<P> The Sun does not have a definite boundary, but its density decreases exponentially with increasing height above the photosphere . For the purpose of measurement, however, the Sun's radius is considered to be the distance from its center to the edge of the photosphere, the apparent visible surface of the Sun . By this measure, the Sun is a near - perfect sphere with an oblateness estimated at about 9 millionths, which means that its polar diameter differs from its equatorial diameter by only 10 kilometres (6.2 mi). The tidal effect of the planets is weak and does not significantly affect the shape of the Sun . The Sun rotates faster at its equator than at its poles . This differential rotation is caused by convective motion due to heat transport and the Coriolis force due to the Sun's rotation . In a frame of reference defined by the stars, the rotational period is approximately 25.6 days at the equator and 33.5 days at the poles . Viewed from Earth as it orbits the Sun, the apparent rotational period of the Sun at its equator is about 28 days . </P> <P> The solar constant is the amount of power that the Sun deposits per unit area that is directly exposed to sunlight . The solar constant is equal to approximately 7003136800000000000 ♠ 1,368 W / m (watts per square meter) at a distance of one astronomical unit (AU) from the Sun (that is, on or near Earth). Sunlight on the surface of Earth is attenuated by Earth's atmosphere, so that less power arrives at the surface (closer to 7003100000000000000 ♠ 1,000 W / m) in clear conditions when the Sun is near the zenith . Sunlight at the top of Earth's atmosphere is composed (by total energy) of about 50% infrared light, 40% visible light, and 10% ultraviolet light . The atmosphere in particular filters out over 70% of solar ultraviolet, especially at the shorter wavelengths . Solar ultraviolet radiation ionizes Earth's dayside upper atmosphere, creating the electrically conducting ionosphere . </P> <P> The Sun's color is white, with a CIE color - space index near (0.3, 0.3), when viewed from space or when the Sun is high in the sky . When measuring all the photons emitted, the Sun is actually emitting more photons in the green portion of the spectrum than any other . When the Sun is low in the sky, atmospheric scattering renders the Sun yellow, red, orange, or magenta . Despite its typical whiteness, most people mentally picture the Sun as yellow; the reasons for this are the subject of debate . The Sun is a G2V star, with G2 indicating its surface temperature of approximately 5,778 K (5,505 ° C, 9,941 ° F), and V that it, like most stars, is a main - sequence star . The average luminance of the Sun is about 1.88 giga candela per square metre, but as viewed through Earth's atmosphere, this is lowered to about 1.44 Gcd / m . However, the luminance is not constant across the disk of the Sun (limb darkening). </P> <P> The Sun is composed primarily of the chemical elements hydrogen and helium; they account for 74.9% and 23.8% of the mass of the Sun in the photosphere, respectively . All heavier elements, called metals in astronomy, account for less than 2% of the mass, with oxygen (roughly 1% of the Sun's mass), carbon (0.3%), neon (0.2%), and iron (0.2%) being the most abundant . </P>

Temperature of the surface of the sun in kelvin