10-10-2012, 12:20 PM
Solar system
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The Solar System or solar system
consists of the Sun and the other
celestial objects gravitationally bound to
it: the eight planets, their 166 known
moons,[1] three dwarf planets (Ceres,
Pluto, and Eris and their four known
moons), and billions of small bodies. This
last category includes asteroids, Kuiper
belt objects, comets, meteoroids, and
interplanetary dust.
In broad terms, the charted regions of
the Solar System consist of the Sun, four
terrestrial inner planets, an asteroid
belt composed of small rocky bodies, four
gas giant outer planets, and a second
belt, called the Kuiper belt, composed of icy objects. Beyond the Kuiper belt
is the scattered disc, the heliopause, and ultimately the hypothetical Oort
cloud.
In order of their distances from the Sun, the planets are Mercury, Venus,
Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Six of the eight planets
are in turn orbited by natural satellites, usually termed "moons" after
Earth's Moon, and each of the outer planets is encircled by planetary rings
of dust and other particles. All the planets except Earth are named after
gods and goddesses from Greco-Roman mythology. The three dwarf planets
are Pluto, the largest known Kuiper belt object; Ceres, the largest object in
the asteroid belt; and Eris, which lies in the scattered disc.
Terminology
Objects orbiting the Sun are divided into
three classes: planets, dwarf planets, and
small Solar System bodies.
A planet is any body in orbit around the
Sun that a) has enough mass to form
itself into a spherical shape and b) has
cleared its immediate neighbourhood of
all smaller objects. There are eight
known planets: Mercury, Venus, Earth,
Mars, Jupiter, Saturn, Uranus, and
Neptune.
On August 24 2006 the International Astronomical Union defined the term
"planet" for the first time, excluding Pluto and reclassifying it under the new
category of dwarf planet along with Eris and Ceres.[2]
A dwarf planet is not required to clear its neighbourhood of other celestial
bodies. Other objects that may become classified as dwarf planets are
Sedna, Orcus, and Quaoar.
From the time of its discovery in 1930 until 2006, Pluto was considered the
Solar System's ninth planet. But in the late 20th and early 21st centuries,
many objects similar to Pluto were discovered in the outer Solar System,
most notably Eris, which is slightly larger than Pluto.
The remainder of the objects in orbit around the Sun are small Solar System
bodies (SSSBs).[3]
Natural satellites, or moons, are those objects in orbit around planets, dwarf
planets and SSSBs, rather than the Sun itself.
A planet's distance from the Sun varies in the course of its year. Its closest
approach to the Sun is called its perihelion, while its farthest distance from
the Sun is called its aphelion.
Astronomers usually measure distances within the Solar System in
astronomical units (AU). One AU is the approximate distance between the
Earth and the Sun, or roughly 149,598,000 km (93,000,000 mi). Pluto is
roughly 38 AU from the Sun while Jupiter lies at roughly 5.2 AU. One light
year, the best known unit of interstellar distance, is roughly 63,240 AU.
Sun
The Sun is the Solar System's parent star, and
far and away its chief component. Its large
mass gives it an interior density high enough to
sustain nuclear fusion, which releases
enormous amounts of energy, mostly radiated
into space as electromagnetic radiation such as
visible light.
The Sun is classified as a moderately large
yellow dwarf, but this name is misleading as,
compared to stars in our galaxy, the Sun is
rather large and bright. Stars are classified by the Hertzsprung-Russell
diagram, a graph which plots the brightness of stars against their surface
temperatures. Generally, hotter stars are brighter. Stars following this
pattern are said to be on the main sequence; the Sun lies right in the middle
of it. However, stars brighter and hotter than the Sun are rare, while stars
dimmer and cooler are common.[28]
It is believed that the Sun's position on
the main sequence puts it in the "prime
of life" for a star, in that it has not yet
exhausted its store of hydrogen for
nuclear fusion. The Sun is growing
brighter; early in its history it was 75
percent as bright as it is today.[29]
Calculations of the ratios of hydrogen
and helium within the Sun suggest it is
halfway through its life cycle. It will
eventually move off the main sequence
and become larger, brighter, cooler and
redder, becoming a red giant in about
five billion years.[30] At that point its
luminosity will be several thousand times
its present value.