Some science behind the scenes

Sun

The physical Sun (Latin: Sol) as opposed to the symbolic SUN and the symbolic Sun and Moon, is the star at the center of the Solar System. The Earth and other matter (including other planets, asteroids, meteoroids, comets, and dust) orbit the Sun, which by itself accounts for about 99.8% of the Solar System's mass. Energy from the Sun, in the form of sunlight and heat, supports almost all life on Earth via photosynthesis, and drives the Earth's climate and weather.

The surface of the Sun consists of hydrogen (about 74% of its mass, or 92% of its volume), helium (about 24-25% of mass, 7% of volume), and trace quantities of other elements, including iron, nickel, oxygen, silicon, sulfur, magnesium, carbon, neon, calcium, and chromium. The Sun has a spectral class of G2V. G2 means that it has a surface temperature of approximately 5,780 K, giving it a white color that often, because of atmospheric scattering, appears yellow when seen from the surface of the Earth. This is a subtractive effect, as the preferential scattering of blue light (causing the sky color) removes enough blue light to leave a residual reddishness that is perceived as yellow. (When low enough in the sky, the Sun appears orange or red, due to this scattering.)

Its spectrum contains lines of ionized and neutral metals as well as very weak hydrogen lines. The V (Roman five) in the spectral class indicates that the Sun, like most stars, is a main sequence star. This means that it generates its energy by nuclear fusion of hydrogen nuclei into helium. There are more than 100 million G2 class stars in our galaxy. Once regarded as a small and relatively insignificant star, the Sun is now known to be brighter than 85% of the stars in the galaxy, most of which are red dwarfs.

The Sun orbits the center of the Milky Way galaxy at a distance of approximately 26,000 light-years from the galactic center, completing one revolution in about 225–250 million years. Its approximate orbital speed is 220 ± 20 kilometers per second (140 ± 12 mi/s). This is equivalent to about one light-year every 1,400 years, and about one AU every 8 days. These measurements of galactic distance and speed are as accurate as we can get given our current knowledge, but will change as we learn more.

The Sun is currently traveling through the Local Interstellar Cloud in the low-density Local Bubble zone of diffuse high-temperature gas, in the inner rim of the Orion Arm of the Milky Way Galaxy, between the larger Perseus and Sagittarius arms of the galaxy. Of the 50 nearest stellar systems within 17 light-years (1.6×1014 km) from the Earth, the Sun ranks 4th in absolute magnitude as a fourth magnitude star (M=4.83).

All matter in the Sun is in the form of gas and plasma because of its high temperatures. This makes it possible for the Sun to rotate faster at its equator (about 25 days) than it does at higher latitudes (about 35 days near its poles). The differential rotation of the Sun's latitudes causes its magnetic field lines to become twisted together over time, causing magnetic field loops to erupt from the Sun's surface and trigger the formation of the Sun's dramatic sunspots and solar prominences.  This twisting action gives rise to the solar dynamo and an 11-year solar cycle of magnetic activity as the Sun's magnetic field reverses itself about every 11 years.

The influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium creates the heliospheric current sheet, which separates regions with magnetic fields pointing in different directions. The plasma in the interplanetary medium is also responsible for the strength of the Sun's magnetic field at the orbit of the Earth. If space were a vacuum, then the Sun's 10-4 tesla magnetic dipole field would reduce with the cube of the distance to about 10-11 tesla. But satellite observations show that it is about 100 times greater at around 10-9 tesla. Magnetohydrodynamic (MHD) theory predicts that the motion of a conducting fluid (such as the interplanetary medium) in a magnetic field induces electric currents, which in turn generate magnetic fields, and in this respect it behaves like an MHD dynamo.

Observations

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