See Also

Sun

|+ The Sun   |+ |- | colspan="2" align="center" | |- ! bgcolor="#ffffc0" colspan="2" align="center" | Observation data |- ! align="left" | Mean distance fromEarth Earth

Earth is the third planet [i] in the solar system [i] in terms of distance from the Sun [i], and the fi ... 

| 149.6 km  |- ! align="left" | Visual brightness | −26.8m |- ! align="left" | Absolute magnitude | 4.8m |- ! align="left" | Spectral classification Stellar classification

In astronomy, stellar classification is a classification of star [i]s based initially on photospheric temperature [i] ... 

| G2V |- ! bgcolor="#ffffc0" colspan="2" align="center" | Orbital characteristics |- ! align="left" | Mean distance fromMilky Way Milky Way

The Milky Way , is a barred spiral galaxy [i] which forms part of the Local Group [i]. ... 

 core | ~2.5 km

Discussions

  Discussion Features

   Ask a question about 'Sun'

   Start a new discussion about 'Sun'

   Answer questions about 'Sun'

   'Sun' discussion forum

Timeline

1275   Chinese China

China is a cultural region [i] and ancient civilization [i] in East Asia [i]. ... 

 astronomers Astronomer

An astronomer or astrophysicist is a person whose area of interest is astronomy [i] or astrophysics [i] ... 

 observe a total eclipse of the Sun in China China

China is a cultural region [i] and ancient civilization [i] in East Asia [i]. ... 

 on March 4.

1735   Mathematical calculations suggest it was on this day that Pluto Pluto

Pluto is the second-largest known dwarf planet [i] in the solar system [i]. ... 

 moved closer to the Sun than Neptune Neptune

Neptune is the eighth and outermost planet [i] in our solar system [i]. ... 

 for the last time before 1979.

1769   James Cook James Cook

Captain [i] James Cook, FRS [i], RN [i] was an English [i] explorer, navigator [i] ... 

 arrives in Tahiti Tahiti

Tahiti is the largest island [i] of French Polynesia [i], located in the archipelago [i] of Society Islands [i] ... 

 on the ship HM Bark ''Endeavour'' HM Bark Endeavour

HMB Endeavour was a small 18th century British [i] sailing ship, famous for ... 

, preparing to observe the solar eclipse Solar eclipse

A solar eclipse occurs when the Moon [i] passes between Earth [i] and the Sun [i], thereby totally or pa ... 

 of the planet Venus Venus

Venus is the second-closest planet [i] to the Sun [i], orbiting it every 224.7 Earth days. ... 

, which took place on June 3rd. After the voyage, the data was found to be inaccurate in determining the distance between the Sun and Earth Earth

Earth is the third planet [i] in the solar system [i] in terms of distance from the Sun [i], and the fi ... 

.

1836   Francis Baily, during an eclipse Eclipse

An is an astronomical event that occurs when one celestial object [i] moves into the shadow of another. ... 

 of the sun, observes the phenomenon named after him as Baily's beads Baily's beads

As the moon "grazes" by the Sun during the eclipse [i], the rugged lunar limb topography allows beads of sunli ... 

.

1859   Solar flare Solar flare

A solar flare is a violent explosion in the Sun [i]'s atmosphere with an energy [i] equivalent to tens o ... 

s first observed on the Sun by English astronomer Richard Carrington.

1866   Calculations indicate Pluto Pluto

Pluto is the second-largest known dwarf planet [i] in the solar system [i]. ... 

 reached its most recent aphelion Apsis

In astronomy [i], an apsis is the point of greatest or least distance of the elliptical orbit [i] of a celestial body [i] ... 

 (furthest point from Sun) on this day. The next aphelion will occur in ''August 2113''.

1982   Syzygy: all nine planet Planet

The International Astronomical Union [i] , the official scientific [i] body for astronomical [i] nomenclature [i] ... 

s align on the same side of the Sun.

1994   An annular eclipse Solar eclipse

A solar eclipse occurs when the Moon [i] passes between Earth [i] and the Sun [i], thereby totally or pa ... 

 of the sun is visible across much of North America North America

North America is a continent [i] in the Earth [i]'s northern hemisphere [i] and almost fully in the western hemisphere [i] ... 

.

1999   Pluto Pluto

Pluto is the second-largest known dwarf planet [i] in the solar system [i]. ... 

, a dwarf planet Dwarf planet

A dwarf planet is a category of celestial bodies [i] in the solar system [i] as introduced in a resolution [i] ... 

 with an eccentric orbit, moves further from the Sun than Neptune Neptune

Neptune is the eighth and outermost planet [i] in our solar system [i]. ... 

. It had been nearer than Neptune since 1979, and will become again in ''2231''.

2000   A rare conjunction Conjunction (astronomy)

Conjunction is a term used in positional astronomy [i] and astrology [i]. ... 

 occurs on the New Moon New moon

The New Moon is the lunar phase [i] that occurs when the Moon, in its monthly orbital motion around Eart ... 

, including all seven of the traditional celestial bodies known from ancient Ancient history

Ancient history is the study of significant cultural and political events from the beginning of human hi... 

 times up until 1781 with the discovery of Uranus Uranus

Uranus is the seventh planet [i] from the Sun [i]. ... 

. The May 2000 conjunction consisted of: the Sun and Moon Moon

The Moon is Earth [i]'s only natural satellite [i]. ... 

, Mercury Mercury (planet)

Mercury is the nearest planet to the sun [i], orbiting at an average distance of about 58 million kilome ... 

, Venus Venus

Venus is the second-closest planet [i] to the Sun [i], orbiting it every 224.7 Earth days. ... 

, Mars Mars

Mars is the fourth planet [i] from the Sun [i] in our solar system [i] and is named after Mars [i] ... 

, Jupiter Jupiter

Jupiter is the fifth planet [i] from the Sun [i] and the largest [i] within the solar system [i] ... 

, and Saturn Saturn

Saturn is the sixth planet [i] from the Sun [i]. ... 

.

   More Events >>


Quotations

Keep your eyes on the sun and you will not see the shadows.

Like our shadows, Our wishes lengthen as our sun declines.

The sun is the king of torches.

       More Quotes >>


Encyclopedia

>|+
|+ The Sun  

|+
|-
| colspan="2" align="center" |

|-
! bgcolor="#ffffc0" colspan="2" align="center" | Observation data
|-
! align="left" | Mean distance from
Earth Earth

Earth is the third planet [i] in the solar system [i] in terms of distance from the Sun [i], and the fi ... 


| 149.6 km


|-
! align="left" | Visual brightness
| −26.8m
|-
! align="left" | Absolute magnitude
| 4.8m
|-
! align="left" | Spectral classification Stellar classification

In astronomy, stellar classification is a classification of star [i]s based initially on photospheric temperature [i] ... 


| G2V
|-
! bgcolor="#ffffc0" colspan="2" align="center" | Orbital characteristics
|-
! align="left" | Mean distance from
Milky Way Milky Way

The Milky Way , is a barred spiral galaxy [i] which forms part of the Local Group [i]. ... 

 core
| ~2.5 km

|-
! align="left" | Galactic Galaxy

A galaxy is a huge gravitationally bound [i] system of star [i]s, interstellar gas and dust [i] ... 

 period
| 2.25-2.50 a
|-
! align="left" | Velocity
| 217 km/s orbit around the center of the Galaxy, 20 km/s relative to average velocity of other stars in stellar neighborhood
|-
! bgcolor="#ffffc0" colspan="2" align="center" | Physical characteristics
|-
! align="left" | Mean diameter
| 1.392 km

|-
! align="left" | Circumference
| 4.373 km

|-
! align="left" | Oblateness Oblate

An oblate spheroid [i] is a rotationally symmetric [i] ellipsoid [i] having a polar ... 


| 9
|-
! align="left" | Surface area
| 6.09 km²

|-
! align="left" | Volume
| 1.41 km³ Cubic metre

The cubic metre is the SI derived unit [i] of volume [i]. ... 



|-
! align="left" | Mass
| 1.988 435 kg Kilogram

The kilogram or kilogramme, is the SI base unit [i] of mass [i]. ... 



|-
! align="left" | Density
| 1.408 g/cm³
|-
! align="left" | Surface gravity Gravitation

In physics [i], gravitation or gravity is the tendency of objects with mass [i] to accelerate [i] ... 


| 273.95 m s-2

|-
! align="left" | Escape velocity
from the surface
| 617.54 km/s
|-
! align="left" | Surface temperature
| 5785 K Kelvin

The Kelvin scale is a temperature [i] scale where absolute zero [i]—the coldest possible temperatu ... 


|-
! align="left" | Temperature of corona Corona

In astronomy, a corona is the luminous plasma [i] "atmosphere [i]" o ... 


| 5 MK
|-
! align="left" | Core temperature
| ~13.6 MK
|-
! align="left" | Luminosity
| 3.827 W
~9 cd
~3 cd

|-
! align="left" | Mean Intensity
| 2.009 W m-2 sr-1
|-
! bgcolor="#ffffc0" colspan="2" align="center" | Rotation Rotation

Rotation is the movement of an object in a circular motion.... 

 characteristics

|-
! align="left" | Obliquity Axial tilt

Axial tilt is an astronomical [i] term regarding the inclination [i] angle of a planet's [i] ... 


| 7.25° 

67.23°

|-
! align="left" | Right ascension Right ascension

Right ascension is the astronomical [i] term for one of the two coordinate [i]s of a point on the celestial sphere [i] ... 


of North pole
| 286.13°

|-
! align="left" | Declination Declination

In astronomy [i], declination is one of the two coordinates of the equatorial coordinate system [i], th ... 


of North pole
| +63.87°

|-
! align="left" | Rotation period
at equator
| 25.3800 days

The Sun orbits the center of the Milky Way Milky Way

The Milky Way , is a barred spiral galaxy [i] which forms part of the Local Group [i]. ... 

 galaxy Galaxy

A galaxy is a huge gravitationally bound [i] system of star [i]s, interstellar gas and dust [i] ... 

 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 Supernova

A supernova is a stellar [i] explosion [i] which produces an extremely bright [i] ... 

. This is suggested by a high abundance of heavy elements such as gold Gold

Gold is a highly sought-after precious metal [i] that for many centuries has been used as money [i], a store of value [i] ... 

 and uranium Uranium

Uranium is a chemical element [i] in the periodic table [i] that has the symbol U and atomic number [i] ... 

 in the solar system; these elements could most plausibly have been produced by endergonic nuclear reactions during a supernova, or by transmutation via neutron Neutron

In physics [i], the neutron is a subatomic particle [i] with no net electric charge [i] and a mass [i] o ... 

 absorption inside a massive second-generation star.

Sunlight is the main source of energy near the surface of Earth. The solar constant Solar constant

The solar constant is the amount of incoming solar radiation [i] per unit area, measured on the outer su ... 

 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 . 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 Photosynthesis

Photosynthesis , generally, is the synthesis of sugar from light [i], carbon dioxide [i] and water, with ... 

 by plants captures the energy of sunlight and converts it to chemical form , while direct heating or electrical conversion by solar cells Solar cell

A solar cell is a semiconductor device [i] that converts photon [i]s into electricity [i]. ... 

 are used by solar power Solar power

Solar power is the technology of obtaining usable energy [i] from the light [i] of the Sun [i]. ... 

 equipment to generate electricity Electricity

Electricity is a general term for the variety of phenomena resulting from the presence and flow of electric charge [i] ... 

 or to do other useful work. The energy stored in petroleum Petroleum

Petroleum or crude oil is a black, dark brown or greenish liquid [i] found in porous rock formati ... 

 and other fossil fuels Fossil fuel

Fossil fuels are hydrocarbon [i]s formed from the remains of dead plants and animals. ... 

 was originally converted from sunlight by photosynthesis in the distant past.

Sunlight has several interesting biological properties. Ultraviolet Ultraviolet

Ultraviolet light is electromagnetic radiation [i] with a wavelength [i] shorter than that of visible l ... 

 light from the Sun has antiseptic Antiseptic

An antiseptic is a substance that prevents the growth and reproduction of various microorganism [i]s o ... 

 properties and can be used to sterilize tools. It also causes sunburn Sunburn

A sunburn is a burn [i] to the skin [i] produced by overexposure to ultraviolet [i] radiation, comm ... 

, and has other medical effects such as the production of Vitamin D Vitamin D

[i] precursor that contributes to the maintenance of normal levels of [[calcium]... 

. Ultraviolet light is strongly attenuated by Earth's atmosphere, so that the amount of UV varies greatly with latitude Latitude

Latitude, usually denoted symbolically by the Greek letter f [i] , gives the location of a place on ... 

 because of 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 Human skin color

Human skin color can range from almost black to nearly colorless in different people [i]. ... 

 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 Analemma

Analemma is a term in astronomy [i] used to describe the plot of the positions of the Sun [i] on the celestial sphere [i] ... 

 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 , there is an East/West component as well. The North/South swing in apparent angle is the main source of seasons Season

A season is one of the major divisions of the year [i], generally based on yearly periodic changes in we ... 

 on Earth.

Life cycle

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 Stellar evolution

In astronomy [i], stellar evolution is the sequence of changes that a star [i] undergoes during its life ... 

 and nucleocosmochronology, is thought to be about 4.57 billion years.

The Sun does not have enough mass to explode as a supernova Supernova

A supernova is a stellar [i] explosion [i] which produces an extremely bright [i] ... 

. Instead, in 4–5 billion years, it will enter a red giant Red giant

According to the Hertzsprung-Russell diagram [i], a red giant is a large non-main sequence [i] star [i] ... 

 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 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.

Following the red giant phase, intense thermal pulsations will cause the Sun to throw off its outer layers, forming a planetary nebula Planetary nebula

A planetary nebula is an astronomical [i] object [i] consisting of a glowi ... 

. The Sun will then evolve into a white dwarf, slowly cooling over eons. This stellar evolution Stellar evolution

In astronomy [i], stellar evolution is the sequence of changes that a star [i] undergoes during its life ... 

 scenario is typical of low- to medium-mass stars.

Structure


While the Sun is an averaged-sized star, it contains approximately 99% of the total mass of the solar system. The Sun is a near-perfect sphere Sphere

A sphere is a perfectly symmetrical [i] geometrical [i] object. ... 

, with an oblateness Oblate

An oblate spheroid [i] is a rotationally symmetric [i] ellipsoid [i] having a polar ... 

 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 , it takes approximately 28 days to complete one full rotation; the centrifugal effect of this slow rotation is 18 million times weaker than the surface gravity at the Sun's equator. Tidal effects from the planets do not significantly affect the shape of the Sun, although the Sun itself orbits the center of mass Barycenter

Sorry, no overview for this topic 

 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 Jupiter

Jupiter is the fifth planet [i] from the Sun [i] and the largest [i] within the solar system [i] ... 

.

The Sun does not have a definite boundary as rocky planets do; the density of its gases drops approximately exponentially Exponential distribution

In probability theory [i] and statistics [i], the exponential distributions are a class of continuous probability distribution [i] ... 

 with increasing distance from the center of the Sun. Nevertheless, the Sun has a well-defined interior structure, described below. The Sun's radius is measured from its center to the edge of the photosphere Photosphere

The photosphere of an astronomical [i] object [i] is the region at which t ... 

. This is simply the layer below which the gases are thick enough to be opaque but above which they are transparent; the photosphere is the surface most readily visible to the naked eye. 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 Seismology

Seismology is the scientific study of earthquake [i]s and the movement of waves through the Earth [i]. ... 

 uses waves generated by earthquake Earthquake

An earthquake is a phenomenon [i] that results from and is powered by the sudden release of stored energ ... 

s to reveal the interior structure of the Earth, the discipline of helioseismology Helioseismology

Helioseismology is the study of the propagation of pressure wave [i]s in the Sun [i].Unlike seismic waves on earth [i] ... 

 makes use of pressure waves traversing the Sun's interior to measure and visualize the Sun's inner structure. Computer modeling of the Sun is also used as a theoretical tool to investigate its deeper layers.

Core

The core of the Sun is considered to extend from the center to about 0.2 solar radii. It has a density of up to 150,000 kg/m3 and a temperature of close to 15,000,000 Kelvins . Energy is produced by exothermic thermonuclear reaction Nuclear fusion

In physics [i], nuclear fusion is the process by which multiple nuclei [i] join together ... 

s that mainly convert hydrogen Hydrogen

|-
| Triple point [i] || 13.8033 K, 7.042 kPa
... 

 into helium Helium

|-
| 3He || 0.000137%* || colspan="4" | He is stable [i] with 1 neutron [i]
... 

, helium Helium

|-
| 3He || 0.000137%* || colspan="4" | He is stable [i] with 1 neutron [i]
... 

 into carbon Carbon

Carbon is a chemical element [i] in the periodic table [i] that has the symbol [i] C' ... 

, carbon Carbon

Carbon is a chemical element [i] in the periodic table [i] that has the symbol [i] C' ... 

 into iron Iron

Iron is a chemical element [i] with the symbol Fe and atomic number [i] 26. ... 

. The core is the only location in the Sun that produces an appreciable amount of heat Heat

In physics [i], heat, symbolized by Q, is defined as energy in transit. ... 

 via fusion: the rest of the star is heated by energy that is transferred outward from the core. 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 Sunlight

Sunlight in the broad sense is the total spectrum [i] of electromagnetic radiation [i] give ... 

 or kinetic energy of particles.

About 8.9 proton Proton

In physics [i], the proton is a subatomic particle [i] with an electric charge [i] of one positive fundamental unit [i] ... 

s are converted into helium nuclei every second, releasing energy at the matter-energy conversion rate of 4.26 million tonnes per second, 383 yottawatts  or 9.15 megatons of TNT Trinitrotoluene

Trinitrotoluene is an explosive [i].... 

 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 photon Photon

In modern physics [i], the photon is the elementary particle [i] responsible for electromagnetic phenomena [i] ... 

s 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. Estimates of the "photon travel time" range from as much as 50 million years to as little as 17,000 years. After a final trip through the convective outer layer to the transparent "surface" of the photosphere, the photons escape as visible light Visible spectrum

The visible spectrum is the portion of the electromagnetic spectrum [i] that is visible [i] ... 

. Each gamma ray in the Sun's core is converted into several million visible light photons before escaping into space. Neutrino Neutrino

The neutrino is an elementary particle [i]. ... 

s are also released by the fusion reactions in the core, but unlike photons they very rarely interact with matter, so almost all are able to escape the Sun immediately. For many years measurements of the number of neutrinos produced in the Sun were much lower than theories predicted Solar neutrino problem

The solar neutrino problem was a major discrepancy between measurements of the numbers of neutrino [i]s ... 

, a problem which was recently resolved through a better understanding of the effects of neutrino oscillation Neutrino oscillation

Neutrino oscillation is a quantum mechanical [i] phenomenon whereby a neutrino [i] cre ... 

.

Radiation zone

From about 0.2 to about 0.7 solar radii, solar material is hot and dense enough that thermal radiation is sufficient to transfer the intense heat of the core outward. In this zone there is no thermal convection Convection

Convection is the transfer of potential energy, for example heat [i], by currents within liquids and gas ... 

; while the material grows cooler as altitude increases, this temperature gradient Gradient

A generalization of these concepts is the gradient in vector calculus [i]; and this article is mostly ab ... 

 is slower than the adiabatic lapse rate Lapse rate

The lapse rate is defined as the rate of change in temperature [i] observed while moving upwards through ... 

 and hence cannot drive convection. Heat is transferred by radiation—ions of hydrogen and helium emit photons Photon

In modern physics [i], the photon is the elementary particle [i] responsible for electromagnetic phenomena [i] ... 

, which travel a brief distance before being reabsorbed by other ions.

Convection zone



From about 0.7 solar radii to the Sun's visible surface, the material in the Sun is not dense enough or hot enough to transfer the heat energy of the interior outward via radiation. As a result, thermal convection occurs as thermal columns Thermal

A thermal column is a column of rising air [i] in the lower altitudes of the Earth's atmosphere [i].... 

 carry hot material to the surface of the Sun. Once the material cools off at the surface, it plunges back downward to the base of the convection zone, to receive more heat from the top of the radiative zone. 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 Supergranulation

Supergranulation is one of the influences on the Sun [i]. ... 

. The turbulent convection of this outer part of the solar interior gives rise to a "small-scale" dynamo that produces magnetic north and south poles all over the surface of the Sun.

Photosphere

The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes opaque to visible light. Above the photosphere visible sunlight is free to propagate into space, and its energy escapes the Sun entirely. The change in opacity is because of the decreasing overall particle density: the photosphere is actually tens to hundreds of kilometers thick, being slightly less opaque than air on Earth. Sunlight has approximately a black-body Black body

In physics [i], a black body is an object that absorbs all electromagnetic radiation [i] that falls onto ... 

 spectrum that indicates its temperature is about 6,000 K Kelvin

The Kelvin scale is a temperature [i] scale where absolute zero [i]—the coldest possible temperatu ... 

 , interspersed with atomic absorption line Spectral line

A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum [i] ... 

s from the tenuous layers above the photosphere. The photosphere has a particle density of about 1023 m−3 .

During early studies of the optical spectrum Visible spectrum

The visible spectrum is the portion of the electromagnetic spectrum [i] that is visible [i] ... 

 of the photosphere, some absorption lines were found that did not correspond to any chemical element Chemical element

A chemical element, often called simply an element, is a substance [i] that can... 

s then known on Earth. In 1868, Norman Lockyer hypothesized that these absorption lines were because of a new element which he dubbed "helium Helium

|-
| 3He || 0.000137%* || colspan="4" | He is stable [i] with 1 neutron [i]
... 

", after the Greek Sun god Helios Helios

In Greek mythology [i] the sun [i] was personified as Hlios or Helius, as it is commonly spelt in Engli ... 

. It was not until 25 years later that helium was isolated on Earth.

Atmosphere



The parts of the Sun above the photosphere are referred to collectively as the solar atmosphere. They can be viewed with telescopes operating across the electromagnetic spectrum Electromagnetic spectrum

The electromagnetic spectrum is the range of all possible electromagnetic radiation [i]. ... 

, from radio through visible light Visible spectrum

The visible spectrum is the portion of the electromagnetic spectrum [i] that is visible [i] ... 

 to gamma rays Gamma ray

Gamma rays are an energetic form of electromagnetic radiation [i] produced by radioactive decay [i] or ... 

, and comprise five principal zones: the temperature minimum, the chromosphere Chromosphere

The chromosphere is a thin layer of the Sun [i]'s atmosphere [i] just above ... 

, the transition region Solar transition region

The solar transition region is a region of the Sun [i]'s atmosphere, between the chromosphere [i] and corona [i] ... 

, the corona Corona

In astronomy, a corona is the luminous plasma [i] "atmosphere [i]" o ... 

, and the heliosphere Heliosphere

The heliosphere is a bubble in space produced by the solar wind [i]. ... 

. The heliosphere, which may be considered the tenuous outer atmosphere of the Sun, extends outward past the orbit of Pluto Pluto

Pluto is the second-largest known dwarf planet [i] in the solar system [i]. ... 

 to the heliopause Heliopause

In astronomy [i], the heliopause is the boundary where the Sun [i]'s solar wind [i] is stopped by the interstellar medium [i] ... 

, where it forms a sharp shock front Shock wave

A shock wave is a type of propagating disturbance.... 

 boundary with the interstellar medium Interstellar medium

The interstellar medium is the name astronomers [i] give to the tenous gas and dust that perva ... 

. The chromosphere, transition region, and corona are much hotter than the surface of the Sun; the reason why is not yet known.

The coolest layer of the Sun is a temperature minimum region about 500 km above the photosphere, with a temperature of about 4,000 K Kelvin

The Kelvin scale is a temperature [i] scale where absolute zero [i]—the coldest possible temperatu ... 

. This part of the Sun is cool enough to support simple molecules such as carbon monoxide Carbon monoxide

Carbon monoxide, with the chemical formula CO, is a colourless, odourless, and tasteless gas [i]. ... 

 and water, which can be detected by their absorption spectra.

Above the temperature minimum layer is a thin layer about 2,000 km thick, dominated by a spectrum of emission and absorption lines. It is called the chromosphere from the Greek root chroma, meaning color, because the chromosphere is visible as a colored flash at the beginning and end of total eclipses of the Sun Solar eclipse

A solar eclipse occurs when the Moon [i] passes between Earth [i] and the Sun [i], thereby totally or pa ... 

. The temperature in the chromosphere increases gradually with altitude, ranging up to around 100,000 K near the top.

Above the chromosphere is a transition region Solar transition region

The solar transition region is a region of the Sun [i]'s atmosphere, between the chromosphere [i] and corona [i] ... 

 in which the temperature rises rapidly from around 100,000 K Kelvin

The Kelvin scale is a temperature [i] scale where absolute zero [i]—the coldest possible temperatu ... 

 to coronal temperatures closer to one million K. The increase is because of a phase transition as helium Helium

|-
| 3He || 0.000137%* || colspan="4" | He is stable [i] with 1 neutron [i]
... 

 within the region becomes fully ionized by the high temperatures. The transition region does not occur at a well-defined altitude. Rather, it forms a kind of nimbus around chromospheric features such as spicule Spicule

Spicules are skeletal [i] structures that occur in most sponge [i]s. ... 

s 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 Outer space

Outer space, also simply called space, refers to the relatively empty regions of the universe [i] ou... 

 by instruments sensitive to the far ultraviolet Ultraviolet

Ultraviolet light is electromagnetic radiation [i] with a wavelength [i] shorter than that of visible l ... 

 portion of the spectrum Electromagnetic spectrum

The electromagnetic spectrum is the range of all possible electromagnetic radiation [i]. ... 

.

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 Solar wind

[i]s, [[carrot]... 

 that fills the solar system Solar System

The Solar System or solar system is the stellar system [i] comprising the Sun [i] and ... 

 and heliosphere Heliosphere

The heliosphere is a bubble in space produced by the solar wind [i]. ... 

. The low corona, which is very near the surface of the Sun, has a particle density of 1014 m−3–1016 m−3. The temperature of the corona is several million kelvin. 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 Magnetic reconnection

Magnetic reconnection is the process whereby magnetic field lines from different magnetic domains are sp... 

.

The heliosphere Heliosphere

The heliosphere is a bubble in space produced by the solar wind [i]. ... 

 extends from approximately 20 solar radii to the outer fringes of the solar system. Its inner boundary is defined as the layer in which the flow of the solar wind Solar wind

[i]s, [[carrot]... 

 becomes superalfvénic—that is, where the flow becomes faster than the speed of Alfvén waves Alfvén wave

An Alfvn wave, named after Hannes Alfvn [i], is a type of magnetohydrodynamic [i] wave [i] ... 

. Turbulence and dynamic forces outside this boundary cannot affect the shape of the solar corona within, because the information can only travel at the speed of Alfvén waves. The solar wind travels outward continuously through the heliosphere, forming the solar magnetic field into a spiral Parker spiral

The influence of the Sun's spiral-shaped magnetic field on the interplanetary medium [i] creates the largest s ... 

 shape, until it impacts the heliopause Heliopause

In astronomy [i], the heliopause is the boundary where the Sun [i]'s solar wind [i] is stopped by the interstellar medium [i] ... 

 more than 50 AU from the Sun. In December 2004, the Voyager 1 probe Voyager program

The Voyager program consists of a pair of unmanned scientific probe [i]s, Voyager 1 [i] and ... 

 passed through a shock front that is thought to be part of the heliopause. Both of the Voyager probes have recorded higher levels of energetic particles as they approach the boundary.

Solar activity


Sunspots and the solar cycle



When observing the Sun with appropriate filtration, the most immediately visible features are usually its sunspot Sunspot

A sunspot is a region on the Sun [i]'s surface that is marked by a lower temperature than its surroundi ... 

s, which are well-defined surface areas that appear darker than their surroundings because of lower temperatures. Sunspots are regions of intense magnetic activity where convection Convection

Convection is the transfer of potential energy, for example heat [i], by currents within liquids and gas ... 

 is inhibited by strong magnetic fields, reducing energy transport from the hot interior to the surface. The magnetic field gives rise to strong heating in the corona, forming active regions that are the source of intense solar flare Solar flare

A solar flare is a violent explosion in the Sun [i]'s atmosphere with an energy [i] equivalent to tens o ... 

s and coronal mass ejection Coronal mass ejection

A coronal mass ejection is an ejection of material from the solar [i] corona [i], observed with a wh ... 

s. The largest sunspots can be tens of thousands of kilometers 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 Solar cycle

The Schwabe [i] solar cycle or Schwabe-Wolf cycle is the eleven-year cycle of sol ... 

. 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 Spörer's law

Sprer's law predicts the variation of sunspot [i] latitudes during a solar cycle [i]. ... 

. 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 Space weather

Space weather is the concept of changing environmental conditions in outer space [i]. ... 

, 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 this period. During this era, which is known as the Maunder minimum Maunder Minimum

The Maunder Minimum is the name given to the period roughly from 1645 [i] to 1715 [i] A.D., when sunspot [i]... 

 or Little Ice Age Little Ice Age

The Little Ice Age was a period of cooling occuring after a warmer era known as the Medieval climate optimum [i] ... 

, Europe experienced very cold temperatures. Earlier extended minima have been discovered through analysis of tree ring Growth ring

Growth rings can be seen in a horizontal cross section [i] cut through the trunk [i] of a tree [i] ... 

s 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 Van Allen radiation belt

The Van Allen Radiation Belt is a torus [i] of energetic [i] charged particle [i]s around Earth [i] ... 

. The Van Allen belts consist of an inner belt composed primarily of proton Proton

In physics [i], the proton is a subatomic particle [i] with an electric charge [i] of one positive fundamental unit [i] ... 

s and an outer belt composed mostly of electron Electron

The electron is a fundamental [i] subatomic particle [i] that carries an electric charge [i]... 

s. Radiation within the Van Allen belts can occasionally damage satellite Satellite

A satellite is any object that orbit [i]s another object . ... 

s 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 Southern Hemisphere

The Southern Hemisphere is the half of a planet's surface that is south [i] of the equator [i].
... 

. In periods of normal solar activity, aurorae can be seen in oval-shaped regions centered on the magnetic pole Magnet

A magnet is an object that has a magnetic field [i]. ... 

s and lying roughly at a geomagnetic latitude Latitude

Latitude, usually denoted symbolically by the Greek letter f [i] , gives the location of a place on ... 

 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 Mexico

The United Mexican States, generally known as Mexico is a country [i] located in North America [i] ... 

.

Theoretical problems


Solar neutrino problem



For many years the number of solar electron neutrino Neutrino

The neutrino is an elementary particle [i]. ... 

s 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 Solar neutrino problem

The solar neutrino problem was a major discrepancy between measurements of the numbers of neutrino [i]s ... 

. 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 Neutrino oscillation

Neutrino oscillation is a quantum mechanical [i] phenomenon whereby a neutrino [i] cre ... 

, that is, change into undetectable tau Neutrino

The neutrino is an elementary particle [i]. ... 

 and muon neutrino Neutrino

The neutrino is an elementary particle [i]. ... 

s 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 Sudbury Neutrino Observatory

The Sudbury Neutrino Observatory is located 6800 feet underground in Inco Limited [i]'s Creighton Mine [i] ... 

 and Kamiokande Super-Kamiokande

Super-Kamiokande, or Super-K for short, is a neutrino observatory [i] in Japan [i]. ... 

. Results from these observatories eventually led to the discovery that neutrinos have a very small rest mass and can indeed oscillate.. Moreover, the Sudbury Neutrino Observatory was able to detect all three types of neutrinos directly, and found that the Sun's total neutrino emission rate agreed with the Standard Solar Model, although only one-third of the neutrinos seen at Earth were of the electron type.

Coronal heating problem

The optical surface of the Sun is known to have a temperature of approximately 6,000 K Kelvin

The Kelvin scale is a temperature [i] scale where absolute zero [i]—the coldest possible temperatu ... 

. 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 direct heat conduction from 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 Wave

[i], often transferring [[energy]... 

 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 Magnetic field

In physics [i], a magnetic field is that part of the electromagnetic field [i] that exists when there is ... 

 heating, in which magnetic energy is continuously built up by photospheric motion and released through magnetic reconnection Magnetic reconnection

Magnetic reconnection is the process whereby magnetic field lines from different magnetic domains are sp... 

 in the form of large solar flare Solar flare

A solar flare is a violent explosion in the Sun [i]'s atmosphere with an energy [i] equivalent to tens o ... 

s 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, Alfvén 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


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 gas Greenhouse gas

Greenhouse gases are gas [i]eous components of the atmosphere [i] that contribute to ... 

es 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 Plasma

Plasma may refer to:
  • Plasma [i], an ionized gas

... 

 because of its high temperatures. This makes it possible for the Sun to rotate faster at its equator than it does at higher latitudes . The differential rotation of the Sun's latitudes causes its magnetic field Magnetic field

In physics [i], a magnetic field is that part of the electromagnetic field [i] that exists when there is ... 

 lines to become twisted together over time, causing magnetic field lo