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Our Sun has inspired mythology in almost
all cultures, including ancient Egyptians, Aztecs, Native
Americans, and Chinese. We now know that the Sun is
a huge, bright sphere of mostly ionized gas, about 4.5
billion years old, and is the closest star to Earth
at a distance of about 150 million km. The next closest
star - Proxima Centauri - is nearly 268,000 times farther
away. There are millions of similar stars in the Milky
Way Galaxy (and billions of galaxies in the universe).
Our Sun supports life on Earth. It powers photosynthesis
in green plants and is ultimately the source of all
food and fossil fuel. The connection and interaction
between the Sun and the Earth drive the seasons, currents
in the ocean, weather, and climate.
The Sun is some 333,400 times more massive
than Earth and contains 99.86 percent of the mass of
the entire solar system. It is held together by gravitational
attraction, producing immense pressure and temperature
at its core (more than a billion times that of the atmosphere
on Earth, with a density about 160 times that of water).
At the core, the temperature is 16 million
degrees kelvin (K), which is sufficient to sustain thermonuclear
fusion reactions. The released energy prevents the collapse
of the Sun and keeps it in gaseous form. The total energy
radiated is 383 billion trillion kilowatts, which is
equivalent to the energy generated by 100 billion tons
of TNT exploding each second.
In addition to the energy-producing solar
core, the interior has two distinct regions: a radiative
zone and a convective zone. From the edge of the core
outward, first through the radiative zone and then through
the convective zone, the temperature decreases from
8 million to 7,000 K. It takes a few hundred thousand
years for photons to escape from the dense core and
reach the surface.
The Sun's "surface," known as the photosphere,
is just the visible 500-km-thick layer from which most
of the Sun's radiation and light finally escape, and
it is the place where sunspots are found. Above the
photosphere lies the chromosphere ("sphere of color")
that may be seen briefly during total solar eclipses
as a reddish rim, caused by hot hydrogen atoms, around
the Sun. Temperature steadily increases with altitude
up to 50,000 K, while density drops to 100,000 times
less than in the photosphere. Above the chromosphere
lies the corona ("crown"), extending outward from the
Sun in the form of the "solar wind" to the edge of the
solar system. The corona is extremely hot - millions
of degrees kelvin. Since it is physically impossible
to transfer thermal energy from the cooler surface of
the Sun to the much hotter corona, the source of coronal
heating has been a scientific mystery for more than
60 years. Scientists believe that energy transfer has
to be in the form of waves or magnetic energy. Likely
solutions have emerged from recent SOHO and TRACE satellite
observations, which found evidence for the upward transfer
of magnetic energy from the Sun's surface toward the
corona above. Researchers in NASA's Sun-Earth Connection
Space Science theme study these myserious phenomena.
Related Links:
Source: NASA
Last Updated : 08.20.2003
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Mean Distance from Earth:
1.495979 x 1013 cm
Radius:
6.955 x 1010 cm
Volume:
1.4122 x 1033 cm3
Mass:
1.989 x 1033 g
Mean Density:
1.409 gm/cm3
Surface Area:
6.087 x 1022 cm2
Surface Gravity:
2.740 x 104 cm/s2
Escape Velocity:
6.177 x 107 cm/s
Sidereal Rotation Period:
25.38 days
Equatorial Inclination:
7 degrees, 15 minutes
Effective Temperature:
5777 K
Age:
4.5 - 4.7 x 109 years
Spectral Type:
G2 V
Synodic Period:
27.2753 days
Velocity Relative to Near Stars:
19.7 km/s
Solar Constant
(Total Solar Irradiance)
1.365 - 1.369 kW/m2
(at the mean distance of the earth from the Sun, about
one AU)
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