or 83 million years, approximately 2.7 times per orbit. 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. [162][163][164], In the early first millennium BC, Babylonian astronomers observed that the Sun's motion along the ecliptic is not uniform, though they did not know why; it is today known that this is due to the movement of Earth in an elliptic orbit around the Sun, with Earth moving faster when it is nearer to the Sun at perihelion and moving slower when it is farther away at aphelion. The Sun is a G-type main-sequence star (G2V) based on its spectral class. This energy, which can take between 10,000 and 170,000 years to escape the core, is the source of the Sun's light and heat. It is calculated that the Sun will become sufficiently large to engulf the current orbits of Mercury and Venus, and render Earth uninhabitable – but not for about five billion years. A special instrument called a coronagraph allows astronomers to view the corona at other times. For a simple dipolar solar magnetic field, with opposite hemispherical polarities on either side of the solar magnetic equator, a thin current sheet is formed in the solar wind. [106], An 11-year sunspot cycle is half of a 22-year Babcock–Leighton dynamo cycle, which corresponds to an oscillatory exchange of energy between toroidal and poloidal solar magnetic fields. From an observation of a transit of Venus in 1032, the Persian astronomer and polymath Ibn Sina concluded that Venus is closer to Earth than the Sun. In the overall gloom, the pupil expands from ~2 mm to ~6 mm, and each retinal cell exposed to the solar image receives up to ten times more light than it would looking at the non-eclipsed Sun. Total solar eclipses won't be around forever! [89], Above the temperature minimum layer is a layer about 2,000 km thick, dominated by a spectrum of emission and absorption lines. Visit us on Twitter It is a nearly perfect sphere of hot plasma,[18][19] heated to incandescence by nuclear fusion reactions in its core, radiating the energy mainly as visible light and infrared radiation. [219] In ancient Greek religion, the sun deity was the male god Helios,[220] but traces of an earlier female solar deity are preserved in Helen of Troy. Because the upper part of the photosphere is cooler than the lower part, an image of the Sun appears brighter in the center than on the edge or limb of the solar disk, in a phenomenon known as limb darkening. The Sun has been an object of veneration in many cultures throughout human history. The number of substellar objects in that volume are expected to be comparable to the number of stars. = In the 1st century AD, Ptolemy estimated the distance as 1,210 times the radius of Earth, approximately 7.71 million kilometers (0.0515 AU).[167]. The English word sun developed from Old English sunne. / [29][30] The Sun is a Population I, or heavy-element-rich,[b] star. The theoretical concept of fusion was developed in the 1930s by the astrophysicists Subrahmanyan Chandrasekhar and Hans Bethe. [185][186], In the 1970s, two Helios spacecraft and the Skylab Apollo Telescope Mount provided scientists with significant new data on solar wind and the solar corona. The remainder of the Sun is heated by this energy as it is transferred outwards through many successive layers, finally to the solar photosphere where it escapes into space through radiation (photons) or advection (massive particles). {\displaystyle 2\Omega /\kappa \approx 1.50.} This motion of the Sun is mainly due to Jupiter, Saturn, Uranus, and Neptune. The solar magnetic field extends well beyond the Sun itself. [175][176] In 1890 Joseph Lockyer, who discovered helium in the solar spectrum, proposed a meteoritic hypothesis for the formation and evolution of the Sun. [165], One of the first people to offer a scientific or philosophical explanation for the Sun was the Greek philosopher Anaxagoras. Its polar field is 1–2 gauss (0.0001–0.0002 T), whereas the field is typically 3,000 gauss (0.3 T) in features on the Sun called sunspots and 10–100 gauss (0.001–0.01 T) in solar prominences. [129], After the red-giant branch the Sun has approximately 120 million years of active life left, but much happens.