Astronomical Terms
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.