Sun

For other uses, see Sun (disambiguation).

The sun   $\bigodot$

Observation data
Mean distance from Earth	149.6×106 km (92.95×106 mi) (8.31 minutes at the speed of light)
Visual brightness (V)	−26.8m
Absolute magnitude	4.8m
Spectral classification	G2V
Orbital characteristics
Mean distance from Milky Way core	~2.5×1017 km (26,000-28,000 light-years)
Galactic period	2.25-2.50×108 a
Velocity	217 km/s orbit around the center of the Galaxy, 20 km/s relative to average velocity of other stars in stellar neighborhood
Physical characteristics
Mean diameter	1.392×106 km (109 Earth diameters)
Circumference	4.373×106 km (342 Earth diameters)
Oblateness	9×10−6
Surface area	6.09×1012 km² (11,900 Earths)
Volume	1.41×1018 km³ (1,300,000 Earths)
Mass	1.9891×1030 kg (332,950 Earths)
Density	1.408 g/cm³
Surface gravity	273.95 m s-2 (27.9 g)
Escape velocity from the surface	617.54 km/s
Surface temperature	5780 K
Temperature of corona	5 MK
Core temperature	~13.6 MK
Luminosity (Lsol)	3.827×1026 W 3.9×1028 lm or 100 lm/W efficacy
Mean Intensity (Isol)	2.009×107 W m-2 sr-1
Rotation characteristics
Obliquity	7.25° (to the ecliptic) 67.23° (to the galactic plane)
Right ascension of North pole	286.13° (19 h 4 min 30 s)
Declination of North pole	+63.87° (63°52' North)
Rotation period at equator	25.3800 days (25 d 9 h 7 min 13 s)
Rotation velocity at equator	7174 km/h
Photospheric composition (by mass)
Hydrogen	73.46 %
Helium	24.85 %
Oxygen	0.77 %
Carbon	0.29 %
Iron	0.16 %
Neon	0.12 %
Nitrogen	0.09 %
Silicon	0.07 %
Magnesium	0.05 %
Sulphur	0.04 %

The Sun is the star at the centre 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.

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 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 is a magnetically active star; it supports a strong, changing magnetic field that varies year-to-year and reverses direction about 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 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 just 6,000 K. Current topics of scientific enquiry 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.

General information

The sun as it appears through a camera lens from the surface of Earth.

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. There are more than 100 million G2 class stars in our galaxy. Due to logarithmic size distribution, the Sun is actually brighter than 85% of the stars in the Galaxy, most of which are red dwarfs. 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. 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. 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. However, Earth's water and most of the atmosphere will be boiled away.

Artist's conception of the remains of artificial structures on the Earth after the Sun enters its red giant phase and grows to roughly 100 times its current size.

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 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. Ultraviolet light is strongly attenuated by Earth's atmosphere, so that the amount of UV varies greatly with latitude due the longer passage of sunlight through the atmosphere at high latitudes. This variation is responsible for many biological adaptations, including variations in human skin color in different regions of the globe.

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.

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: $\bigodot$. Some ancient peoples of the world considered it a planet.

Origin

See Solar System: Origin and Evolution

Structure

Main article: structure of the Sun

Solar activity

Sunspots and the solar cycle

File:Sunspotcloseinset.png

Sunspot group 9393, one of the largest recorded in recent years

History of the number of observed sunspots during the last 250 years, which shows the ~11 year solar cycle.

When observing the Sun with appropriate filtration, the most immediately visible features are usually its sunspots, which are well-defined surface areas that appear darker than their surroundings due to lower temperatures. Sunspots are regions of intense magnetic activity where energy transport is inhibited by strong magnetic fields. They are often the source of intense flares and coronal mass ejections. The largest sunspots can be tens of thousands of kilometres across.

The number of sunspots visible on the Sun is not constant, but varies over a 10-12 year cycle known as the Solar cycle. At a typical solar minimum, few sunspots are visible, and occasionally none at all can be seen. Those that do appear are at high solar latitudes. As the sunspot cycle progresses, the number of sunspots increases and they move closer to the equator of the Sun, a phenomenon described by Spörer's law. Sunspots usually exist as pairs with opposite magnetic polarity. 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. Solar minima tend to be correlated with colder temperatures, and longer than average solar cycles tend to be correlated with hotter temperatures. In the 17th century, the solar cycle appears to have stopped entirely for several decades; very few sunspots were observed during the period. 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.

Effects on Earth

Solar activity has several effects on the Earth and its surroundings. Because the Earth has a magnetic field, charged particles from the solar wind cannot impact the atmosphere directly, but are instead deflected by the magnetic field and aggregate to form the Van Allen belts. The Van Allen belts consist of an inner belt composed primarily of protons and an outer belt composed mostly of electrons. Radiation within the Van Allen belts can occasionally damage satellites passing through them.

The Van Allen belts form arcs around the Earth with their tips near the north and south poles. 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 centred 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. Aurorae borealis have been observed from locales as far south as Mexico.

Theoretical problems

Solar neutrino problem

File:High Resolution Solar Spectrum.jpg

Extremely high resolution spectrum of the Sun showing thousands of elemental absorption lines (Fraunhofer lines).

For many years the number of solar electron neutrinos detected on Earth was only a third of the number expected, according to theories describing the nuclear reactions in the Sun. This anomalous result was termed the solar neutrino problem. 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. Results from these observatories eventually led to the discovery that neutrinos have a very small rest mass and can indeed oscillate.

Coronal heating problem

The optical surface of the Sun (the photosphere) is known to have a temperature of approximately 6,000 K. Above it lies the solar corona at a temperature of 1,000,000 K. The high temperature of the corona shows that it is heated by something other than the photosphere.

It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere, and two main mechanisms have been proposed to explain coronal heating. The first is wave heating, in which sound, gravitational and magnetohydrodynamic waves are produced by turbulence in the convection zone. These waves travel upward and dissipate in the corona, depositing their energy in the ambient gas in the form of heat. 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. All waves except Alfven waves have been found to dissipate or refract before reaching the corona. In addition, Alfven waves do not easily dissipate in the corona. Current research focus has therefore shifted towards flare heating mechanisms. One possible candidate to explain coronal heating is continuous flaring at small scales, but this remains an open topic of investigation.

Faint young sun problem

Main article: Faint young sun paradox

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. Such a weak star would not have been able to sustain liquid water on the Earth's surface, and thus life should not have been able to develop. However, the geological record demonstrates that the Earth has remained at a fairly constant temperature throughout its history, and in fact that the young Earth was somewhat warmer than 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

File:Heliospheric-current-sheet.jpg

The heliospheric current sheet extends to the outer reaches of the Solar System, and results from the influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium [1]

All matter in the Sun is in the form of gas and plasma due to 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 (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. 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 (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

See history of solar observation.

Sun observation and eye damage

File:SOHO solar flare sun large 20031026 0119 eit 304.png

Large solar flare recorded by the SOHO/EIT telescope using UV light from the He⁺ emission line at 30.4 nm.

Sunlight is very bright, and looking directly at the Sun with the naked eye for brief periods can be painful, but is generally not hazardous. Looking directly at the Sun causes phosphene visual artifacts and temporary partial blindness. It also delivers about 4 milliwatts of sunlight to the retina, slightly heating it and potentially (though not normally) damaging it. 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.

Viewing the Sun through light-concentrating optics such as binoculars is very hazardous without an attenuating (ND) filter to dim the sunlight. Using a proper filter is important as some improvised filters pass UV rays that can damage the eye at high brightness levels. Unfiltered binoculars can deliver over 500 times more sunlight to the retina than does the naked eye, killing retinal cells almost instantly. Even brief glances at the midday Sun through unfiltered binoculars can cause permanent blindness. One way to view the Sun safely is by projecting an image onto a screen using binoculars or a small telescope.

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. During partial eclipses most sunlight is blocked by the Moon passing in front of the Sun, but the uncovered parts of the photosphere have the same surface brightness as during a normal day. In the overall gloom, the pupil expands from ~2 mm to ~6 mm, and each retinal cell exposed to the solar image receives about ten times more light than it would looking at the non-eclipsed sun. 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. Hazy conditions, atmospheric dust, and high humidity contribute to this atmospheric attenuation.

External links

• Current SOHO snapshots
• Far-Side Helioseismic Holography from Stanford
• NASA Eclipse homepage
• Nasa SOHO (Solar & Heliospheric Observatory) satellite FAQ
• Solar Sounds from Stanford
• Spaceweather.com
• Eric Weisstein's World of Astronomy - Sun
• The Position of the Sun
• A collection of solar movies
• The Institute for Solar Physics- Movies of Sunspots and spicules
• NASA/Marshall Solar Physics website
• Solar Position Algorithm and documentation from the National Renewable Energy Laboratory
• libnova - a celestial mechanics and astronomical calculation library
• NASA Podcast

References

• Thompson, M.J. (2004), Solar interior: Helioseismology and the Sun's interior, Astronomy & Geophysics, v. 45, p. 4.21-4.25
• White, T. J., M. A. Mainster, P. W. Wilson, and J. H. Tips, Chorioretinal temperature increases from solar observation, Bulletin of Mathematical Biophysics 33, 1-17 (1971)

See also

• List of Solar System bodies formerly considered planets