Astronomical Terms
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The
 Milky Way, the galaxy containing our solar system, is about 100,000 
light-years in diameter and about 10,000 light-years thick. 
  
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Aphelion:  see 
Orbit. 
 Apogee: see 
Orbit.
  
 Black hole:
  the theoretical end-product of the total gravitational collapse of a 
massive star or group of stars. Crushed even smaller than the incredibly
 dense neutron star, the black hole may become so dense that not even 
light can escape its gravitational field. In 1996, astronomers found 
strong evidence for a massive black hole at the center of the Milky Way.
 Recent evidence suggests that black holes are so common that they 
probably exist at the core of nearly all galaxies. 
 
Conjunction: the
 alignment of two celestial objects at the same celestial longitude.  
Conjunction of the Moon and planets is often determined with reference 
to the Sun.  For example, Saturn is said to be in conjunction with the 
Sun when Saturn and Earth are aligned on opposite sides of the Sun. 
 Mercury and Venus, the two planets with orbits within Earth's orbit, 
have two positions of conjunction.  Mercury, for example, is said to be 
in 
inferior conjunction when the Sun and Earth are aligned on opposite sides of Mercury.  Mercury is in 
superior conjunction when Mercury and Earth are aligned on opposite sides of the Sun.  
 
Dwarf planet: see 
Planet. 
 Elongation:
  the angular distance between two points in the sky as measured from a 
third point.  The elongation of Mercury, for example, is the angular 
distance between Mercury and the Sun as measured from Earth.  Planets 
whose orbits are outside Earth's can have elongations between 0° and 
180°.  (When a planet's elongation is 0°, it is at conjunction; when it 
is 180°, it is at opposition.)  Because Mercury and Venus are within 
Earth's orbit, their greatest elongations measured from Earth are 28° 
and 47°, respectively. 
 
Galaxy: gas
 and millions of stars held together by gravity.  All that you can see 
in the sky (with a very few exceptions) belongs to our galaxy—a system 
of roughly 200 billion stars. The exceptions you can see are other 
galaxies. Our own galaxy, the rim of which we see as the “
Milky Way,”
 is about 100,000 light-years in diameter and about 10,000 light-years 
in thickness. Its shape is roughly that of a thick lens; more precisely,
 it is a 
spiral nebula, a term first used for other galaxies when
 they were discovered and before it was realized that these were 
separate and distinct galaxies. Astronomers have estimated that the 
universe could contain 40 to 50 billion galaxies. In 2004, the Hubble 
Space Telescope and observers at the Keck Observatory in Hawaii 
discovered a new galaxy 13 billion light-years from Earth. 
 
Neutron star: 
 an extremely dense star with a powerful gravitational pull.  Some 
neutron stars pulse radio waves into space as they spin; these are known
 as 
pulsars.
 
Occultation: the eclipse of one celestial object by another.  For example, a star is occulted when the Moon passes between it and Earth. 
 
Opposition: the
 alignment of two celestial objects when their longitude differs by 
180°.  Opposition of the Moon and planets is often determined with 
reference to the Sun.  For example, Saturn is said to be at opposition 
when Saturn and the Sun are aligned on opposite sides of Earth.  Only 
the planets whose orbits lie outside Earth's can be in opposition to the
 Sun. 
 
Orbit:  the path traveled by an object in space. The term comes from the Latin 
orbis, which means “circle” or “disk,” and 
orbita,
 “orbit.” Theoretically, there are four mathematical figures, or models,
 of possible orbits: two are open (hyperbola and parabola) and two are 
closed (ellipse and circle), but in reality all closed orbits are 
ellipses. Ellipses can be nearly circular, as are the orbits of most 
planets, or very elongated, as are the orbits of most comets, but the 
orbit revolves around a fixed, or 
focal, point. In our solar 
system, the Sun's gravitational pull keeps the planets in their 
elliptical orbits; the planets hold their moons in place similarly. For 
planets, the point of the orbit closest to the Sun is the 
perihelion, and the point farthest from the Sun is the 
aphelion. For orbits around Earth, the point of closest proximity is the 
perigee; the farthest point is the 
apogee.  See also 
Retrograde. 
 Perigee: see 
Orbit.
 Perihelion: see 
Orbit. 
 Planet: the International Astronomical Union (IAU) issued the definition for planet (from the Greek 
planetes,
 “wanderers”) at their General Assembly in August 2006. A planet is a 
body that (a) is in orbit around the Sun, (b) is massive enough that its
 self-gravity gives it a nearly-spherical shape, and (c) has cleared the
 neighborhood around its orbit. A body that fulfills the first two 
criteria but not the third is a 
dwarf planet, provided that it (d) is not a satellite.
 While the exact definition of “clearing the neighborhood” was not 
established at press time, the eight planets from Mercury through 
Neptune have either assimilated or repulsed most other objects in their 
orbits, and each has more mass than the combined total of everything 
else in its area. The same cannot be said for Pluto, which has now been 
reclassified as a dwarf planet. There are currently eight planets and 
three dwarf planets recognized in the solar system, and more dwarf 
planets are expected to be admitted.
 In 1994, Dr. Alexander 
Wolszcan, an astronomer at Pennsylvania State University, presented 
convincing evidence of the first known planets to exist outside our 
solar system. These particular  
extrasolar planets circle a 
pulsar, or exploded star, in the constellation 
Virgo. 
 In 1995, several of these
 extrasolar planets
 were discovered orbiting stars similar to our Sun. Swiss astronomers 
found the first extrasolar planet (HD 209458b, nicknamed “Osiris”) to 
circle a normal Sun-like star. As of May 2006, 170 such planets have 
been discovered. 
 In Feb. 2004, using the 
Hubble Space Telescope,
 a team of scientists at the Institut d'Astrophysique de Paris announced
 that they had discovered oxygen and carbon in the atmosphere of 
“Osiris.” 
 In Aug. 2004, NASA and the National Science Foundation
 announced the discovery of two new planets, the smallest yet found, 
about the size of Neptune. The discovery opens up the possibility of 
smaller, Earth-sized extrasolar planets.
 In April 2005, a team of American and European astronomers reported that the first image of an 
extrasolar planet had been made. The planet is orbiting a brown dwarf near the constellation Hydra, 230 million light-years from Earth. 
 
Pulsar:
 a celestial object, believed to be a rapidly spinning neutron star, 
that emits intense bursts of radio waves at regular intervals.
 
Quasar:
 “quasi-stellar” object. Originally thought to be peculiar stars in our 
own galaxy, quasars are now believed to be the most remote objects in 
the universe. 
 Quasars emit tremendous amounts of light and 
microwave radiation. Although they are not much bigger than Earth's 
solar system, quasars pour out 100 to 1,000 times as much light as an 
entire galaxy containing a hundred billion stars. It is believed that 
quasars are powered by massive black holes that suck up billions of 
stars.
 
Retrograde:
 describes the clockwise orbit or rotation of a planet or other 
celestial object, which is in the direction opposite to Earth and most 
celestial bodies.  As viewed from a position in space north of the solar
 system (from some great distance above Earth's North Pole), all the 
planets revolve counterclockwise around the Sun, and all but Venus, 
Uranus, and Pluto rotate counterclockwise on their own axes.  These 
three planets have retrograde motion.
 Sometimes 
retrograde
 is also used to describe apparent backward motion as viewed from Earth.
  This motion happens when two objects rotate at different speeds around
 another fixed object.  For example, the planet Mars appears to be 
retrograde when Earth overtakes and passes by it as they both move 
around the Sun.  
 
Satellite (or
 moon): an object in orbit around a planet. Until the discovery of Jupiter's four main moons by 
Galileo Galilei, celestial objects in orbit around a planet were called 
moons.  However, upon Galilei's discovery, Johannes 
Kepler (in a letter to Galileo) suggested 
satellite (from the Latin 
satelles, which means “attendant”) as a general term for such objects. The word 
satellite is used interchangeably with 
moon, and astronomers speak and write about the moons of Neptune, Saturn, etc. The term 
satellite is also used to describe man-made devices of any size that are launched into orbit.  
 
Small Solar System Objects:
 at the 2006 IAU General Assembly, solar system bodies not defined as 
planets, dwarf planets, or satellites were placed in this category. 
These include most asteroids, most Trans-Neptunian Objects, comets, and 
other small bodies. 
 
Star: 
 a celestial object consisting of intensely hot gases held together by 
gravity. Stars derive their energy from nuclear reactions going on in 
their interiors, generating heat and light.  Stars are very large.  Our 
Sun  has a diameter of 865,400 mi—a comparatively small star.  
 A dwarf star is a small star that is of relatively low mass and average or below-average luminosity.  The Sun is a 
yellow dwarf,
 which is in its main sequence, or prime of life.  This means that 
nuclear reactions of hydrogen maintain its size and temperature.  By 
contrast, a
 white dwarf is a star at the end of its life, with low luminosity, small size, and very high density.
 A 
red giant
 is a star nearing the end of its life.  When a star begins to lose 
hydrogen and burn helium instead, it gradually collapses, and its outer 
region begins to expand and cool.  The light we see from these stars is 
red because of their cooler temperature. There are also red super 
giants, which are even more massive.  
 A 
brown dwarf
 lacks the mass to generate nuclear fusion like a true star, but it is 
also too massive and hot to be a planet.  A brown dwarf usually cools 
into a dark, practically invisible object.  The existence of brown 
dwarfs, also called 
failed stars, was confirmed in Nov. 1995 when
 astronomers at Palomar Observatory in California took the first 
photograph of this mysterious object.
 
Supernova:
  a celestial phenomenon in which a star explodes, releasing a great 
burst of light. There are two basic types of supernova.  Type Ia happens
 when a white dwarf star draws large amounts of matter from a nearby 
star until it can no longer support itself and collapses.  The second 
more well-known kind of supernova, type IIa, is the result of the 
collapse of a massive star.  (Massive is a classification for a star 
that is at least eight times the size of our Sun.)  Once the star's 
nuclear fuel is exhausted, if its core is heavy enough, the star will 
collapse in on itself, releasing a huge amount of energy (the 
supernova), which may be brighter than the star's host galaxy. 
 
On Feb. 24, 1987, Canadian astronomer Ian Shelter at the Las Campanas 
Observatory in Chile discovered a supernova—an exploding star—from a  
photograph taken on Feb. 23 of the Large Magellanic Cloud, a galaxy some
 160,000 light-years away from Earth. Astronomers believe that the dying
 star was Sanduleak –69°202, a 10-million-year-old blue supergiant.
 Supernova 1987A was the closest and best-studied supernova in almost 
400 years. One was previously observed by Johannes Kepler in 1604, four 
years before the  telescope was invented.