Sun
From The Book of THoTH (Leaves of Wisdom)
The Sun is the star at the center of Earth's solar system. The Earth and other matter (including other planets, asteroids, meteoroids, comets and dust) orbit the Sun, which by itself accounts for more than 99% of the solar system's mass. Energy from the Sun has supported almost all life on Earth through photosynthesis since the appearance of the first organisms. Humans use sunlight for eyesight, warmth, growing crops, and powering solar cells. Heat generated by sunlight (insolation) drives climate and the weather cycle on Earth.
About 74% of the Sun's mass is hydrogen, 25% is helium, and the rest is made up of trace quantities of heavier elements. The Sun is about 4.6 billion (109) years old and is about halfway through its main sequence evolution, during which nuclear fusion reactions in its core fuse hydrogen into helium. Approximately 5 million tons of matter are converted into energy within the Sun's core every second, producing neutrinos and solar radiation. In about 5 billion years, the Sun will evolve into a red giant and then a white dwarf, creating a planetary nebula in the process.
The Sun is a magnetically active star; it supports a strong, changing magnetic field that varies year-to-year and reverses direction approximately every eleven years. The Sun's magnetic field gives rise to many effects that are collectively called solar activity, including sunspots on the surface of the Sun, solar flares, and variations in the solar wind that carries material through the solar system. Effects of solar activity on the Earth include auroras at moderate to high latitudes as well as disruption of radio communications and electric power. Solar activity is thought to have played a large role in the formation and evolution of the solar system, and strongly affects the structure of Earth's outer atmosphere.
Although it is the nearest star to Earth and has been intensively studied by scientists, many questions about the Sun remain unanswered, such as why its outer atmosphere has a temperature of over 1 million K when its visible surface (the photosphere) has a temperature of just 6,000 K. Current topics of scientific study include the sun's regular cycle of sunspot activity, the physics and origin of solar flares and prominences, the magnetic interaction between the chromosphere and the corona, and the origin of the solar wind.
The Sun has a spectral class of G2V. "G2" means that it has a surface temperature of approximately 5,500 K, giving it a yellow color, and that its spectrum contains lines of ionized and neutral metals as well as very weak hydrogen lines. The "V" suffix 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 and is in a state of hydrostatic balance, neither contracting nor expanding over time.
The Sun will spend a total of approximately 10 billion years as a main sequence star. Its current age, determined using computer models of stellar evolution and nucleocosmochronology, is thought to be about 4.57 billion years.[2] The Sun orbits the center of the Milky Way galaxy at a distance of about 25,000 to 28,000 light-years from the galactic center, completing one revolution in about 225–250 million years. The orbital speed is 220 km/s, equivalent to one light-year every 1,400 years, and one AU every 8 days.
The Sun is a third generation star, whose formation may have been triggered by shockwaves from a nearby supernova. This is suggested by a high abundance of heavy elements such as gold and uranium in the solar system; these elements could most plausibly have been produced by endergonic nuclear reactions during a supernova, or by transmutation via neutron absorption inside a massive second-generation star.
The Sun does not have enough mass to explode as a supernova, and its mass is below the Chandrasekhar limit. Instead, in 4–5 billion years, it will enter a red giant phase, its outer layers expanding as the hydrogen fuel in the core is consumed and the core contracts and heats up. Helium fusion will begin when the core temperature reaches about 3×108 K. While it is likely that the expansion of the outer layers of the Sun will reach the current position of Earth's orbit, recent research suggests that mass lost from the Sun earlier in its red giant phase will cause the Earth's orbit to move further out, preventing it from being engulfed. Following the red giant phase, intense thermal pulsations will cause the Sun to throw off its outer layers, forming a planetary nebula. The Sun will then evolve into a white dwarf, slowly cooling over eons. This stellar evolution scenario is typical of low- to medium-mass stars.
Sunlight is the main source of energy near the surface of Earth. 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 1,370 watts per square meter of area at a distance of one AU from the Sun (that is, on or near Earth). Sunlight on the surface of Earth is attenuated by the Earth's atmosphere so that less power arrives at the surface—closer to 1,000 watts per directly exposed square meter in clear conditions when the Sun is near its zenith. This energy can be harnessed via a variety of natural and synthetic processes—photosynthesis by plants captures the energy of sunlight and converts it to chemical form (oxygen and reduced carbon compounds), while direct heating or electrical conversion by solar cells are used by solar power equipment to generate electricity or to do other useful work. The energy stored in petroleum and other fossil fuels was originally converted from sunlight by photosynthesis in the distant past.
Observed from Earth, the path of the Sun across the sky varies throughout the year. The shape described by the Sun's position, considered at the same time each day for a complete year, is called the analemma and resembles a figure 8 aligned along a North/South axis. While the most obvious variation in the Sun's apparent position through the year is a North/South swing over 47 degrees of angle (due to the 23.5-degree tilt of the Earth with respect to the Sun), there is an East/West component as well. The North/South swing in apparent angle is the main source of seasons on Earth.
