transmitted images of Mars and its moons back to Earth.

VIRGO [Abbreviation: Vir] Virgo (the virgin) is a constellation of the zodiac. It is located along the ecliptic between Leo and Libra. Virgo is seen along the ecliptic. The brightest star in Virgo is Spica (meaning "ear of wheat"). Virgo is the second-largest constellation (of the 88 constellations), after Hydra.

VIRGO CLUSTER The Virgo cluster is a massive cluster of over 100 galaxies (including M61, M87, M90, and M100) and a lot of very hot, X-ray emitting gas. This cluster is located mostly within the constellation Virgo. This cluster is roughly 60 million light years from Earth and is the closest cluster of galaxies to our galaxy (the Milky Way Galaxy). From Earth, the Virgo cluster spans over 5 degrees in the sky; this is about 10 times the angle made by a full Moon.

VISCOSITY Viscosity is the measure of a material's resistance to flow. Viscosity is a result of the internal friction of the material's molecules. Materials with a high viscosity do not flow readily; materials with a low viscosity are more fluid.

VISIBLE WAVELENGTHS Visible wavelengths are the parts of the electromagnetic radiation that we can see. They range from red (longer wavelengths, about 700 nanometers) to violet (shorter wavelengths, about 400 nanometers).

VISUAL BINARY A visual binary is a binary star pair that are far enough apart to be visible as separate objects.

VISUAL LIMITING MAGNITUDE The visual limiting magnitude is the magnitude of the dimmest star that you can see by the zenith (overhead).

Star Visual (Absolute) Magnitude Apparent Magnitude Distance from Earth (light-years) The Sun +4.8 -26.72 . Sirius +1.4 -1.46 8.6 Canopus -2.5 -0.72 74 Rigel Kentaurus +4.4 -0.27 4.3 Arcturus +0.2 -0.04 34 Vega +0.6 0.03 25 Capella +0.4 +0.08 41 Rigel -8.1 +0.12 900 Betelgeuse -7.2 +0.7 1,500 Altair +2.3 +0.77 16 Deneb -7.2 +1.25 1,500 Proxima Centauri +15.5 +11.05 (var.) 4.3 VISUAL MAGNITUDE Visual (apparent) magnitude is a measure of the brightness of a celestial object as seen from Earth. The lower the number, the brighter the object. Negative numbers indicate extreme brightness. The full moon has an apparent magnitude of -12.6; the sun's is -26.8. We can see objects up to 6th magnitude without a telescope. Apparent magnitude is abbreviated m. This system of rating the brightness of celestial objects was developed by the Greek astronomer Hipparchus in 120 B.C.

VLA VLA (Very Large Array) is a set of 27 linked radio telescope dishes in New Mexico, USA. Each of the metal dishes is 82 feet (25 m) in diameter. It is the most sensitive radio telescope in the world.

VMO VMO is short for Very Massive Object. VMO's are over 100 times more massive than our Sun. Black holes, for example, are VMO's.

VOLCANO When a volcano erupts, it spews out lava and gases from deep inside a planet (or moon).

VOYAGER NASA launched the two Voyager missions in 1977 to explore the solar system. They transmitted images of the outer planets and their moons back to Earth.