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Last Update:  10-23-2004

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Sources:  Zoom Astronomy / StarDate On-line / Imagine the Universe / Eric Weisstein's World of Physics

A

a.m. - The abbreviation a.m. stands for ante-meridiem (before the Sun has crossed the line) and p.m. for post-meridiem (after the Sun has crossed the line).  At 12 noon, the Sun is at its highest point in the sky and directly over the meridian.  It is therefore neither "ante-" nor "post-".

absolute zero - At absolute zero, a hypothetical temperature, all molecular movement stops. All actual temperatures are above absolute zero. Absolute zero would occur at -273.16°C, -459.69°F, or 0 K.

accretion disk - A relatively flat sheet of gas and dust surrounding a newborn star, a black hole, or any massive object growing in size by attracting material.

active galactic nuclei (AGN) - Active galactic nuclei are a class of galaxies which spew massive amounts of energy from their centers, far more than ordinary galaxies. Many astronomers believe supermassive black holes may lie at the center of these galaxies and power their explosive energy output.

altitude - Altitude is a measurement in mapping astronomical objects on the celestial sphere (the sky as visible from Earth). Altitude is the angle of the object from the observer's horizon. If an object is on the horizon, its altitude is 0 degrees. If it is at the observer's zenith, its altitude os 90 degrees. To find an object in the sky, two coordinates are needed, its azimuth and its altitude.

angular distance - Angular distance is the measure of an arc (a segment of the circumference of a circle). Angular distance measures the proportion of a circle that the arc in question consists of. Angular distance is expressed in degrees, radians, arc minutes (one-sixtieth of a degree), or arc seconds (one-sixtieth of a minute). For example, latitude is the angular distance north or south of the equator measured in degrees, and longitude is the angular distance east or west of the prime meridian (which goes through Greenwich, England), measured in degrees.

angular momentum - A quantity obtained by multiplying the mass of an orbiting body by its velocity and the radius of its orbit. According to the conservation laws of physics, the angular momentum of any orbiting body must remain constant at all points in the orbit, i.e., it cannot be created or destroyed. If the orbit is elliptical the radius will vary. Since the mass is constant, the velocity changes. Thus planets in elliptical orbits travel faster at periastron and more slowly at apastron.  A spinning body also possesses spin angular momentum.

annular eclipse - An eclipse of the Sun in which the Moon does not completely mask the Sun's light, resulting in a ring of light around the Moon at the peak of the eclipse.

apastron - The point of greatest separation between two stars which are in orbit around each other. See binary stars. Opposite of periastron. 

aphelion - The point of greatest separation between a planet and the Sun.

apoapsis - The point in an orbit when the two objects are farthest apart. Special names are given to this orbital point for commonly used systems: see apastron, aphelion and apogee.

apogee - The point of greatest separation between an orbiting satellite or the Moon and the Earth.

apparent magnitude - The brightness of a star or other celestial body as measured from Earth.

arc - An arc is a segment along the circumference of a circle.

arc minute - An arc minute is a measure of angularity and is equal to one-sixtieth of a degree (there are 360 degrees in a circle).

arc second - An arc second is equal to one-sixtieth of an arc-minute.

asteroid - Small, irregularly shaped rocks of iron and nickel that are scattered in an orbital ring around the Sun between the orbits of Mars and Jupiter. 

asteroid belt - A ring of asteroid containing perhaps thousands of big rocks several kilometers in diameter, and millions of smaller chunks. Asteroids orbit between 2 and 3.5 Astronomical Units from the Sun, between the orbits of Mars and Jupiter.

astronomical unit (AU) - The average distance between the Earth and Sun.  One AU is equal to 92,955,806 miles or 149,597,870 kilometers.

astronomy - Astronomy is the scientific study of space, including the planets, stars, galaxies, comets, and nebulae.

astrophysics - The part of astronomy that deals principally with the physics of stars, stellar systems, and interstellar material.

aurora - Beautiful ribbons of light caused by the interaction of high-energy particles in the solar wind and the Earth's magnetic field.  These are common in both extreme northern (aurora borealis or Northern Lights) and southern latitudes (aurora australis), near Earth's poles.

autumnal equinox - Equinoxes are days in which daytime and nighttime are of equal duration. The two yearly equinoxes occur when the Sun crosses the celestial equator. The autumnal equinox occurs around September 21; the vernal (spring) equinox occurs around March 21.

axial tilt - Axial tilt, or obliquity to the ecliptic, is the tilt of a planet's axis from perpendicular to the plane of the ecliptic. Another way of looking at it is the angle between the plane of the planet's orbit and that of the equator.

azimuth - Azimuth is a measurement in mapping astronomical objects on the celestial sphere (the sky as visible from Earth). Azimuth is the angle of the object from the observer's north point (projected onto the horizon). If an object is due north, its azimuth is 0 degrees. If it is due east, its azimuth is 90 degrees, etc. To find an object in the sky, two coordinates are needed, its altitude and its azimuth.

B

Barycenter - the center of mass of two or more bodies which are orbiting each other, and is the point around which both of them orbit.

Big Bang - A theory of cosmology in which the expansion of the universe is presumed to have begun with a primeval explosion (referred to as the "Big Bang").

binary stars - Binary stars are two stars that orbit around a common center of mass. An X-ray binary is a special case where one of the stars is a collapsed object such as a white dwarf, neutron star, or black hole, and the separation between the stars is small enough so that matter is transferred from the normal star to the compact star star, producing X-rays in the process.

black dwarf - A non-radiating ball of gas resulting from either a white dwarf that has radiated all its energy or gas which has contracted but contains too little mass to begin nuclear fusion.

black hole - A black hole is a singularity in space-time from which light and matter cannot escape; an object so massive that the escape velocity is greater than the speed of light.

Blue Moon - When a single month has two full moons, the second full moon is called a Blue Moon. Another definition of the blue moon is the third full moon that occurs in a season of the year which has four full moons (usually each season has only three full moons.)

blueshift - An apparent shift toward shorter wavelengths of spectral lines in the radiation emitted by an object caused by motion between the object and the observer which decreases the distance between them. See also Doppler Effect.

brown dwarf - A small, premature star that does not shine with the energy of a nuclear fusion reaction at its core.

C

Cassini Division - The Cassini Division is the main, dark division between Saturn's largest rings. This gap is 2,920 miles (4,700 km) wide and is 73,010 miles (117,500 km) from the center of Saturn. It was discovered by G.D. Cassini in 1675.

celestial coordinates - Celestial coordinates are pairs of numbers (right ascension and declination) which are used to locate objects on the celestial sphere. They are similar to the coordinates of longitude and latitude on Earth.

celestial equator - The projection of Earth's equator onto the sky.  If you were standing on Earth's equator, the celestial equator would extend from eastern horizon, directly over your head (zenith), then to the western horizon.   Astronomers use an imaginary projection of Earth's latitude and longitude, called the celestial sphere, to locate planets, stars, star clusters, nebulae and galaxies.

celestial pole - The projection of Earth's rotation axis poles onto the sky. In the northern hemisphere, the celestial pole is within a degree of Polaris, the North Star.

celestial sphere - An imaginary sphere of the sky centered on the Earth.

chromosphere - Literally meaning "sphere of color," the chromosphere is an outer layer of the solar atmosphere sandwiched between the photosphere and the corona. Prominent features of the chromosphere include spicules, and bright hydrogen alpha emission lines, and calcium H and K emission lines. These emission lines suggest a temperature for the chromosphere higher than the photosphere.

cluster - A cluster is a group of stars or galaxies. Our own galactic cluster, the Local Group, is about 5 million light-years across and contains about 30 galaxies (the largest of which are the Andromeda galaxy, Triangulum, and our Milky Way). The next-closest cluster is the Virgo Cluster, which is about 60 million light-years from Earth.

collimate - To collimate a telescope, one lines up the optical components, the lenses, mirrors, prisms, and eyepieces, into their proper positions. Collimating maximizes image quality.

coma - A coma is the roughly spherical blob of gas that surrounds the nucleus of a comet. The coma is comprised of water vapor, carbon dioxide gas, and other neutral gases that are given off by the solid nucleus. The coma and the nucleus form the head of a comet.

comet - An icy, "dirty snowball" that orbits the Sun.  A majority of comets orbit well beyond Pluto in a halo surrounding the solar system.  Comet size ranges from a few meters to a few kilometers in diameter.  Upon a close encounter with the Sun, comet ices vaporize creating a coma and long tail that always points away from the Sun.

conjunction - A geometrical arrangement of the Earth, Sun and another planet(s) in a line, with the planet(s) behind the Sun.  The complementary arrangement is called opposition, when the planet(s) are behind Earth.

constellation - A meaningful pattern of stars visible with the unaided eye, literally meaning "group of stars" in Latin.  Americans know Northern Hemisphere constellations by the names given them by ancient Babylonians and Greeks.

corona - The outer layer of the Sun visible during a total solar eclipse. Compared with the photosphere, the corona has a high temperature (1,000,000 degrees) and low density (a million, million times lower).

cosmology - The study of the structure and evolution of energy, matter, space and time past, present and future.

cosmos - The cosmos refers to a universe that is a complete, harmonious, and orderly system.

D

dark matter - Theoretical non-luminous matter that has eluded detection by all present means, except through gravitational interaction with luminous objects.  Astronomers and astrophysicists calculate that dark matter comprises more than 90 percent of the universe.  Perhaps most of the universe exists as undetected clumps of interstellar dust and gas, plus cold dark stellar cores that collapsed billions of years ago.  

declination - One of two measurements of the position of an object in the sky. Declination measures in degrees from 0 to 90 the position of a celestial object north or south of the celestial equator, just like latitude measures positions on the Earth north or south of the equator.  The other coordinate is right ascension.

Deep Space Network - NASA's Deep Space Network (DSN) is an international network of antennas that communicate with spacecraft and perform radio astronomy and radar astronomy observations.

degree - A degree is a measure of temperature or angle. There are 360 degrees in a circle. Each degree is divided into 60 minutes, represented by the apostrophe symbol, '. 1°=60'.

density - Density is a measure of how massive an object is per unit volume. For example, iron is more dense than wood; one cubic meter of iron weighs more than one cubic meter of wood.

Doppler Effect - A measurable shift in the wavelength of a traveling wave caused by the relative motion of the source and observer. On the straight-aways of the Indianapolis 500, the sound of cars whizzing by an observer changes pitch from high to low. As the car approaches at more than 200 m.p.h., its motion compresses the sound wavelength (high pitch, or short wavelengths); the opposite occurs after the car rushes by (low pitch, or long wavelengths). Light behaves in somewhat the same way. Stars with relative motion toward an astronomer appear blue, i.e. the entire spectrum of star light shifts toward the blue end of the spectrum or is blue-shifted. Stars speeding away from the astronomer appear red, i.e. the entire spectrum is red-shifted. This is how astronomers know that the universe is expanding -- most everything is red-shifted.

E

earthshine - Earthshine is the faint light that is reflected from the Earth onto the dark part of the Moon.

eclipse - Total or partial masking of a celestial body by another along the line of sight. Solar eclipses result from the Moon blocking the Sun relative to the Earth; thus Earth, Moon and Sun all lie on a line.  Lunar eclipses work the same way in a different order: Moon, Earth and Sun all on a line.  In this case the Earth's shadow hides the Moon from view.

ecliptic - The Sun's path in the sky.  Planets and the Moon closely follow this path, since their orbital planes are nearly aligned with Earth's orbital plane.  The ecliptic is tilted 23.5 degrees from the celestial equator.

electromagnetic spectrum - The electromagnetic spectrum is full range of electromagnetic radiation, including: gamma rays, X-rays, UV rays, visible light, infrared, microwaves, and radio waves (in order of decreasing energy and increasing wavelength).

elongation - The angle between a planet and the Sun in the sky.  Greatest elongations refer to planets inside the Earth's orbit Venus and Mercury.  The greatest elongation of Venus is 46 degrees east (evening star) or west (morning star) of the Sun, while Mercury is 23 degrees east or west of the Sun.  For outer planets, see opposition and conjunction.

ephemeris - An ephemeris is a table listing the spatial coordinates of celestial bodies and spacecraft as a function of time.

equinox - The time of year when the Sun appears in the sky at the intersection of the ecliptic and the celestial equator.  Two such points exist: The vernal equinox (approx. March 21) marks the start of spring.  The autumnal equinox (approx. Sept. 20) marks the start of autumn.  At each of these points, the Sun rises due east and sets due west.

escape velocity - The minimum speed necessary to escape the gravitational pull of a celestial body.  A rocket launched from Kennedy Space Center must accelerate to 11.18 km/s, or 18,300 m.p.h. to escape Earth's gravitational pull.

event horizon - A black hole point of no return. Any light or matter crossing this boundary is doomed by the hole's gravity. Beyond this point, the escape velocity is greater than the speed of light, the ultimate speed limit in space.

F

focal length - The distance between a lens or mirror and its focal point.  This length is usually stated in terms of the lens or mirror diameter f/4 means that the focal length is four times the diameter of the optic.

frame dragging - see Lense-Thirring Effect.

fusion - The process in which atomic nuclei collide so fast that they stick together and emit a large amount of energy. In the center of most stars, hydrogen fuses into helium. The energy emitted by fusion supports the star's enormous mass from collapsing in on itself, and causes the star to glow.

G

galaxy - Vast islands of stars, gas, and dust that populate the universe by the billions. Galactic size and structure range from subtle ellipticals to grand pinwheel spirals with the mass of at least 100 billion stars. Instead of randomly scattered throughout the universe, galaxies clump together in web-like structures, or cosmic foam. Despite immense luminosity, dark matter prevails as the primary galactic mass component.

globular cluster - A ball of a few hundred thousand stars that resides in the halo of our galaxy and other galaxies.  These clusters contain stars that evolve separately from galactic stars; thus, globular clusters hold the original recipe of the galaxy.  About 250 globular clusters form a spherical halo around our galaxy.  M13, a bright globular cluster in Hercules, is a classic example easily seen with binoculars or a small telescope.

gravity - The most feeble of the four fundamental forces in the universe that affect all matter.  Gravity is an attractive force.  The magnitude of gravitational attraction depends directly on mass and inversely on distance squared.  For instance, the gravitational attraction between you and the Earth is much greater than that between you and the Sun, even though the Sun is far more massive than the Earth (333,000 times).  The distance separating you from the Sun weakens the mutual gravitational attraction so that the Earth's gravitational pull on you at Earth's surface is 1653 times greater than the Sun's.

Great Attractor - The Great Attractor is a giant group of roughly 100,000 galaxies beyond our Local Group of galaxies. No one has yet seen the galaxies of the Great Attractor, but the effect on nearby galaxy clusters has been detected.

Great Red Spot - Jupiter's Great Red Spot is a huge, long-lasting storm in the atmosphere of the Southern Hemisphere of Jupiter (near the equator). Its color is actually pink to orange. This whirlwind varies in size and color from year to year. It is about 17,000 miles (28,000 km) long and 9,000 miles (14,000 km) wide; it is the biggest storm in this solar system. It is so big that the Earth would fit in it with room to spare. It was discovered in 1664 by Robert Hooke.

H

Harvest Moon - The Harvest moon is the full moon that appears closest in time to the Autumnal Equinox, occurring in late September or early October.

heliosphere - The heliosphere is an area centered around the Sun over which the effect of the solar wind extends. The heliosphere extends beyond the orbit of Pluto.

horizon - The horizon is an imaginary circle that delimits the sky and the Earth, or an extension of the plane of the observer (at an altitude of 0 degrees).

Hubble Constant - The present expansion rate of the universe, measured in units of km/s/Mpc or kilometers per second per megaparsec. Astronomers calculate the distance to a galaxy, then observe its Doppler shift. Once this information is plotted on a graph, the astronomer repeats these observations for several other galaxies. After plotting the distances and Doppler shifts from several observations, a relationship appears: As distance increases, so does the magnitude of the Doppler shift. That relationship is H. In order to check validity, astronomers compose various observation techniques that independently derive H.

I

inclination - Inclination is the angle between the plane of an orbit and the plane of the ecliptic. Orbital inclination is abbreviated as i.

inertia - Inertia is a property of matter: a mass at rest remains at rest and a mass in motion remains in motion as long as no outside force acts upon it.

interstellar dust - Interstellar dust is composed of microscopic bits (on the order of a micron in diameter) of carbon and/or silicates. The origin of interstellar dust in unknown, but it seems to be associated with young stars. Interstellar dust is not at all like the dust we have in our houses (which is mostly bits of organic debris and lint).

ionosphere - The ionosphere is one of the highest layer of the Earth's atmosphere. The ionosphere starts at about 43-50 miles (70-80 km) high and continues for hundreds of miles (about 400 miles = 640 km).  Auroras occur in the ionosphere.

J

jansky - The jansky (abbreviated Jy) is a unit of radio flux density (the rate of flow of radio waves); 1 Jy = 10^-26 watts/m²/Hz. A typical radio source has a spectral flux density of roughly 1 Jy. The jansky was named to honor Karl Gothe Jansky who developed radio astronomy in 1932.   A common term used in the movie Contact.

Jovian - Jovian means of or relating to the planet Jupiter.

K

Kelvin temperature - Kelvin is a temperature scale designed so that 0K is defined as absolute zero and the size of one unit is the same as the size of one degree Celsius. Water freezes at 273.16K; water boils at 373.16K. This temperature scale was designed by Lord Kelvin (William Thompson, 1824 - 1907). [ K = C + 273°, F = 9/5C + 32°]

Kuiper Belt - The Kuiper belt is a region beyond Neptune in which at least 70,000 small objects orbit. This belt is located from 30 to 50 (?) A.U.'s and was discovered in 1992. The Kuiper belt may be the source of the short-period comets (like Halley's comet). The Kuiper belt was named for the Dutch-American astronomer Gerard P. Kuiper, who predicted its existence in 1951.

L

Lagrange Points - Points in the vicinity of two massive bodies (such as the Earth and the Moon) where each others' respective gravities balance. There are five, labeled L1 through L5. L1, L2, and L3 lie along the centerline between the centers of mass between the two masses; L1 is on the inward side of the secondary, L2 is on the outward side of the secondary; and L3 is on the outward side of the primary. L4 and L5, the so-called Trojan points, lie along the orbit of the secondary around the primary, sixty degrees ahead and behind of the secondary.  L1 through L3 are points of unstable equilibrium; any disturbance will move a test particle there out of the Lagrange point. L4 and L5 are points of stable equilibrium, provided that the mass of the secondary is less than about 1/25.96 the mass of the primary. These points are stable because centrifugal pseudo-forces work against gravity to cancel it out.

latitude - Latitude is the angular distance north or south of the equator of a celestial object.

Lense-Thirring Effect - An effect predicted by general relativity and also known as "frame dragging" in which the orbit of a small body orbiting around a rotating massive one is slightly warped by the rotation. The effect was first predicted by Austrian physicists Joseph Lense and Hans Thirring in 1918. The possible detection of frame dragging around neutron stars has been tentatively reported by Italian astronomers in Nov. 1997. Stronger experimental confirmation of this effect was made using black hole observations by NASA's Rossi X-Ray Timing Explorer spacecraft.  As an example, the effect being investigated is similar to what happens when a tennis ball is placed in a bowl of honey and the ball is slowly rotated.  The honey immediately contacting and surrounding the ball also moves in the direction of the tennis ball's spin, but the movement of the honey is diminished the further you get away from the rotating ball.  Detailed analysis of this effect is the purpose of NASA's Gravity Probe B mission.  The Lense-Thirring effect results in the orbital precession of X-ray emitting gas near the black hole, causing the X-ray emission to peak at periods which match the frame-dragging predictions. 

libration (of the Moon) - any one of those small periodical changes in the position of the moon's surface relatively to the earth, in consequence of which narrow portions at opposite limbs become visible or invisible alternately. It receives different names according to the manner in which it takes place; as: (a) Libration in longitude, that which, depending on the place of the moon in its elliptic orbit, causes small portions near the eastern and western borders alternately to appear and disappear each month. (b) Libration in latitude, that which depends on the varying position of the moon's axis in respect to the spectator, causing the alternate appearance and disappearance of either pole. (c) Diurnal or parallactic libration, that which brings into view on the upper limb, at rising and setting, some parts not in the average visible hemisphere.

light - Light is a type of energy (and the tiny part of the electromagnetic spectrum that we can see). The fastest that light can travel is 186,300 miles per second. Visible light has a wavelength from 10^-7m to 10^-8m.

light-year - The distance light travels in one year, about 5.88 trillion miles or almost 800 times the diameter of our solar system.  The nearest star is a mere four light-years away, while the nearest galaxy lies 2.2 million light-years from Earth.

Local Group - Our galactic neighborhood, including the Milky Way, Andromeda Galaxy, M33, and more than 25 smaller galaxies.  The Local Group appears to be a suburb of a supercluster of galaxies that lies 60 million light-years away, in the direction of the constellation Virgo (Virgo Supercluster).

longitude - Longitude is the angular distance east or west of the prime meridian of a celestial object.

lunar eclipse - An event that occurs when the Moon, Earth and Sun form a line in space.  The Moon travels through Earth's shadow.

lunar phases - The Moon passes through its phases every 29.5 days. The same phase will fall on the same date of the year according to the Saros cycle, every 18 years, 11 days, and eight hours.

lunation - the time that elapses between successive new moons, averaging 29 days, 12 hours, 44 minutes; a lunar month.

M

magnetic field - A force field generated by moving electrical charges.  An electrical current running through a loop of wire generates a magnetic field.  The strength of the field depends on the current and area of the wire loop.  Plasma churning through the atmosphere of the Sun drives powerful magnetic fields that sometimes produce cool magnetic storms called sunspots.  A magnetic dynamo underneath Earth's crust generates a magnetic field around Earth.

magnetosphere - The magnetosphere is the Earth's magnetic environment. The Earth is a huge dipole (2-pole) magnet. The Earth's magnetic field is probably cause by its molten iron-nickel core. This magnetic field is aligned with the north and south poles, and has reversed many times during geologic history. Charged particles become trapped on the field lines of the magnetosphere. William Gilbert hypothesized that the Earth was a giant magnet in 1600. Thomas Gold proposed the name "magnetosphere" in 1959. The Earth's magnetosphere extends far into space and is influenced by the solar wind (ions and electrons emitted from the sun). It extends into space from 60 to 37,280 miles (100 to 60,000 km) towards the Sun, and over 186,500 miles (300,000 km) away from the Sun, forming the Earth's magnetotail.

magnitude - A measure of brightness, as perceived by the human eye. Think of magnitude as indicating faintness, not brightness. Increasing magnitudes refers to fainter and fainter objects. The magnitude and apparent brightness of a star are related in a logarithmic fashion. For every five steps in magnitude, the apparent brightness of a star, galaxy, or nebula changes by a factor of 100. For example, we receive 100 times more light energy from Vega -- a zero-magnitude star -- than from Eta Ursa Minor -- a fifth-magnitude star in the Little Dipper. Under the clearest, darkest skies, your eye cannot see stars fainter than sixth magnitude. With the aid of binoculars, the human eye can detect 10th-magnitude stars.

The magnitude scale is organized logarithmically because that's the way our eyes perceive brightness, whether it's light bulbs or stars. For instance, your eye would perceive the same brightness difference between a 25- and 50-watt light bulb as it would between a 100- and 200-watt light bulb. Likewise with stars, your eye would detect the same brightness difference between a first- and a second-magnitude star as it would between a second- and third magnitude star. If you repeat this exercise down to a sixth-magnitude star, the first-magnitude star's brightness (amount of light received on Earth) is 100 times the sixth-magnitude star.

mare - Latin for "sea," it is an area of recent basalt rock on the surface of the Moon created by oozing lava. Perhaps a violent impact cracked the surface open. The maria formed while lava bled from the cracks and healed the impact crater. Maria are the most crater-free regions on the Moon's surface.

mass - The total matter content of an object.  Also a physical measure of inertia.  Newton's law states that mass is related to force and acceleration: m = F/a.  Einstein says that mass and space are somehow related, because mass warps space and space directs the motion of mass.

maximum elongation - A planet's greatest angular separation east or west of the Sun in the sky.  The maximum elongation for Venus and Mercury is 46 and 23 degrees, respectively.  At these points in the sky, Venus and Mercury are visible for the longest period of time.

meridian - The meridian is an imaginary north-south line in the sky that passes through the observer's zenith.

mesosphere - The mesosphere is the atmospheric layer between the stratosphere and the ionosphere. The mesosphere is characterized by temperatures that quickly decrease as height increases. The mesosphere extends from between 31 and 50 miles (17 to 80 kilometers) above the earth's surface.

Messier Objects - During the late 18th century (1759-1781), the French astronomer Charles Messier made a list of 103 fuzzy objects in space in order not to mistake star clusters, galaxies, and nebulae for comets (for which he was searching). More objects were added later, bringing the total to 110. In it, M1 is the Crab Nebula.

meteor - A shooting star; a meteoroid that travels between 11.18 and 72 km/s and vaporizes within Earth's atmosphere because of intense friction between it and the air.

meteor shower - A spectacular display of meteors streaking through the sky at rates between several to hundreds per hour.  Whenever Earth intercepts a stream of comet debris in orbit around the Sun, some of the debris falls into Earth's atmosphere, producing the meteor shower.  Meteor velocities fall within a range of 11 km/s and 72 km/s.  The first number is Earth's escape velocity, and the second is Earth's orbital velocity plus solar system escape velocity at Earth's orbit.  Any meteor traveling at a speed greater than 72 km/s must have originated outside the solar system.  Earth's gravity accelerates anything at relative rest to the escape velocity as it falls from outer space to Earth's surface.  Thus meteors within this velocity range are all members of the solar system.

meteor storm - A meteor storm is a very intense meteor shower.

meteorite - A space rock that strikes the surface of the Earth.  Most meteorites are classified as stony because of their mineral composition, which may include olivine, pyroxene, serpentine, sulfates, organic compounds, iron, and nickel.

meteoroid - A tiny grain of silicate or metal between 1 and 10 millimeters across that orbits the Sun. Sometimes meteoroids fall into Earth's atmosphere and become meteors.

Milky Way - Our galaxy.  The Milky Way is a spiral galaxy with four major arms containing young bright stars, gas and dust.  The mass of the Milky Way is estimated to be between 4 x 10^11 and 10^12 solar masses.  The luminous diameter of our galaxy is some 80,000 light-years with our Sun orbiting about 25,000 light-years from the galactic center.  However, the large-scale rotation of galactic matter suggests an even larger diameter extending beyond 120,000 light-years.  This data indicates that galactic mass is not centrally located in the core; rather, the bulk is spread out beyond the Sun's galactic orbit.  Perhaps only 10 percent of the galaxy glows as stars; the remainder is nearly invisible. (see dark matter and galaxy)

N

nautical mile - A nautical mile is equal to one-sixtieth of a degree of the earth's circumference and measures exactly 6,080.2 feet. That makes a lot of sense when one is navigating on the high seas because no fancy conversions are required between systems of measurement. At the equator 60 nautical miles is one degree of latitude. If you understand distance in terms of land miles, then 100 are equivalent to about 87 nautical miles. More familiar with the metric system? Then 100 kilometres equal 54 nautical miles.

On land however, the problem of measurement was quite different. Ancient Rome wanted to ensure its legions were doing a full day's march on their campaigns, and so they decided to define a mile as being 1,000 paces. By the time the legions had made their mark on Britain, those thousand marching paces had become 1,760 yards - the length of a statute mile to the present day.

neap tides - Neap tides are especially weak tides. They occur when the gravitational forces of the Moon and the sun are perpendicular to one another (with respect to the Earth). Neap tides occur during quarter moons.

nebula (or nebulae)- Astronomers have observed four nebula types: H II regions, reflection nebulae, planetary nebulae, and supernova remnants:

neutron star - In stellar evolution, one step beyond white dwarf. Neutron stars are named after their composition: neutrons. Stars with core masses between than 1.4 and 2 solar masses collapse, combining the electrons and protons into neutrons. Neutron star density averages 10^14 grams/cubic centimeter. A full bathtub of neutron star (instead of water) weighs about two Mt. Everests. A solar mass neutron star is between 10 and 15 km wide, with a liquid neutron core and an atmosphere of iron. Some neutron stars, called pulsars, spin rapidly at about one revolution a second and sustain a powerful magnetic field. Stars born with 8 to 20 solar masses blow most of their mass away into interstellar space (supernova), then evolve into neutron stars. Heavier stars collapses into a black hole.

NGC - The NGC (New General Catalog) is a list of over 13,000 deep-sky celestial objects. It was developed in 1888. For example, the Great Nebula in Orion is NCG 1976 (and M42). NGC4414 (pictured above) is a spiral galaxy 60 million light-years away.

nova - An explosive increase in stellar brightness by a factor of one hundred thousand to one million. Most novas occur in closely bound binary star systems, with one member being a white dwarf. The white dwarf pulls material (mostly hydrogen) that lies outside the companion star's Roche lobe. As hydrogen falls onto the white dwarf, the surface temperature and pressure increase until the hydrogen shell suddenly explodes in runaway thermonuclear reaction. Most of the material falls back onto the white dwarf. This is a recurring process that occurs as long as a steady stream of hydrogen flows to the white dwarf.

O

objective lens - The primary light gathering optic of a refraction telescope, located opposite of the eyepiece.

occultation - An eclipse of a celestial object by the Moon or another solar system body.

open cluster - A gravitationally bound group of approximately 100 to 1,000 stars born of the same material at about the same time.

opposition - A planet's position when it is 180 degrees east or west of the Sun in the sky ... or a planet is in opposition when the Earth is exactly between that planet and the sun. 

orbit - The motion of a massive body around another body, governed by the force of gravity.  Planets in our solar system follow an orbit around the Sun, as first noted by Johannes Kepler, in the shape of an ellipse.  An orbit is actually composed of two motions: one directly toward the other body (planet or star...) and the another that points away.  In the case of a circular orbit, these two component motions are orthogonal (90 degrees apart, or at right angles).  

orbital period - The duration of one orbit. For instance, Earth's orbital period around the Sun is one year, and the Moon orbits Earth in 27.3 days.

orbital plane - A plane containing at least two masses orbiting around a common center of gravity.  Earth and Sun orbit each other in an orbital plane, as do Earth and Moon.  However, the Earth-Moon orbital plane crosses the Earth-Sun orbital plane at a five-degree angle.

ozone layer - The ozone layer is a region of the stratosphere which contains most (about 90%) of the Earth's atmospheric ozone. It is about10-25 miles (15-40 km) above the Earth's surface. The ozone layer shields the Earth from Ultraviolet B rays that come from the Sun. The ozone layer is becoming depleted, and there is an "ozone hole" over Antarctica.

P

p.m. - The abbreviation a.m. stands for ante-meridiem (before the Sun has crossed the line) and p.m. for post-meridiem (after the Sun has crossed the line).  At 12 noon, the Sun is at its highest point in the sky and directly over the meridian.  It is therefore neither "ante-" nor "post-".

parallax - An angle formed by measuring the position of an object from two points at opposite ends of a baseline. Astronomers measure the parallax of a star using Earth's orbital diameter as a baseline. The astronomer observes and photographs a star at a six-month interval, when Earth is at opposite ends of an imaginary baseline defined by the width of its orbit around the Sun. Similarly, you can use the distance between your eyes as a baseline. Look at a distant object, say a telephone pole, with one eye closed then the other to see a parallax shift.

Astronomers look for a parallax shift in two star images as a way of estimating a star's distance from Earth. The greater the shift with respect to background stars, the closer the star. This sort of measurement is applicable to only nearby stars, no more than a few hundred light-years away. At 300 light-years, the angular shift is equivalent to a U.S. quarter seen at a distance of 300 miles, or 500 kilometers.

parsec - A distance unit that forms a parallax angle of one arc second(1/3600 degree) in the sky, equivalent to 3.26 light-years. (parallax second).

penumbra (eclipse) - a partial shadow cast by the Earth or Moon that surrounds the umbra, or total shadow. During a total lunar eclipse, the Moon passes through the Earth's penumbra, then into the umbra where the Moon "disappears". During a solar eclipse, the Earth passes through the Moon's shadow. Penumbra also refers to the immediate region surrounding a sunspot umbra, where the temperatures are significantly lower than the photosphere temperature (5800 degrees K).

periapsis - The point in an orbit when two objects are closest together. Special names are given to this point for commonly used systems: see periastron, perihelion, and perigee. The opposite of apoapsis.

periastron - The point of closest approach of two stars, as in a binary star orbit. Opposite of apastron.

perigee - The point of minimum orbital distance from the Earth and maximum orbital velocity around the Earth.

perihelion - The point of minimum orbital distance from the Sun and maximum orbital velocity around the Sun.

photosphere - An outer layer of a star where most photons escape. The temperature of the photosphere is called the effective temperature of the star. Astronomers analyze the composition of the photosphere with a spectrograph and telescope in order to model the interior of the star.

planet - A large celestial body that orbits a star and does not shine on its own.

planetary nebula - A shell of gas ejected from stars like our Sun at the end of their lifetime. This gas continues to expand out from the remaining white dwarf.   also see nebulae 

plasma - A plasma is an extremely hot gas that is composed of free-floating ions (atomic nuclei stripped of some electrons - making the ions positively charged) and free electrons (negatively charged). A plasma behaves much differently than a neutral gas, and is considered the fourth state of matter. A plasma conducts electrical currents. Stars are composed of plasma.

pole star - A pole star is a star that is located almost due north or due south and is useful for navigation. Polaris is the pole star of the Northern Hemisphere.

precession - The "wobble" of the Earth's rotational axis.  One complete wobble takes 26,000 years. Precession changes the pole star as seen from Earth.  Thuban was the pole star while the Egyptians built the Pyramids in Egypt. The motion of precession rotated the Earth's axis away from Thuban towards Polaris. 13,000 years from now, Earth's rotational axis will point at Vega.

prime focus - A reflecting telescope's primary mirror focal point. Astronomers place cameras at prime focus to record bright, wide field images.

proper motion - The motion of a celestial object across our sky. The proper motions of stars visible to the unaided eye are caused by the relative motions of our solar system and stars as we orbit around the center of the Milky Way.

pulsar - A spinning neutron star with a magnetic field on the order of one trillion Gauss. This magnetic field accelerates electrons along the magnetic poles, which then radiate photons that form a beam of light projecting along the poles. If the beam shines toward the Earth, astronomers see a flickering beacon all over the electromagnetic spectrum. The Crab Nebula pulsar pulses 30 times a second. The light curve shows a strong pulse followed by a weak pulse. The strength difference between the primary and secondary pulses is probably due to the direction of the pulsar's magnetic field.  More about pulsars.

Q

quasar - A quasi-stellar radio source or quasi-stellar object radiating extraordinary power from a small region in space, having the following characteristics: super galactic luminosity of 10 to 100,000 times that of the Milky Way; compact power-source dimensions, on the order of a galactic core; extreme distance from Earth; red shift indicating a velocity approaching 90 percent of the speed of light; residence inside the core of a galaxies.

R

radiant - A radiant is the point in the sky from which a meteor shower seems to be coming. For example, the Geminids meteor shower seems to come from the constellation Gemini.

radio galaxy - A radio-luminous galaxy.  Radio galaxies are a part of a larger group of galaxies called active galaxies.  The "active" refers to the fact that the observed radiation source is non-thermal: Glowing matter does not radiate this "active" luminous component.  Furthermore, radio galaxies are named so because they are "louder" at radio wavelengths than at visible light wavelengths.  This indicates tremendous activity within radio galaxies...something must be driving the non-thermal emission.  Radio telescopes show lobes of radio emission extending millions of light-years, centered at the nucleus of some radio galaxies, called extended radio galaxies.  Centaurus A is a nearby example of an extended radio galaxy that features two outer lobes 650,000 and 1,350,000 light-years in diameter.  Compact radio galaxies emit radio lobes not much larger that the galactic nucleus.

radio telescope - Instead of gathering and focusing visual wavelength light, a radio telescope collects radio wavelength light with a huge dish-shaped antenna.  An array of radio telescopes called the Very Large Array lies 80 kilometers west of Soccoro, New Mexico.  VLA radio telescopes are arranged in a "Y" pattern with each arm supporting nine 25-meter parabolic dishes.  These radio telescopes probe the galaxy and universe where optical telescopes cannot.  Astronomers have mapped the hydrogen content of our galaxy, and discovered enormous activity and structures in other galaxies invisible to optical telescopes.

red giant - A state of stellar evolution beyond the main-sequence life of a star. A red giant core is degenerate ionized helium, surrounded by a shell of hydrogen fusion, that expands the outer atmosphere in response to higher core temperatures.  The hydrogen fusing shell eats through the surrounding atmosphere and deposits helium onto the shrinking core.  The ballooning atmosphere cools and glows red; hence red giant.  The Sun will become a red giant the size of Earth's orbit in five to six billion years.  Once the helium core reaches 100 million degrees, it explosively begins fusing helium.  The birth of the active helium core is called the helium flash.  The Sun as a red giant will fuse helium for about 2 billion years after the helium flash.

redshift - The Doppler Effect shifting light wavelengths to the red end of the spectrum because of the motion of the source.  A redshift indicates a receding source of light, with the magnitude of the shift corresponding to the velocity of the source.  Astronomers relate red shifts to a calibrated distance in order to stretch their cosmic yard stick beyond the local realm of galaxies.  Planetary astronomers observe small redshifts and blue shifts in stars that unseen planets orbit.

refraction - A change in the direction of light that depends on incident angle, wavelength (color), and the material.  Raindrops refract light into rainbows.  Each color is a different wavelength that refracts at a unique angle, forming a band of color in order of wavelength: red, orange, yellow, green, blue, indigo, and violet.  A prism behaves the same way using glass instead of water.  The refraction happens at the boundary between two different, transparent materials.

Right Ascension - The position of a celestial object in the sky east of the vernal equinox along the celestial equator. Sidereal hours, minutes and seconds divide the celestial equator from 0 to 24 sidereal hours of right ascension. Keep in mind that the celestial equator is a projection of Earth's equator onto the sky, and Earth's rotation period is 24 sidereal hours (23.9345 solar mean hours). Thus dividing the celestial equator into sidereal hours, minutes and seconds instead of 360 degrees makes some intuitive sense. Set 1 sidereal hour equals 15 degrees to convert between right ascension and degrees.

S

sidereal clock - A clock that measures sidereal time, from 0 to 24 sidereal hours. This clock measures time with respect to the motion of stars, not the Sun: One sidereal day equals 24 sidereal hours equals 23.9345 solar mean hours equals 23 hours, :56 minutes, and 4.091 seconds in mean solar time. Thus, a sidereal clock ticks just a bit faster than a solar mean clock. The difference between solar and sidereal time depends on the apparent motion of stars or the apparent motion of the Sun in the sky.

sidereal time - Measurement of time based on the position of the vernal equinox instead of the Sun. A sidereal clock does not suffer the irregularities of a solar clock caused by Earth's elliptical orbit. A sidereal clock tracks sidereal time from 0 to 24 sidereal hours. The time reading on the sidereal clock equals the right ascension meridian that lies at the zenith, directly overhead. Astronomers prefer this time keeping method, because sidereal time is fixed to stars. An astronomer compares the right ascension of an object and sidereal time to figure out the optimum observing time.

singularity - A theoretical point at the core of a black hole, where the curvature of space-time is infinite. Einstein's General Theory of Relativity predicts that such points, which could be cosmic potholes to advanced space travelers, may exist in the space and time of our universe. Anything falling into a black hole meets certain doom. Once past the black hole's event horizon, the curvature of space-time is so great that nothing can escape -- not even light. Beyond the event horizon, space-time curvature increases toward the singularity. Gravitational tidal forces also are unimaginably strong and will shred anything into thin strands of super-dense spaghetti.

solar cycle - The solar cycle is the roughly 11-year, quasi-periodic variation in the frequency or number of sunspots, solar flares, and other solar activity.

solar day - A solar day is the amount of time that passes between two subsequent times when the Sun reaches its highest position above the horizon (passing through the meridian). The solar day varies greatly through the year, so the mean solar day is used instead. This is calculated as the average of all of the solar days in one year. A solar day is exactly 24 hours (since this is how we define hour, as 1/24 of a day); a solar day is slightly longer than a sidereal day. A solar day is not the time it takes for the Earth to rotate on its axis (that is the sidereal day).

solar eclipse - An arrangement (in order) of the Earth, Moon, and Sun where the Moon blocks the light from the Sun to an observer on Earth. The Moon's shadow does not completely contain the Earth. Only a narrow shadow cone of totality sweeps across the Earth's surface (umbra) surrounded by a partial shadow (penumbra). The solar eclipse alignment of Earth, Moon and Sun does not occur every month, because the Moon's orbit is tilted five degrees from a plane containing the Earth and Sun.

solar flare - A violent eruption of plasma from the chromosphere whipped up by intense magnetic activity. Temperatures quickly soar to 20 million degrees as the flare rises thousands of kilometers above the chromosphere. Large flares release 10^25 Joules, or about the energy of a few million volcanic eruptions on the Earth. Sunspots and solar-flare frequency are strongly related. Flares often disturb the atmosphere electrically, thus interfering with radio transmissions. The aurora borealis and aurora australis is a result of flare activity injecting energetic particles into Earth's magnetic field.

solar maximum - A solar maximum is the highest level in solar activity (like flares, prominences, sunspots, soronal holes, etc.), and occurs between consecutive solar minima.

solar minimum - A solar minimum is a low level in solar activity (like flares, prominences, sunspots, soronal holes, etc.), and occurs between consecutive solar maxima.

solar plume - A solar plume is a long, feathery jet of high-speed electrified gas that is expelled from the Sun's corona. Solar plumes emanate near the Sun's poles and travel over 13 million miles (21 million km) into space.

solar prominence - A solar prominence is an arc of gas that erupts from the surface of the Sun. Prominences can loop hundreds of thousands of miles into space and can last for many months.

solar wind - A flow of atomic nuclei and electrons from the Sun that travels at an average speed of 400 km/s.

solstice - An event in the Earth's orbit where the Sun in the sky at solar noon reaches its highest or lowest altitude during the year. Solstices occur within a few days of June 20 (summer solstice) and December 21 (winter solstice). The day of the summer solstice is the longest day of the year; the opposite is true for the winter solstice.

space-time - A four-dimensional coordinate system, or reference frame, with three space axis (x, y, z) and one time axis (t). A point in this reference frame is called an event, something that happens in space and time. Physicists and astrophysicists prefer a space-time reference frame to visualize complex events and interactions. Einstein explained that a constant exists between all reference frames, regardless of their motion relative to each other.

spectral line - Light given off at a specific frequency by an atom or molecule. Every different type of atom or molecule gives off light at its own unique set of frequencies; thus, astronomers can look for gas containing a particular atom or molecule by tuning the telescope to one of the gas's characteristic frequencies. For example, carbon monoxide (CO) has a spectral line at 115 Gigahertz (or a wavelength of 2.7 mm).

spectrograph - An astronomical (or optical?) instrument that diffracts light with a grating or into its component colors for spectral analysis of chemical absorption or emission lines. Usually, a CCD camera records the spectrum, transmits the image to a computer that writes the data to a hard drive or tape. The astronomer is free to look at the spectrum on the computer monitor, then analyze the image.

spectroscopy - Astronomers seek the recipe of stars, galaxies, and other celestial objects through spectroscopy. Because each atomic element absorbs and emits light in a unique set of wavelengths, the astronomer can sift through the spectrum of a star and determine what elements are present in the star's atmosphere. The spectrum is evidence of whatever is glowing, or radiating light. With spectroscopy astronomers relate the appearance of the spectrum to the physical processes inside the star.

From the shapes and depths of spectral lines, the astronomer can calculate fundamental qualities of a star, like how fast the gases churn through the stellar atmosphere, or the star's effective temperature. An astronomer may also be interested in correlations between the amounts (abundance) of certain elements and the physical behavior of the star, or the age of the star, or the abundances of other elements. For instance, compared with the Sun, stars with low amounts of iron are also low in almost every other element with respect to hydrogen.

Perhaps you have wondered why stars appear in colors from red to blue and white. These are the dominant colors of the star's spectrum, where the star radiates most of its energy. Effective temperature, or the temperature of the star's photosphere, is related to the color of the star. Stars with cooler stellar atmospheres than our Sun appear orange and red. Those with hotter temperatures are blue and white.

In addition, stars that convert hydrogen into helium and energy in their cores (main sequence stars) show a relation with mass. Hot main sequence stars are more massive than cool main sequence stars. This correlation does not apply once the star "burns up" its primary hydrogen fuel, and evolves off the main sequence.

spectrum - A wavelength or frequency band of electromagnetic radiation that contains information about the radiating source of energy. Optical astronomers rely on the spectrum of light from red to blue (visible spectrum) for detailed information carried by absorption or emission lines, color appearance and brightness. An optical instrument called a spectrograph refracts the light into a spectrum of color with a grating or prism.   About spectra.

speed of light - The maximum velocity for everything in the universe; 186,000 miles a second or fast enough to go to the Moon and back in less than three seconds.   Also see light-year.

speed of sound - The speed of sound is the speed at which sound moves through air. At sea level and standard atmospheric pressure, the speed of sound is 1116.45 feet per second (340.294 meters per second).

spicule - A structural component of the chromosphere that erupts like a plasma geyser driven by dense magnetic fields. Spicules rise between 3,000 and 10,000 kilometers above the magnetic mesh beneath. At the boundary between the photosphere and chromosphere, spicules look like peach fuzz. Spicules live short, dynamic lives of a few minutes but spew plasma at speeds of more than ten kilometers a second.

star - A dense, glowing ball of hydrogen, helium and trace quantities of heavier elements that shines with the energy released from a hydrogen thermonuclear fusion reaction inside the star's core. Stars appear in colors that range among red, orange, yellow, blue and white, set by the mass and photosphere temperature of the star. Stars are born, live and die within a metropolis populated with billions of stars called a galaxy. They may live for millions, or billions of years depending on their mass.

star cluster - A bunch of stars (ranging in number from a few to hundreds of thousands) which are bound to each other by their mutual gravitational attraction.

stratosphere - The stratosphere is the atmospheric layer between the troposphere and the mesosphere. The stratosphere is characterized by a slight temperature increase with altitude and the absence of clouds. The stratosphere extends between 11 and 31 miles (17 to 50 kilometers) above the earth's surface. The earth's ozone layer is located in the stratosphere.

statute mile - A unit of distance equal to 1.609 km (0.869 nautical mile, or 5280 ft.).

stellar classification - Stars are given a designation consisting of a letter and a number according to the nature of their spectral lines which corresponds roughly to surface temperature. The classes are: O, B, A, F, G, K, and M; O stars are the hottest; M the coolest. The numbers are simply subdivisions of the major classes. The classes are oddly sequenced because they were assigned long ago before we understood their relationship to temperature. O and B stars are rare but very bright; M stars are numerous but dim. The Sun is designated G2.

stellar nursery - A stellar nursery is a nebula ( a large cloud of hydrogen gas in space) in which star formation is occurring (stars are formed from gas). These nebulae are frequently illuminated by ultraviolet light which is emitted from the newborn stars. One example of a stellar nursery is the Eagle nebula.

stellar wind - The ejection of gas off the surface of a star. Many different types of stars, including our Sun, have stellar winds; however, a star's wind is strongest near the end of its life when it has consumed most of its fuel.

summer solstice - The solstices are days when the Sun reaches its farthest northern and southern declinations. The winter solstice occurs on December 21 and marks the beginning of winter (this is the shortest day of the year). The summer solstice occurs on June 21 and marks the beginning of summer (this is the longest day of the year).

sunspot - A solar magnetic disturbance that cools a region on the photosphere. Strong, dense magnetic fields generated by circulating plasma sometimes surge through the photosphere, creating the sunspot. The temperature falls around 1000 degrees Kelvin in the sunspot region, which darkens the photosphere. The dark center is called the umbra, surrounded by a dim filamentary penumbra. Sunspots range from Earth-size "pimples" to swollen scars halfway across the surface. Sunspot activity generally follows an 11-year cycle, called the sunspot cycle.

supercluster - An enormous chain of galaxies linked by their mutual gravity. Superclusters look as if they ride on the surfaces of bubbles. Our galaxy is a peripheral member of the Virgo Supercluster, centered around a giant elliptical galaxy M87. Luminous matter does not bind this cluster: Dark matter outweighs luminous matter 10 to 1. Our galaxy seems to be drifting toward the Virgo Supercluster at about 250 km/s.

supergiant - A star of at least eight solar masses that has evolved off the main sequence and depleted a significant portion of its hydrogen fuel supply. Helium becomes the new fuel, and "burns" at a much higher temperature in the core while hydrogen continues to "burn" in a surrounding shell. This shell expands the outer atmosphere of the star to over 10 up to 1000 solar radii. The entire life of the star from main sequence to supergiant is only a few million years.

supernova - A violent stellar explosion that releases energy exceeding the luminosity of an entire galaxy or the radiated energy from the Sun over one billion years. Astronomers divide supernovae into two groups: Type I and Type II. Low mass stars most likely produce Type I supernovae, stars of eight or more solar masses become Type II. The frequency and location of Type I suggest that white dwarfs stars within a binary system may blow off accumulated material as a supernova. Type II spectra reveal hydrogen lines, while Type I do not. Finally, Type I light curves peak then smoothly taper, while a type II light curve resembles a bumped and bruised Type I light curve. Astronomers use supernova data to estimate a distance to the supernova. Most observed supernova lie within other galaxies, thus astronomers can check distance calibrations and the expansion rate of the universe, or Hubble Constant. In addition, supernova observations show the relative element amounts blown into the interstellar medium. This information helps astronomers understand a galaxy's "metabolism."   About supernovae.

supernova remnant - The remainder of the stellar atmosphere that the supernova blows away into interstellar space. Astronomers see spectral emission of elements processed in the late stages of stellar evolution, like nitrogen, oxygen, and neon. The material is still hot and rushes through space at hundreds of kilometers per second.  More about supernova remnants.

T

terminator - A hard, shadow edge marking the boundary between the night and day side of a moon or planet. Along this jagged boundary on the Moon, you can clearly see another dimension to mountains and . Sometimes, towering crater rings still catch a bit of sunlight as they slip behind the terminator.

terrestrial planets - The four inner planets of our solar system: Mercury, Venus, Earth and Mars. They consist mainly of rocky material: iron, sulfur, magnesium, silicon, nickel. No H nor He exists in the atmospheres of these planets. Daytime temperatures range from 600 K (621 F) on Mercury to 300 K (81 F) on Mars. In addition, the most striking difference between the terrestrial planets and outer solar system planets lies in average density.

But the outer planets dominate in size, ranging from Jupiter (11.21 Earth diameters) to Neptune (3.88 Earth diameters). Pluto is the smallest planet in our solar system, 0.18 the Earth's diameter and even labeling Pluto a planet has recently been debated.

thermonuclear fusion - A star's light or photon engine.  Every star on the main sequence converts mass into energy through thermonuclear fusion. The process demands high temperature and pressure in order to fuse the hydrogen into helium, due to powerful electromagnetic forces separating neighboring H atoms.

tidal force - The difference between gravitational forces on opposite sides of an object, like a planet or moon. Gravity is a force that depends on both the involved masses and distance between the masses. The Moon-Earth gravitational interaction results in sea tides on Earth. Elsewhere in the solar system, the magnitude of gravitational force on Jupiter's moon, Io, is greatest on the Jupiter side of Io, and least on the backside. The difference between the two forces compresses and stretches Io, which in turn heats up Io's interior. As a result, Io is the most geologically active body in the solar system.

tide - A tide is a periodic rise and fall of large bodies of water. Tides are caused by the gravitational interaction between the Earth and the Moon. The gravitational attraction of the moon causes the oceans to bulge out in the direction of the moon. Another bulge occurs on the opposite side, since the Earth is also being pulled toward the moon (and away from the water on the far side). Since the earth is rotating while this is happening, two tides occur each day. Isaac Newton was the first person to explain tides scientifically.

total solar eclipse - Results from the Moon completely blocking out the Sun in the sky (see eclipse). The Moon's shadow narrowly projects onto the Earth, making this a rare experience.

transit - The time at which a celestial object is highest in the sky. This is the best time to view the object. At transit, the object's light travels through the thinnest possible layer of Earth's atmosphere, which can distort an object's appearance. At times of rise and set, when an object is nearest the horizon, its light travels through the most atmosphere.

trophosphere - The troposphere is the lowest region in the Earth's (or any planet's) atmosphere. On the Earth, it goes from ground (or water) level up to about 11 miles (17 kilometers) high. The weather and clouds occur in the troposphere. In the troposphere, the temperature generally decreases as altitude increases.

U

umbra - the darkest shadow region cast during an eclipse. If you stood within the Moon's umbra during a solar eclipse, the sun would "disappear" behind the Moon. The Moon disappears from veiw when it passes through Earth's umbra during a total lunar eclipse. This word also refers to the darkest region of a sunspot, where the sunspot tempertures are lowest. (see sunspot)

universe - The universe is everything, all matter and energy that is in existence.

Universal Time - Official Earth time. Universal Time (UT) is local Greenwich Meridian time at 0 degrees longitude. Astronomers depend on a consistent time standard in order to accurately record and report their observations. The United States is divided into time zones. From east to west, they are: Atlantic, Eastern, Central, Mountain, and Pacific.

V

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.

variable star - A star whose brightness changes periodically. These changes may relate to several different situations, from a luminosity change to an eclipsing dark companion. Luminosity changes relate to internal structure and stellar processes. For instance, the period of luminosity change of a Cepheid variable star is directly related to the Cepheid's luminosity. Thus, all the astronomer has to do is record the Cepheid's light curve, measure the period, match the period to the luminosity, then calculate the distance to the Cepheid. When Cepheids are located inside another galaxy, the astronomer can measure the distance to the galaxy.

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.

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.

W

wavelength - The distance between adjacent peaks in a series of periodic waves. Also see electromagnetic spectrum.

white dwarf - A dense, degenerate, Earth-size stellar core remnant, that does not exceed 1.4 solar masses. White dwarfs spin rapidly (1 rev/10 sec), and radiate at high temperatures (4000 - 100,000 degrees), resulting from rapid gravitational collapse. White dwarf magnetic fields can reach a billion times that of the Sun.

winter solstice - The solstices are the days when the Sun reaches its farthest northern and southern declinations. The winter solstice occurs on December 21 and marks the beginning of winter (this is the shortest day of the year) in the Northern Hemisphere. The summer solstice occurs on June 21 and marks the beginning of summer (this is the longest day of the year) in the Northern Hemisphere.

X,Y,Z

year - A planetary year is the length of time it takes that planet to revolve around the sun.

zenith - A point in the sky directly above an observer. The atmosphere is thinnest at the zenith, and thickest at the horizon. Thus astronomers prefer to observe an object when it is closest to the zenith.

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.

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