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The central object of our Solar System and the nearest star to the Earth. Its basic characteristics are: mass

99×10³⁰ kg; radius 696 000 km/432 500 mi; mean density 4 g/cm³; mean rotation period 2 4 days; luminosity 85×10²⁴ J/s. Its average distance from Earth is 150 million km/93 million mi, and on account of this proximity it is studied more than any other star. The source of its energy is nuclear reactions in the central core (temperature 15 million K, relative density 155) extending to a quarter of the solar radius and including half the mass. Our Sun is nearly 5000 million years old, and is about halfway through its expected life-cycle. Every second it annihilates 5 million tonnes of matter, to maintain power output of 39 × 10²⁶ watts of energy.otheruses

The Sun is the star of our 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?in the form of insolation from sunlight?supports almost all life on Earth via photosynthesis, and drives the Earth's climate and weather.

The Sun is sometimes referred to by its Latin name Sol or by its Greek name Helios. Its astrological and astronomical symbol is a circle with a point at its center: bigodot.

The sun's history and destiny

The Sun is about 4.6 billion years old and is about halfway through its main-sequence evolution, during which nuclear fusion reactions in its core fuse hydrogen into helium. Each second, more than 4 million tonnes of matter are converted into energy within the Sun's core, 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'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 carry material through the solar system. The effects of solar activity on Earth include auroras at moderate to high latitudes, and the 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 a million K while its visible surface (the photosphere) has a temperature of less than 6,000 K. Current topics of scientific inquiry 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.

Overview

About 74% of the Sun's mass is hydrogen, 25% is helium, and the rest is made up of trace quantities of heavier elements. "G2" means that it has a surface temperature of approximately 5,500 K, giving it a white color, which because of atmospheric scattering appears yellow. 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. Because of logarithmic size distribution, the Sun is actually brighter than 85% of the stars in the Galaxy, most of which are red dwarfs. www.space.com/scienceastronomy/060130_mm_single_stars.html

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 217 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.

Sunlight is the main source of energy near the surface of 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 the 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.

Sunlight has several interesting biological properties. Ultraviolet light from the Sun has antiseptic properties and can be used to sterilize tools. It also causes sunburn, and has other medical effects such as the production of Vitamin D. Its current age, determined using computer models of stellar evolution and nucleocosmochronology, is thought to be about 4.57 billion years.

The Sun does not have enough mass to explode as a supernova. 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 km. While the Sun does not rotate as a solid body (the rotational period is 25 days at the equator and about 35 days at the poles), it takes approximately 28 days to complete one full rotation; Tidal effects from the planets do not significantly affect the shape of the Sun, although the Sun itself orbits the center of mass of the solar system, which is located nearly a solar radius away from the center of the Sun mostly because of the large mass of Jupiter.

The Sun does not have a definite boundary as rocky planets do; This is simply the layer below which the gases are thick enough to be opaque but above which they are transparent; Most of the Sun's mass lies within about 0.7 radii of the center.

The solar interior is not directly observable, and the Sun itself is opaque to electromagnetic radiation. However, just as seismology uses waves generated by earthquakes to reveal the interior structure of the Earth, the discipline of helioseismology makes use of pressure waves (infrasound) traversing the Sun's interior to measure and visualize the Sun's inner structure. Energy is produced by exothermic thermonuclear reactions (nuclear fusion) that mainly convert hydrogen into helium, helium into carbon, carbon into iron. All of the energy produced by fusion in the core must travel through many successive layers to the solar photosphere before it escapes into space as sunlight or kinetic energy of particles.

About 8.9 protons (hydrogen nuclei) are converted into helium nuclei every second, releasing energy at the matter-energy conversion rate of 4.26 million tonnes per second, 383 yottawatts (383 W) or 9.15 megatons of TNT per second. The rate of nuclear fusion depends strongly on density, so the fusion rate in the core is in a self-correcting equilibrium: a slightly higher rate of fusion would cause the core to heat up more and expand slightly against the weight of the outer layers, reducing the fusion rate and correcting the perturbation; and a slightly lower rate would cause the core to cool and shrink slightly, increasing the fusion rate and again reverting it to its present level.

The high-energy photons (gamma and X-rays) released in fusion reactions take a long time to reach the Sun's surface, slowed down by the indirect path taken, as well as by constant absorption and reemission at lower energies in the solar mantle. For many years measurements of the number of neutrinos produced in the Sun were much lower than theories predicted, a problem which was recently resolved through a better understanding of the effects of neutrino oscillation. while the material grows cooler as altitude increases, this temperature gradient is slower than the adiabatic lapse rate and hence cannot drive convection. Heat is transferred by radiation—ions of hydrogen and helium emit photons, which travel a brief distance before being reabsorbed by other ions. Convective overshoot is thought to occur at the base of the convection zone, carrying turbulent downflows into the outer layers of the radiative zone.

The thermal columns in the convection zone form an imprint on the surface of the Sun, in the form of the solar granulation and supergranulation. Sunlight has approximately a black-body spectrum that indicates its temperature is about 6,000 K (10,340°F / 5,727 °C), interspersed with atomic absorption lines from the tenuous layers above the photosphere. The photosphere has a particle density of about 10²³ m⁻³ (this is about 1% of the particle density of Earth's atmosphere at sea level).

During early studies of the optical spectrum of the photosphere, some absorption lines were found that did not correspond to any chemical elements then known on Earth. In 1868, Norman Lockyer hypothesized that these absorption lines were because of a new element which he dubbed "helium", after the Greek Sun god Helios. They can be viewed with telescopes operating across the electromagnetic spectrum, from radio through visible light to gamma rays, and comprise five principal zones: the temperature minimum, the chromosphere, the transition region, the corona, and the heliosphere. The heliosphere, which may be considered the tenuous outer atmosphere of the Sun, extends outward past the orbit of Pluto to the heliopause, where it forms a sharp shock front boundary with the interstellar medium. The increase is because of a phase transition as helium within the region becomes fully ionized by the high temperatures. Rather, it forms a kind of nimbus around chromospheric features such as spicules and filaments, and is in constant, chaotic motion. The transition region is not easily visible from Earth's surface, but is readily observable from space by instruments sensitive to the far ultraviolet portion of the spectrum.

The corona is the extended outer atmosphere of the Sun, which is much larger in volume than the Sun itself. The corona merges smoothly with the solar wind that fills the solar system and heliosphere. While no complete theory yet exists to account for the temperature of the corona, at least some of its heat is known to be from magnetic reconnection.

The heliosphere extends from approximately 20 solar radii (0.1 AU) to the outer fringes of the solar system. Its inner boundary is defined as the layer in which the flow of the solar wind becomes superalfvénic—that is, where the flow becomes faster than the speed of Alfvén waves. The solar wind travels outward continuously through the heliosphere, forming the solar magnetic field into a spiral shape, until it impacts the heliopause more than 50 AU from the Sun. In December 2004, the Voyager 1 probe passed through a shock front that is thought to be part of the heliopause. The magnetic field gives rise to strong heating in the corona, forming active regions that are the source of intense solar flares and coronal mass ejections. The polarity of the leading sunspot alternates every solar cycle, so that it will be a north magnetic pole in one solar cycle and a south magnetic pole in the next.

The solar cycle has a great influence on space weather, and seems also to have a strong influence on the Earth's climate. During this era, which is known as the Maunder minimum or Little Ice Age, Europe experienced very cold temperatures. Earlier extended minima have been discovered through analysis of tree rings and also appear to have coincided with lower-than-average global temperatures. The Van Allen belts consist of an inner belt composed primarily of protons and an outer belt composed mostly of electrons. The most energetic particles can 'leak out' of the belts and strike the Earth's upper atmosphere, causing auroras, known as aurorae borealis in the northern hemisphere and aurorae australis in the southern hemisphere. In periods of normal solar activity, aurorae can be seen in oval-shaped regions centered on the magnetic poles and lying roughly at a geomagnetic latitude of 65°, but at times of high solar activity the auroral oval can expand greatly, moving towards the equator. Theories proposed to resolve the problem either tried to reduce the temperature of the Sun's interior to explain the lower neutrino flux, or posited that electron neutrinos could oscillate, that is, change into undetectable tau and muon neutrinos as they traveled between the Sun and the Earth. Several neutrino observatories were built in the 1980s to measure the solar neutrino flux as accurately as possible, including the Sudbury Neutrino Observatory and Kamiokande.

Coronal heating problem

The optical surface of the Sun (the photosphere) is known to have a temperature of approximately 6,000 K. The other is magnetic heating, in which magnetic energy is continuously built up by photospheric motion and released through magnetic reconnection in the form of large solar flares and myriad similar but smaller events.

Currently, it is unclear whether waves are an efficient heating mechanism.

Faint young sun problem

Theoretical models of the sun's development suggest that 3.8 to 2.5 billion years ago, during the Archean period, the Sun was only about 75% as bright as it is today. The general consensus among scientists is that the young Earth's atmosphere contained much larger quantities of greenhouse gases (such as carbon dioxide and/or ammonia) than are present today, which trapped enough heat to compensate for the lesser amount of solar energy reaching the planet.

Magnetic field

All matter in the Sun is in the form of gas and plasma because of its high temperatures. 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 (see magnetic reconnection). 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. Magnetohydrodynamic (MHD) theory predicts that the motion of a conducting fluid (e.g., the interplanetary medium) in a magnetic field, induces electric currents which in turn generates magnetic fields, and in this respect it behaves like an MHD dynamo.

History of solar observation

Early understanding of the Sun

Humanity's most fundamental understanding of the Sun is as the luminous disk in the heavens, whose presence above the horizon creates day and whose absence causes night. In many prehistoric and ancient cultures, the Sun was thought to be a solar deity or other supernatural phenomenon, and worship of the Sun was central to civilizations such as the Inca of South America and the Aztecs of what is now Mexico. for example, stone megaliths accurately mark the summer solstice (some of the most prominent megaliths are located in Nabta Playa, Egypt, and at Stonehenge in England); the pyramid of El Castillo at Chichén Itzá in Mexico is designed to cast shadows in the shape of serpents climbing the pyramid at the vernal and autumn equinoxes. With respect to the fixed stars, the Sun appears from Earth to revolve once a year along the ecliptic through the zodiac, and so the Sun was considered by Greek astronomers to be one of the seven planets (Greek planetes, "wanderer"), after which the seven days of the week are named in some languages.

Development of modern scientific understanding

One of the first people in the Western world to offer a scientific explanation for the sun was the Greek philosopher Anaxagoras, who reasoned that it was a giant flaming ball of metal even larger than the Peloponnesus, and not the chariot of Helios. For teaching this heresy, he was imprisoned by the authorities and sentenced to death (though later released through the intervention of Pericles). In the early 17th century, Galileo pioneered telescopic observations of the Sun, making some of the first known observations of sunspots and positing that they were on the surface of the Sun rather than small objects passing between the Earth and the Sun. Isaac Newton observed the Sun's light using a prism, and showed that it was made up of light of many colors, while in 1800 William Herschel discovered infrared radiation beyond the red part of the solar spectrum. The 1800s saw spectroscopic studies of the Sun advance, and Joseph von Fraunhofer made the first observations of absorption lines in the spectrum, the strongest of which are still often referred to as Fraunhofer lines.

In the early years of the modern scientific era, the source of the Sun's energy was a significant puzzle. Lord Kelvin suggested that the Sun was a gradually cooling liquid body that was radiating an internal store of heat. Kelvin and Hermann von Helmholtz then proposed the Kelvin-Helmholtz mechanism to explain the energy output. Ernest Rutherford suggested that the energy could be maintained by an internal source of heat, and suggested radioactive decay as the source. However it would be Albert Einstein who would provide the essential clue to the source of a Sun's energy with his mass-energy relation E=mc².
In 1920 Sir Arthur Eddington proposed that the pressures and temperatures at the core of the Sun could produce a nuclear fusion reaction that merged hydrogen into helium, resulting in a production of energy from the net change in mass. This theoretical concept was developed
in the 1930s by the astrophysicists Subrahmanyan Chandrasekhar and Hans Bethe.

Solar space missions

The first satellites designed to observe the Sun were NASA's Pioneers 5, 6, 7, 8 and 9, which were launched between 1959 and 1968. Pioneer 9 operated for a particularly long period of time, transmitting data until 1987.

In the 1970s, Helios 1 and the Skylab Apollo Telescope Mount provided scientists with significant new data on solar wind and the solar corona. The Helios 1 satellite was a joint U.S.-German probe that studied the solar wind from an orbit carrying the spacecraft inside Mercury's orbit at perihelion. Discoveries included the first observations of coronal mass ejections, then called "coronal transients", and of coronal holes, now known to be intimately associated with the solar wind.

In 1980, the Solar Maximum Mission was launched by NASA. This spacecraft was designed to observe gamma rays, X-rays and UV radiation from solar flares during a time of high solar activity. In 1984 Space Shuttle Challenger mission STS-41C retrieved the satellite and repaired its electronics before re-releasing it into orbit. The Solar Maximum Mission subsequently acquired thousands of images of the solar corona before re-entering the Earth's atmosphere in June 1989.

Japan's Yohkoh (Sunbeam) satellite, launched in 1991, observed solar flares at X-ray wavelengths. It was destroyed by atmospheric reentry in 2005.

One of the most important solar missions to date has been the Solar and Heliospheric Observatory, jointly built by the European Space Agency and NASA and launched on December 2, 1995. In addition to its direct solar observation, SOHO has enabled the discovery of large numbers of comets, mostly very tiny sungrazing comets which incinerate as they pass the Sun.

All these satellites have observed the Sun from the plane of the ecliptic, and so have only observed its equatorial regions in detail. Once Ulysses was in its scheduled orbit, it began observing the solar wind and magnetic field strength at high solar latitudes, finding that the solar wind from high latitudes was moving at about 750 km/s (slower than expected), and that there were large magnetic waves emerging from high latitudes which scattered galactic cosmic rays.

Elemental abundances in the photosphere are well known from spectroscopic studies, but the composition of the interior of the Sun is more poorly understood. A solar wind sample return mission, Genesis, was designed to allow astronomers to directly measure the composition of solar material. Genesis returned to Earth in 2004 but was damaged by a crash landing after its parachute failed to deploy on reentry into Earth's atmosphere. UV exposure gradually yellows the lens of the eye over a period of years and can cause cataracts, but those depend on general exposure to solar UV, not on whether one looks directly at the Sun.

When looking at the sun, either with or without optical aid, using a proper filter is important as some improvised filters pass UV or IR rays that can damage the eye at high brightness levels. For example, any kind of photographic color film transmits IR light and must not be used.

Viewing the Sun through light-concentrating optics such as binoculars is very hazardous without an attenuating (ND) filter to dim the sunlight. These filters can be made of mirrored glass or metallised plastic film.

Partial solar eclipses are hazardous to view because the eye's pupil is not adapted to the unusually high visual contrast: the pupil dilates according to the total amount of light in the field of view, not by the brightest object in the field. This can damage or kill those cells, resulting in small permanent blind spots for the viewer. The hazard is insidious for inexperienced observers and for children, because there is no perception of pain: it is not immediately obvious that one's vision is being destroyed.

During sunrise and sunset, sunlight is attenuated through rayleigh and mie scattering of light by a particularly long passage through Earth's atmosphere, and the direct Sun is sometimes faint enough to be viewed directly without discomfort or safely with binoculars (provided there is no risk of bright sunlight suddenly appearing in a break between clouds).

See also

• List of Solar System bodies formerly considered planets
  
• Formation and evolution of the solar system
  

References