| Planets |
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Moons |
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Earth
- Moon (Luna) -
Mars
- Deimos
- Phobos
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Jupiter
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Calisto
- Europa
- Ganymede
- Io
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Saturn
- Dione
- Enceladus
- Iapetus
- Mimas
- Phoebe
- Rhea
- Tethys
- Titan
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Uranus
- Ariel
- Miranda
- Oberon
- Titania
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Neptune
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Proteus
- Triton
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Pluto
- Charon
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Galaxies |
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The Sun |
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The Sun is the star at
the center of the Solar System. The Earth and other
matter (including other planets, asteroids, meteoroids,
comets, and dust) orbit the Sun, which by itself
accounts for about 99.8% of the Solar System's mass.
Energy from the Sun, in the form of sunlight and heat,
supports almost all life on Earth via photosynthesis,
and drives the Earth's climate and weather.
The surface of the Sun consists of hydrogen (about 74%
of its mass, or 92% of its volume), helium (about 24-25%
of mass, 7% of volume), and trace quantities of other
elements, including iron, nickel, oxygen, silicon,
sulfur, magnesium, carbon, neon, calcium, and chromium.
The Sun has a spectral class of G2V. G2 means that it
has a surface temperature of approximately 5,780 K (9900
degrees Fahrenheit), giving it a white color that often, because of
atmospheric scattering, appears yellow when seen from
the surface of the Earth. This is a subtractive effect,
as the preferential scattering of shorter wavelength
light removes enough violet and blue light, leaving a
range of frequencies that is perceived by the human eye
as yellow. It is this scattering of light at the blue
end of the spectrum that gives the surrounding sky its
color. When the Sun is low in the sky, even more light
is scattered so that the Sun appears orange or even red.
A rare optical phenomenon may occur shortly after sunset
or before sunrise, known as a green flash. The flash is
caused by light from the sun just below the horizon
being bent (usually through a temperature inversion)
towards the observer. Light of shorter wavelengths
(violet, blue, green) is bent more than that of longer
wavelengths (yellow, orange, red) but the violet and
blue light is scattered more leaving light that is
perceived as green.
The Sun's spectrum contains lines of ionized and neutral
metals as well as very weak hydrogen lines. The V (Roman
five) in the spectral class indicates that the Sun, like
most stars, is a main sequence star. This means that it
generates its energy by nuclear fusion of hydrogen
nuclei into helium. There are more than 100 million G2
class stars in our galaxy. Once regarded as a small and
relatively insignificant star, the Sun is now known to
be brighter than 85% of the stars in the galaxy, most of
which are red dwarfs.
The Sun orbits the center of the Milky Way galaxy at a
distance of approximately 26,000 light-years from the
galactic center, completing one revolution in about
225–250 million years. Its approximate orbital speed is
220 ± 20 kilometers per second (140 ± 12 mi/s). This is
equivalent to about one light-year every 1,400 years,
and about one AU every 8 days. These measurements of
galactic distance and speed are as accurate as we can
get given our current knowledge, but will change as we
learn more.
The Sun is currently traveling through the Local
Interstellar Cloud in the low-density Local Bubble zone
of diffuse high-temperature gas, in the inner rim of the
Orion Arm of the Milky Way Galaxy, between the larger
Perseus and Sagittarius arms of the galaxy. Of the 50
nearest stellar systems within 17 light-years (1.6×1014
km) from the Earth, the Sun ranks 4th in absolute
magnitude as a fourth magnitude star (M=4.83). |
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