A sun is the star at the center of a solar system. Our
sun is usually referred to as the Sun, and is occasionally
referred to as Sol to distinguish it from other "suns".
Planet Earth orbits the Sun, as do innumerable other bodies
including other planets, asteroids, meteoroids, comets
and dust. The primary stellar body around which an object
orbits is called its sun, and stars in a multiple star
system are referred to as the "suns" of bodies
in that system. The Sun is a main sequence star, with
a spectral class of G2, meaning that it is somewhat more
massive and hotter than the average star but far smaller
than a blue giant star. A G2 star is on the main sequence,
and has a lifetime of about 10 billion years (10 Ga),
and the Sun formed about 5 Ga (5 billion years) ago, as
determined by nucleocosmochronology and computer models
of stellar evolution. The Sun orbits the center of the
Milky Way galaxy at a distance of about 25,000 to 28,000
light-years from the galactic centre, completing one revolution
in about 226 Ma (226 million years). The orbital speed
is 217 km/s, i.e. 1 light-year in ca. 1400 years, and
1 AU in 8 days.
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