Glossary
ACCRETION
Accumulation of dust and gas into larger bodies such as stars, planets and moons.
ALBEDO
An object's albedo is the ratio of the amount of solar radiation reflected from an object to the total amount it receives. An object with a high albedo is shinier than an object with a low albedo. A white, completely reflecting object has an albedo of 1.0; a black object with no reflectivity has an albedo of 0.0 (zero).
APASTRON
The apastron is the point of greatest separation of two stars, as in a binary star system orbit.
APHELION
For an object (a planet or comet) orbiting the Sun, the aphelion is the point in its orbit which is farthest from the sun.
APOAPSIS
For an object orbiting a celestial body, the apoapsis is the point in its orbit which is farthest from that body.
ASTERISM
An asterism is a collection of stars (within a constellation) that forms an apparent pattern from Earth. Some familiar asterisms include the Big Dipper, the Little Dipper, Pleiades, Trapezium, and the Summer Triangle.
ASTEROID
An asteroid is a large rocky object or very small planet (planetoid). Most asteroids orbit the Sun in the asteroid belt between Mars and Jupiter. A few asteroids approach the Sun more closely. An asteroid impact with the Earth may have caused the extinction of the dinosaurs. The largest asteroid and first one ever discovered (by Giuseppe Piazzi on January 1, 1801) is Ceres, whose mass is equal to over one-third of the 2.3 x 1021 kg estimated total mass of all the 3,000 catalogued asteroids.
ASTROMETRY
Astrometry is the precise measurement of the position and motion of astronomical objects, often with respect to standard star catalogues.
AU
AU stands for "astronomical unit." An AU is equal to the mean (average) distance from the Earth to the sun, about 93 million miles (150 million km = 500 light-seconds).
BACKGROUND RADIATION
Background radiation is the microwaves permeating the universe that are probably the remnants of the Big Bang. This background radiation accounts for a temperature of 2.7 K in space.
BARRED SPIRAL GALAXY
A barred spiral galaxy (abbreviated SB) is a spiral galaxy whose center is elongated or bar-shaped.
BIG BANG THEORY
The big bang theory states that the universe began as a tiny but powerful explosion of space-time roughly 13.7 billion years ago (plus or minus 1 percent). In 1965, Arno Penzias and Robert Wilson used a horn antenna (in Crawford Hill, N.J.) and discovered the cosmic microwave background (CMB) radiation with a temperature of 2.7 Kelvin; this CMB was left-over from the early moments of the universe after the Big Bang (this was predicted by George Gamow and Ralph Alpher, in 1948). Contrast it with the Steady-State Theory.
BINARY STAR
A binary star is really two stars that rotate around a common center of mass. About half of all stars are in a group of at least two stars.
BLACK HOLE
A black hole is a massive object (or region) in space that is so dense that within a certain radius (the Schwarzschild radius, which determines the event horizon), its gravitational field does not let anything escape from it, not even light. It is thought that giant stars (those with a mass over 3 times the mass of the Sun) will evolve into red supergiants, then supernova, and then black holes. The phrase 'black hole' was coined by the physicist John Archibald Wheeler; before Wheeler, black holes were called 'frozen stars.' Astronomers think that there may be a black hole at the center of each galaxy.
CELESTIAL EQUATOR
The celestial equator is the projection of the Earth's equator onto the celestial sphere.
CELESTIAL POLES
The celestial poles are the projection of the Earth's poles onto the celestial sphere.
CELESTIAL SPHERE
The celestial sphere is an imaginary sphere whose center is the Earth. This sphere is used by astronomers to map celestial objects.
COMET
A comet is a celestial body that orbits around the sun. It is made up of a nucleus (solid, frozen ice, gas and dust), a gaseous coma (water vapor, CO2, and other gases) and a tail (dust and ionized gases). Its long tail of gas and dust always points away from the sun, because of the force of the solar wind. The tail can be up to 250 million km long, and is most of what we see. Comets are only visible when they're near the sun in their highly eccentric orbits.
CONJUNCTION
Conjunction is the apparent close approach of a planet to the Sun (or another planet), from the point of view of an observer on the Earth. A planet is in conjunction when the Sun is exactly between that planet and the Earth or, for Mercury and Venus (the two inferior planets), when that planet, the Sun, and the Earth are lined up. Mercury and Venus have two positions of conjunction: when either planet is directly between the earth and the Sun, it is in inferior conjunction; when either planet is on the far side of the Sun from the earth, it is in superior conjunction. During conjunction, a planet cannot be seen from Earth (unless it is in transit); it is either behind the Sun or is lost in the glare of the Sun.
CONSTELLATION
A constellation is a group of stars that, when seen from Earth, it forms a shape that along the history, it has suggested different animals or mythological beings for different cultures.
Since International Astronomical Union convention in 1930, the sky map was divided and fixed in 88 constellations.
CORONAGRAPH
A special instrument which blocks light from the disk of the Sun in order to study the faint solar atmosphere. In other words, makes an artificial eclipse.
CLUSTER (star)
A cluster is a group of stars that share characteristics as age and motion in the galaxy, because were formed together and keep a gravitational relationship. There are two types of clusters:- OPEN CLUSTER. An open cluster is a loose collection of up to about 1,000 relatively young stars that formed around the same time. An open cluster is about 10 parsecs across. Examples include the Pleiades and Hyades.
- GLOBULAR CLUSTER. A globular star cluster is a spherical group of up to a million stars held together by gravity. These remote objects lie mostly around the central bulge of spiral galaxies. The brightest globular cluster is Omega Centauri (in the constellation Centaurus); it is easily seen by the naked eye and is magnitude 4.
DECLINATION
Declination is a celestial coordinate that is used to measure the degrees of latitude above or below the celestial equator on the celestial sphere.
DOPPLER SHIFT
The Doppler shift (or Doppler Effect) is an increase or decrease in wavelength as the object emitting the wave moves relative to the observer. For example, a train whistle seems to be higher in pitch when the train is approaching you (the waves are compressed, shortening the wavelength), and lower in pitch when it is traveling away from you (the waves are elongated, lengthening the wavelength). The same thing happens with light waves when the light source is coming or going relative to us. For example, when a star is travelling away from Earth, its light appears redder (the light waves are elongated, lengthening the wavelength); this is called the red shift. The expansion of the universe was discovered when E. Hubble observed that the light from almost all other galaxies was red-shifted. The Doppler effect was named for Johann Christian Doppler (November 29, 1803-March 17, 1853), who first realized that it existed (1842).
DOUBLE STAR
A double star is two stars that appear close to one another in the sky. Some are true binaries (see Binary Star); others just appear together from the Earth because they are both in the same line-of-sight (Optical binary).
DWARF PLANET
A dwarf planet is an object that orbits a sun, has enough mass to give it a nearly round shape, is not a satellite of another object, and has not "cleared the neighbourhood" of its own orbit. This definition was established by the International Astronomical Union (IAU) on August 24, 2006. Pluto is considered to be a dwarf planet because objects in the Kuiper Belt are within its orbit. Ceres (the asteroid) is also classified as a dwarf planet. Another dwarf planet is Eris, formerly called Xena.
DWARF STAR
Dwarf stars are relatively small stars, up to 20 times larger than our sun and up to 20,000 times brighter. Our sun is a dwarf star.
ECCENTRICITY
Eccentricity is a measure of how an orbit deviates from circular. A perfectly circular orbit has an eccentricity of zero; an eccentricit between 0 and 1 represents an elliptical orbit. A parabolic orbit has an eccentricity equal to 1; a hyperbolic orbit has an eccentricity greater than 1. Neptune, Venus, and Earth are the planets with the least eccentric orbits in our solar system. Pluto and Eris are the dwarf planets with the most eccentric orbits in our solar system.
ECLIPSE
An eclipse happens when the moon blocks the Sun (Solar eclipse) or the Earth's shadow falls on the moon (Lunar eclipse).
TOTAL ECLIPSE
A total eclipse happens when the moon entirely blocks the Sun or the Earth's shadow totally blocks our view of the moon. A total solar eclipse can only happen during a new Moon. During a total eclipse, the Moon's shadow moves across the Earth at speeds over 1,000 mph (1,600 kph). Because of the relative size and distance of the Sun, Earth and the moon, the Earth is the only planet in our Solar System that experiences total solar eclipses.
PARTIAL ECLIPSE
A partial eclipse happens when the moon partially blocks the sun or the Earth's shadow partially blocks our view of the moon.
ECLIPTIC
The ecliptic is the plane defined by the Earth's orbit around the Sun; conversely, in the course of a year, the sun traces a path in the sky along the ecliptic. Most of the planets in our solar system appear close to the ecliptic plane from Earth. The Earth's axis is tilted at a 23.5° from the ecliptic (which causes the seasons).
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).
ELLIPTICAL GALAXY
An elliptical galaxy is a galaxy that has the shape of an ellipse. It is also called an "E" or "E-type" galaxy. M87 and M32 are elliptical galaxies.
EPHEMERIS
An ephemeris is a table listing the spatial coordinates of celestial bodies and spacecraft as a function of time.
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 autumnal equinox occurs on September 23, 2003; the vernal equinox occurs on March 20, 2003.
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GALACTIC HALO
The term galactic halo is used to denote an extended, roughly spherical component of a galaxy, which extends beyond the main, visible component. It can refer to any of several distinct components which share these properties:
the galactic spheroid (stars)
the galactic corona (hot gas, i.e. a plasma)
the dark matter halo
The distinction between the halo and the main body of the galaxy is clearest in spiral galaxies, where the spherical shape contrasts with the flat disc. In an elliptical galaxy, there is no sharp transition between the body of the galaxy and the halo.
GALAXY
A galaxy is a huge group of stars and other celestial bodies bound together by gravitational forces. There are spiral, elliptical, and irregularly shaped galaxies. Our Sun and solar system are a small part of the Milky Way Galaxy.
GALAXIES CLUSTER
A cluster is a group of galaxies. Our own galactic cluster, the Local Group, is about 5 million light-years across and contains about 40 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.
GRAVITY
Gravity is a physical force that pulls objects together. Every bit of mass produces a gravitational force; this force attracts all other masses. The more massive an object, the stronger the gravitational force. Newton formulated the laws of gravity.
GRAVITATIONAL CONSTANT
The gravitational constant (abbreviated G) is the constant of proportionality in Newton's equation (formulated in 1666) that describes the gravitational attraction between objects; their gravitational attraction (F) depends only on their masses and the distance between them, according to the formula F = Gm1m2 / r2. Henry Cavendish, in 1798, determined the numerical value of G to be 6.668 x 10-8 dynes cm2/g2.
HELIOPAUSE
The point in which finishes the solar gravity influence. In that point the solar wind meets the interstellar medium or solar wind from other stars.
HELIOSPHERE
The space within the boundary of the heliopause containing the Sun and all the solar system bodies.
HR
HR stands For Harvard Revised. One of many stars catalogues in astronomy. The initials HR is used followed by 4 digits to designate a star. In fact, one star can have designations as much as catalogues there are. For example: Betelgeuse, its proper name, is Aplha Orionis in Bayer’s designation, HR 2061 in this catalogue and SAO 113271 in the Smithsonian Astrophysical Observatory Catalogue, and so on.
INTERFEROMETRY
In interferometry, the data from two separate telescopes are used simultaneously to create a very good image (much better than either telescope alone).
INCLINATION
Inclination is the angle between the plane of an orbit and the plane of the ecliptic. Orbital inclination is abbreviated as i.
IRREGULAR GALAXY
An irregular galaxy is a galaxy with no rotational symmetry (it is neither spital, elliptical nor lenticular). Irregular galaxies usually contain only 100 million to 10 billion stars. Some are cloud-lie (with no apparent structure) and others are more standard shapes that have been disturbed. For example, the Magellanic Cloud is an irregular galaxy.
JPL
JPL stands for Jet Propulsion Laboratory. JPL is managed for NASA by the California Institute of Technology. JPL develops robotic devices for exploring the Solar System. JPL's main site is in the San Gabriel Mountains near Pasadena, California.
KBO’s
Kuiper Belt Objects There are two types, Classic and Scatter Disk Objects (see Scatter Disk).
KBO’s Classics. They are those whose orbits tend to be slightly eccentric and most of them remain within the disk of the Kuiper Belt. p. ex. Orcus.
Scatter disk objects. They are those whose orbits are so large and eccentric that it takes them most of their journey out of the plane of the Kuiper Belt disk. p. ex. Sedna.
LAGRANGE POINTS
Lagrange points (named for Josef Lagrange, the Italian-French mathematician who discovered them) are a set of five special points that occur between two large objects. At these points, a small orbiting object can orbit at a constant distance from both larger masses. This happens because at those five points, the gravitational force of the large objects is exactly equal to the centripetal force required to rotate with the objects. Three of the points are unstable (L1, L2, and L3) and two are stable (L4 and L5). Each of the stable Lagrange points forms an equilateral triangle with the two large masses.
LIGHT-YEAR
A light-year is the distance that light can travel in one year in a vacuum, which is about 5,880,000,000,000 miles or 63,240 AU or 9.46053 x 1012 kilometers.
MAGNITUDE
Magnitude is a measure of brightness of celestial objects. Lower numbers represent brighter objects than higher numbers; very bright star are 1st magnitude, less bright stars are 2nd magnitude, etc. The magnitude scale is logarithmic; a difference in magnitude of 5 is a 100-fold increase in brightness (the difference in each successive magnitude is a factor of about 2.512 times). This system of rating the brightness of celestial objects was developed by the Greek astronomer Hipparchus in 120 B.C. The current system was developed in 1850 by the English astronomer Norman Robert Pogson.
Apparent magnitude is the visible brightness of an object from Earth. Absolute magnitude is the brightness the object would have if seen from a distance of 10 parsecs (32.6 light-years) from Earth. Bolometric magnitude includes a star's entire spectrum of radiation, not just the visible light.
MAIN SEQUENCE STARS
Main sequence stars are the central band of stars on the Hertzsprung-Russell Diagram. These stars' energy comes from nuclear fusion, as they convert Hydrogen to Helium. Most stars are Main Sequence Stars. For these stars, the hotter they are the brighter. The sun is a typical Main Sequence star.
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, M2 is a globular cluster in Aquarius, M3 is a globular cluster in Canes Venatici, etc.
METEOR
A meteor is a meteoroid that has entered the Earth's atmosphere, usually making a fiery trail as it falls. It is sometimes called a shooting star. Most burn up before hitting the Earth.
METEORITE
A meteorite is a meteor that has fallen to Earth. Meteorites are either stone, iron, or stony-iron.
MILKY WAY GALAXY
The Milky Way Galaxy is a spiral galaxy; our sun and solar system are a small part of it. Most of the stars that we can see are in the Milky Way Galaxy. The main plane of the Milky Way looks like a faint band of white in the night sky. The Milky Way is about 100,000 light-years in diameter and 1,000 light-years thick. There are about 2 x 10 11 stars in the Milky Way. This spiral galaxy formed about 14 billion years ago. It takes the sun roughly 250 million years to orbit once around the Milky Way. The Earth is about 26,000 light-years from the center of the Milky Way Galaxy. The major arms of the Milky Way galaxy are the Perseus Arm, Sagittarius Arm, Centaurus Arm, and Cygnus Arm; our Solar System is in a minor arm called the Orion Spur.
MINOR PLANET
A minor planet is another name for an asteroid.
MULTIPLE SYSTEM
An orbiting each other group of three or more stars, due to their mutual gravitational attraction. Well known examples are Alpha Centauri and Castor of Gemini. (See also Binary Star)
NADIR
The nadir is the point directly below an observer.
NEBULA
A nebula is a huge, diffuse cloud of gas and dust in intergalactic space. The gas in nebulae (the plural of nebula) is mostly hydrogen gas (H2). The plural of nebula is nebulae or nebulas.
NGC - NEW GENERAL CATALOG
The 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).
NUCLEAR BULGE
The nuclear bulge is the central, spherical part of a spiral galaxy. It is surrounded by a disk-shaped mass of stars with spiral arms.
OCCULTATION
Occultation is when a smaller astronomical body passes behind a larger astronomical body (wholly obscuring its view). One example of occultation is when a planet passes behind the Sun (from our perspective) and it is hidden from our view. (see transit)
OPPOSITION
A planet is in opposition when the Earth is exactly between that planet and the sun. Mercury and Venus cannot be in opposition.
ORBIT
An orbit is a closed path that an object takes as it revolves around another body. Orbits are generally elliptical but may be perturbed by the presence of yet other bodies and may even form unusual figures.
ORBITAL RESONANCE
A state in which the orbital period of one body is related to that of another by a simple integer fraction, such as 1/2, 2/3, or 3/5.
PARALLAX
Parallax is the apparent change in the position of a star that is caused only by the motion of the Earth as it orbits the Sun. Friedrich Wilhelm Bessel first detected the parallax "motion" of a star in 1838 observing the star 61 Cygni. The parallax is used to calculate the distances of the sun nearest stars.
PARSEC
A parsec is a unit of distance that is equal to 3.26 light-years or 3.085678 x1013 kilometers. It is the distance at which a star would have a parallax of 1 second of arc.
PERIGEE
For an object orbiting the Earth, the perigee is the point in each orbit which is closest to the Earth. When the perigee varies a small amount from orbit to orbit. The closest perigee is called the proxigee.
PERIHELION
The perihelion is a planet or comet's closest approach to the Sun. The Earth is at perihelion (the Earth is closest to the Sun) in January.
PHOTOMETRY
Photometry is the measurement of apparent magnitudes of astronomical objects, like stars.
PLANET
A planet is a large celestial body that orbits a sun (or another star), has enough mass to give it a nearly round shape, is not a satellite of another object, and has "cleared the neighbourhood" of its own orbit. This definition was established by the International Astronomical Union (IAU) on August 24, 2006. There are eight planets orbiting the sun in our solar system.
PLANETARY NEBULA
A planetary nebula is a nebula formed from by a shell of gas which was ejected from a certain kind of extremely hot star. As the giant star explodes, the core of the star is exposed. Planetary nebulae have nothing to do with planets. The Hourglass Nebula is a planetary nebula.
PLUTINO
A Plutino (meaning "little Pluto") is a main Kuiper belt objects, ones that have a 2:3 resonance with Neptune (as Pluto is); a 2:3 resonace means that the Plutino completes 2 orbits around the Sun in the same time that it takes Neptune to complete 3 orbits around the Sun. About one-quarter of the Kuiper Belt objects (also called transneptunian objects) are Plutinos. Compare with cubewano.
PRECESSION
Precession is the small wobbling motion around the Earth's axis that the Earth makes as it spins (just like the wobbling motion of a spinning top). This causes the Earth's axis to point in a slightly different direction over time; Polaris is our pole star now, but this is not permanent because of precession. Precession is caused by the gravitational influences of the Sun and the Moon and has a period of 25,800 years.
PROPER MOTION
Proper motion is the actual motion of a star across the sky (not toward or away from the Earth). This motion is due to the orbit of the star in the Milky Way Galaxy. The proper motions of a star is the distance that it moves across the sky each year. This distance is so tiny, that it is measured in arcseconds per year. The star with the most proper motion is Barnard's Star (the star system second-closest to us), which moves only10 arcseconds per year (1/180 of the apparent width of the full moon).
PULSAR
A pulsar is a rapidly spinning neutron star that emits energy in pulses. Pulsars were discovered in 1967 by S. Jocelyn Bell Burnell (1943- ), who was a Cambridge University astronomy graduate student at the time. Her graduate advisor (Anthony Hewish) was given a share of the 1974 Nobel Prize, but Bell was ignored. No one had any idea what these unusual objects were at the time, so the name little green men (LGM) was used. Soon, Thomas Gold suggested that pulsars were rapidly-spinning neutron stars, the remnants of a supernova.
PROTOPLANETARY DISK
A protoplanetary disk is a rotating disk of dust that surrounds the central core of a developing solar system. This disk eventually coalesces into planets that orbit the star (which forms from the central core).
QUASAR
A quasar (more recently known as a QSO, Quasi-Stellar Object) is a distant star-sized energy source in space with excess of ultraviolet. Some of these QSO's gives off large amounts of radiation, including radio waves and X-rays (but some are radio-quiet). The word quasar is short for quasi-stellar radio objects. Quasars were first detected by Maartin Schmidt and Allan R. Sandage in 1963 -1964.
RADIO ASTRONOMY
Radio astronomy involves exploring space by examining radio waves from outer space. Radio astronomy was pioneered by Karl Gothe Jansky, who in1932 first detected radio waves from a cosmic source - in the central region of the Milky Way Galaxy. Gote Reber (a ham radio operator) made the first true radio telescope (usiing a 32-foot diameter parabolic dish to focus the radio waves) after reading of Jansky's discoveries.
RED SHIFT
The red shift is an increase in the wavelength of the light that is emitted from an object that is moving away from us. This increase in wavelength makes the object appear to be redder than it is. For example, when a star is travelling away from Earth, its light appears redder (the light waves are elongated, lengthening the wavelength). The expansion of the universe was discovered when E. Hubble observed that the light from almost all other galaxies was red-shifted. Compare with blue shift.
REVOLUTION
Revolution is the movement of one object around another. For example, the revolution of the Earth around the Sun takes one year.
RIGHT ASCENSION
Right ascension is a celestial coordinate that is used to measure the degrees of longitude on the on the celestial sphere. Zero degrees of right ascension is the position of the Sun during the vernal (spring) equinox (March 21).
ROCHE LOBE
The volume around a star in a binary system in which, if you were to release a particle, it would fall back onto the surface of that star. A particle released above the Roche lobe of either star will, in general, occupy the circumbinary region that surrounds both stars.
ROTATION
When an object rotates, it turns around a central point or axis. One planetary day is defined as the time it takes a planet to rotate around its axis.
SAO NUMBER
The SAO number is one of many ways of identifying or referring to a particular star. The SAO (Smithsonian Astrophysical Observatory) cataloged 258,996 stars and assigned them numbers; these numbers are the stars' SAO numbers, and are in the form: SAO ####. For example, the star Vega is SAO 67174.
SATELLITE
Satellites are objects that orbit a planet or a moon. Many man-made satellites and one natural satellite (the Moon) orbit the Earth.
SCATTERED DISK
A disk of ice-rock objects in high eccentricity orbits in the ecliptic plane beyond Neptune. Scattered Disk objects (SDOs) may be escapees from the Kuiper Belt and/or scattered Uranus-Neptune planetesimals.
SPECTROSCOPIC BINARY
A binary star system in which the two components are so close together (see close binary), or so far from the Sun, that they cannot be resolved simply by looking at them, even through a powerful telescope. Their binary nature can, however, be established because of the Doppler shift of their spectral lines. The first spectroscopic system discovered was Mizar (Zeta Ursae Majoris) in 1889.
SPECKLE INTERFEROMETRY
Is one method of speckle imaging technique. The principle is to take very short exposure images of astronomical targets, and then process the images to remove the effects of astronomical seeing.
One type of speckle interferometry called speckle masking involves calculation of the bispectrum or closure phases from each of the short exposures. The "average bispectrum" can then be calculated and then inverted to obtain an image. This works particularly well using aperture masks. In this arrangement the telescope aperture is blocked by astronomers apart from a few holes which allow light through, creating a small optical interferometer with better resolving power than the telescope would otherwise have. This aperture masking technique was pioneered by the Cavendish Astrophysics Group.
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).
SPECTROSCOPY
Spectroscopy is a scientific technique in which the visible light coming from objects (like stars and nebulae) is examined to determine the object's composition, temperature, density, and velocity.
SPIRAL GALAXY
Spiral galaxies are galaxies with a central, dense area and spiraling arms. There are two types of spiral galaxies, "S" (normal spiral) and "SB" (barred spiral, with an elongated center). The Milky Way and M31 (commonly known as Andromeda Galaxy) are two of a multitude of known spiral galaxies.
STAR
Each star in the sky is a glowing ball of gas. Our sun is a medium-sized star. The first stars in the Universe appeared about 200 million years after the Big Bang (which occurred about 13.7 billon years).
SUPERNOVA
A supernova is a huge explosion that occurs at the end of a mid- to heavy-weight star's life. A supernova releases a tremendous amount of energy, expelling the outer layers of the star and becoming extremely bright. What remains is a neutron star (from a middle-weight star, mass between 1.5 to 3 times the mass of the Sun) or a black hole (from a supergiant star, mass over 3 times the mass of the Sun).
SURFACE GRAVITY
Surface gravity is the strength of the gravitational field (the acceleration due to gravity) at the surface of the planet.
SYNCHRONOUS ORBIT/ROTATION
A satellite is in synchronous orbit (also called synchronous rotation) when its orbital period is the same as its period of rotation about its axis. The Moon is in a synchronous orbit, so the same side of the moon always faces Earth. Observing from the moon, the Earth is always in the same spot. Sometimes the term synchronous orbit is used to mean geosynchronous or geostationary (causing confusion).
TELESCOPE
A telescope is a device that makes faraway objects appear closer and larger, allowing us to see distant objects in space. The first refracting telescope was invented by Hans Lippershey in 1608. Early telescopes used glass lenses and/or mirrors to detect visible light. Later telescopes gathered electromagnetic radiation from the entire spectrum, from radio waves to gamma rays.
TIDAL FORCES
Tidal forces are the gravitational pull exerted upon planetary objects by nearby planets, moons, or stars.
TNO
TNO stands for Trans Neptunian Object. These small planetesimals made of rock and ice orbit our Sun past the planet Neptune. They are Kuiper Belt objects left over from the formation of the solar system. Pluto may be a TNO, albeit a rather massive one.
ULTRAVIOLET RAYS
Ultraviolet rays are a type of electromagnetic radiation with very short wavelengths (below those of the color blue). Ultraviolet rays are invisible to us. The ozone layer traps much of the Sun's ultraviolet energy coming through Earth's atmosphere.
ULTRAVIOLET TELESCOPE
An ultraviolet telescope recieves UV rays (a type of electromagnetic radiation with very short wavelengths) from space. Since the ozone layer traps much of the Sun's ultraviolet energy coming through Earth's atmosphere, orbiting UV telescopes like the Hopkins Ultraviolet Telescope (HUT) are now used.
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.
VERNAL EQUINOX
Is the Spring Equinox (see Equinox). The vernal equinox occurs on March 21; the autumnal equinox occurs on September 21.
VISUAL BINARY
A visual binary is a binary star pair that are far enough apart to be visible as separate objects.
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X-RAYS
X-rays are a type of electromagnetic radiation (between ultraviolet light and gamma rays in wavelength, frequency, and energy) - basically, it's light that is way past the blue-violet end of the visible spectrum - we cannot see it. They have short wavelength (and high frequency) as compared to visible light. Each photon of X-ray radiation has a lot of energy. X-rays can go through most solid objects. X-ray images of celestial objects are one way of learning about their high-energy properties. For example, the sun's corona emits X-rays, especially over sunspots. The Einstein X-ray satellite was launched in 1978 to survey celestial X-ray sources.
X-RAY ASTRONOMY
X-ray astronomy studies celestial objects by looking at the x-rays that they give off.
X-RAY BINARY STAR
X-ray binary stars are a special type of binary star in which one of the stars is a collapsed object such as a white dwarf, neutron star, or black hole. As matter is stripped from the normal star, it falls into the collapsed star, producing X-rays.
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ZENITH
An observer's zenith is the point directly overhead.
ZODIAC
The zodiac is a band of 12 constellations along the ecliptic. The constellations in the zodiac include: Capricorn (the goat), Aquarius (the water bearer), Pisces (the fish), Aries (the ram), Taurus (the bull), Gemini (the twins), Cancer (the crab), Leo (the lion), Virgo (the virgin), Libra (the balance), Scorpio (the scorpion), and Sagittarius (the archer).