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The Sun

The Sun Sun symbol.svg
Sun white.jpg
Sun with sunspots and limb darkening as seen in visible light with solar filter.
Observation data
Mean distance
from Earth
1 au1.496×108 km
8 min 19 s at light speed
Visual brightness (V) −26.74
Absolute magnitude 4.83
Spectral classification G2V
Metallicity Z = 0.0122
Angular size 31.6–32.7 minutes of arc
Adjectives Solar
Orbital characteristics
Mean distance
from Milky Way core
≈ 2.7×1017 km
27,200 light-years
Galactic period (2.25–2.50)×108yr
Velocity ≈ 220 km/s (orbit around the center of the Milky Way)
≈ 20 km/s (relative to average velocity of other stars in stellar neighborhood)
≈ 370 km/s (relative to the cosmic microwave background)
Physical characteristics
Equatorial radius 695,700 km
109 R
Equatorial circumference 4.379×106 km
109 × Earth
Flattening 9×10−6
Surface area 6.09×1012 km2
12,000 × Earth
Volume 1.41×1018 km3
1,300,000 × Earth
Mass (1.98855±0.00025)×1030 kg
333,000 M
Average density 1.408 g/cm3
0.255 × Earth
Center density (modeled) 162.2 g/cm3
12.4 × Earth
Equatorial surface gravity 274.0 m/s2
27.94 g
27,542.29 cgs
28 × Earth
Moment of inertia factor 0.070 (estimate)
Escape velocity
(from the surface)
617.7 km/s
55 × Earth
Temperature Center (modeled): 1.57×107 K
Photosphere (effective): 5,772 K
Corona: ≈ 5×106 K
Luminosity (Lsol) 3.828×1026 W
≈ 3.75×1028 lm
≈ 98 lm/W efficacy
Mean radiance (Isol) 2.009×107 W·m−2·sr−1
Age ≈ 4.6 billion years
Rotation characteristics
Obliquity 7.25°
(to the ecliptic)
67.23°
(to the galactic plane)
Right ascension
of North pole
286.13°
19 h 4 min 30 s
Declination
of North pole
+63.87°
63° 52' North
Sidereal rotation period
(at equator)
25.05 d
(at 16° latitude) 25.38 d
25 d 9 h 7 min 12 s
(at poles) 34.4 d
Rotation velocity
(at equator)
7.189×103 km/h
Photospheric composition (by mass)
Hydrogen 73.46%
Helium 24.85%
Oxygen 0.77%
Carbon 0.29%
Iron 0.16%
Neon 0.12%
Nitrogen 0.09%
Silicon 0.07%
Magnesium 0.05%
Sulfur 0.04%

The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. It is by far the most important source of energy for life on Earth. Its diameter is about 109 times that of Earth, and its mass is about 330,000 times that of Earth, accounting for about 99.86% of the total mass of the Solar System. About three quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron.

The Sun is a G-type main-sequence star (G2V) based on its spectral class. As such, it is informally referred to as a yellow dwarf. It formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core. It is thought that almost all stars form by this process.

The Sun is roughly middle-aged; it has not changed dramatically for more than four billion years, and will remain fairly stable for more than another five billion years. After hydrogen fusion in its core has diminished to the point at which it is no longer in hydrostatic equilibrium, the core of the Sun will experience a marked increase in density and temperature while its outer layers expand to eventually become a red giant. It is calculated that the Sun will become sufficiently large to engulf the current orbits of Mercury and Venus, and render Earth uninhabitable.


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