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5 lines
2.6 KiB
Plaintext
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields. [ 12 ] [ 13 ] It has a diameter of about 1 , 392 , 684 km ( 865 , 374 mi ) , [ 5 ] around 109 times that of Earth , and its mass ( 1.989×1030 kilograms , approximately 330 , 000 times the mass of Earth ) accounts for about 99.86 % of the total mass of the Solar System. [ 14 ] Chemically , about three quarters of the Sun 's mass consists of hydrogen , while the rest is mostly helium. The remaining 1.69 % ( equal to 5 , 600 times the mass of Earth ) consists of heavier elements , including oxygen , carbon , neon and iron , among others. [ 15 ]
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The Sun formed about 4.567 billion [ a ] [ 16 ] years ago from the gravitational collapse of a region within a large molecular cloud. Most of the matter gathered in the center , while the rest flattened into an orbiting disk that would become the Solar System. The central mass became increasingly hot and dense , eventually initiating thermonuclear fusion in its core. It is thought that almost all stars form by this process. The Sun is a G-type main-sequence star ( G2V ) based on spectral class and it is informally designated as a yellow dwarf because its visible radiation is most intense in the yellow-green portion of the spectrum , and although it is actually white in color , from the surface of the Earth it may appear yellow because of atmospheric scattering of blue light. [ 17 ] In the spectral class label , G2 indicates its surface temperature , of approximately 5778 K ( 5505 °C ) , and V indicates that the Sun , like most stars , is a main-sequence star , and thus generates its energy by nuclear fusion of hydrogen nuclei into helium. In its core , the Sun fuses about 620 million metric tons of hydrogen each second. [ 18 ] [ 19 ]
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Once regarded by astronomers as a small and relatively insignificant star , the Sun is now thought to be brighter than about 85 % of the stars in the Milky Way , most of which are red dwarfs. [ 20 ] [ 21 ] The absolute magnitude of the Sun is +4.83 ; however , as the star closest to Earth , the Sun is by far the brightest object in the sky with an apparent magnitude of −26.74. [ 22 ] [ 23 ] This is about 13 billion times brighter than the next brightest star , Sirius , with an apparent magnitude of −1.46. The Sun 's hot corona continuously expands in space creating the solar wind , a stream of charged particles that extends to the heliopause at roughly 100 astronomical units. The bubble in the interstellar medium formed by the solar wind , the heliosphere , is the largest continuous structure in the Solar System. [ 24 ] [ 25 ]
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