Sun is the star at the center of the Solar System. It is a nearly
perfect sphere of hot plasma, with internal convective motion
that generates a magnetic field via a dynamo process. It is by far
the most important source of energy for life on Earth. Its diameter is
about 1.39 million kilometers, i.e. 109 times that of Earth, and its
mass is about 330,000 times that of Earth, accounting for about 99.86%
of the total mass of the Solar System. About three quarters of the
Sun's mass consists of hydrogen (~73%); the rest is mostly helium
(~25%), with much smaller quantities of heavier elements, including
oxygen, carbon, neon, and iron.
Sun is a
G-type main-sequence star
G-type main-sequence star (G2V) based on its spectral
class. As such, it is informally referred to as a yellow dwarf. It
formed approximately 4.6 billion[a] years ago from the
gravitational collapse of matter within a region of a large molecular
cloud. Most of this matter gathered in the center, whereas the rest
flattened into an orbiting disk that became the Solar System. The
central mass became so hot and dense that it eventually initiated
nuclear fusion in its core. It is thought that almost all stars form
by this process.
Sun is roughly middle-aged; it has not changed dramatically for
more than four billion[a] years, and will remain fairly stable for
more than another five billion years. After hydrogen fusion in its
core has diminished to the point at which it is no longer in
hydrostatic equilibrium, the core of the
Sun will experience a marked
increase in density and temperature while its outer layers expand to
eventually become a red giant. It is calculated that the
become sufficiently large to engulf the current orbits of Mercury and
Venus, and render
The enormous effect of the
Earth has been recognized since
prehistoric times, and the
Sun has been regarded by some cultures as a
deity. The synodic rotation of
Earth and its orbit around the
the basis of solar calendars, one of which is the predominant calendar
in use today.
1 Name and etymology
1.1 Religious aspects
4.1 Singly ionized iron-group elements
4.2 Isotopic composition
5 Structure and energy production
5.2 Radiative zone
5.4 Convective zone
Photons and neutrinos
6 Magnetism and activity
6.1 Magnetic field
6.2 Variation in activity
6.3 Long-term change
7.2 Main sequence
7.3 After core hydrogen exhaustion
8 Motion and location
Orbit in Milky Way
9 Theoretical problems
9.1 Coronal heating problem
9.2 Faint young
10 History of observation
10.1 Early understanding
10.2 Development of scientific understanding
10.3 Solar space missions
11 Observation and effects
12 Planetary system
13 See also
16 Further reading
17 External links
Name and etymology
The English proper name
Sun developed from
Old English sunne and may
be related to south. Cognates to English sun appear in other Germanic
Old Frisian sunne, sonne,
Old Saxon sunna, Middle
Dutch sonne, modern Dutch zon,
Old High German
Old High German sunna, modern German
Old Norse sunna, and Gothic sunnō. All Germanic terms for the
Sun stem from
The English weekday name
Sunday stems from
Old English (Sunnandæg;
"Sun's day", from before 700) and is ultimately a result of a Germanic
interpretation of Latin dies solis, itself a translation of the Greek
ἡμέρα ἡλίου (hēméra hēlíou). The Latin name for
the Sun, Sol, is not common in general English language use; the
adjectival form is the related word solar. The term sol is
also used by planetary astronomers to refer to the duration of a solar
day on another planet, such as Mars. A mean
Earth solar day is
approximately 24 hours, whereas a mean Martian 'sol' is 24 hours, 39
minutes, and 35.244 seconds.
Main article: Solar deity
Solar deities play a major role in many world religions and
mythologies. The ancient Sumerians believed that the sun was
Utu, the god of justice and twin brother of Inanna, the Queen
of Heaven, who was identified as the planet Venus. Later, Utu
was identified with the
East Semitic god Shamash.
regarded as a helper-deity, who aided those in distress, and, in
iconography, he is usually portrayed with a long beard and clutching a
saw, which represented his role as the dispenser of justice.
From at least the
4th Dynasty of Ancient Egypt, the
Sun was worshipped
as the god Ra, portrayed as a falcon-headed divinity surmounted by the
solar disk, and surrounded by a serpent. In the New Empire period, the
Sun became identified with the dung beetle, whose spherical ball of
dung was identified with the Sun. In the form of the
Sun disc Aten,
Sun had a brief resurgence during the
Amarna Period when it again
became the preeminent, if not only, divinity for the Pharaoh
In Proto-Indo-European religion, the sun was personified as the
goddess *Seh2ul. Derivatives of this goddess in
Indo-European languages include the
Old Norse Sól,
Gaulish Sulis, Lithuanian Saulė, and Slavic Solntse. In ancient
Greek religion, the sun deity was the male god Helios, but traces
of an earlier female solar deity are preserved in Helen of Troy.
In later times,
Helios was syncretized with Apollo.
In the Bible, Malachi 4:2 mentions the "
Sun of Righteousness"
(sometimes translated as the "
Sun of Justice"), which some
Christians have interpreted as a reference to the Messiah
(Christ). In ancient Roman culture,
Sunday was the day of the Sun
god. It was adopted as the
Sabbath day by Christians who did not have
a Jewish background. The symbol of light was a pagan device adopted by
Christians, and perhaps the most important one that did not come from
Jewish traditions. In paganism, the
Sun was a source of life, giving
warmth and illumination to mankind. It was the center of a popular
cult among Romans, who would stand at dawn to catch the first rays of
sunshine as they prayed. The celebration of the winter solstice (which
influenced Christmas) was part of the Roman cult of the unconquered
Sun (Sol Invictus). Christian churches were built with an orientation
so that the congregation faced toward the sunrise in the East.
Aztec god of the sun, was usually depicted holding
arrows and a shield and was closely associated with the practice
of human sacrifice. The sun goddess
Amaterasu is the most
important deity in the
Shinto religion, and she is believed to
be the direct ancestor of all Japanese emperors.
Sun is a
G-type main-sequence star
G-type main-sequence star that comprises about 99.86% of
the mass of the Solar System. The
Sun has an absolute magnitude of
+4.83, estimated to be brighter than about 85% of the stars in the
Milky Way, most of which are red dwarfs. The
Sun is a
Population I, or heavy-element-rich,[b] star. The formation of the
Sun may have been triggered by shockwaves from one or more nearby
supernovae. This is suggested by a high abundance of heavy
elements in the Solar System, such as gold and uranium, relative to
the abundances of these elements in so-called Population II,
heavy-element-poor, stars. The heavy elements could most plausibly
have been produced by endothermic nuclear reactions during a
supernova, or by transmutation through neutron absorption within a
massive second-generation star.
Sun is by far the brightest object in the Earth's sky, with an
apparent magnitude of −26.74. This is about 13 billion times
brighter than the next brightest star, Sirius, which has an apparent
magnitude of −1.46. The mean distance of the Sun's center to Earth's
center is approximately 1 astronomical unit (about
150,000,000 km; 93,000,000 mi), though the distance varies
Earth moves from perihelion in January to aphelion in July. At
this average distance, light travels from the Sun's horizon to Earth's
horizon in about 8 minutes and 19 seconds, while light from the
closest points of the
Earth takes about two seconds less. The
energy of this sunlight supports almost all life[c] on
photosynthesis, and drives
Earth's climate and weather.
Sun does not have a definite boundary, but its density decreases
exponentially with increasing height above the photosphere. For
the purpose of measurement, however, the Sun's radius is considered to
be the distance from its center to the edge of the photosphere, the
apparent visible surface of the Sun. By this measure, 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 kilometres (6.2 mi). The tidal
effect of the planets is weak and does not significantly affect the
shape of the Sun. The
Sun rotates faster at its equator than at
its poles. This differential rotation is caused by convective motion
due to heat transport and the Coriolis force due to the Sun's
rotation. In a frame of reference defined by the stars, the rotational
period is approximately 25.6 days at the equator and 33.5 days at the
poles. Viewed from
Earth as it orbits the Sun, the apparent rotational
period of the
Sun at its equator is about 28 days.
Main article: Sunlight
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 7003136800000000000♠1,368 W/m2 (watts
per square meter) at a distance of one astronomical unit (AU) from the
Sun (that is, on or near Earth).
Sunlight on the surface of Earth
is attenuated by Earth's atmosphere, so that less power arrives at the
surface (closer to 7003100000000000000♠1,000 W/m2) in clear
conditions when the
Sun is near the zenith.
Sunlight at the top of
Earth's atmosphere is composed (by total energy) of about 50% infrared
light, 40% visible light, and 10% ultraviolet light. The
atmosphere in particular filters out over 70% of solar ultraviolet,
especially at the shorter wavelengths. Solar ultraviolet radiation
ionizes Earth's dayside upper atmosphere, creating the electrically
The Sun's color is white, with a CIE color-space index near (0.3,
0.3), when viewed from space or when the
Sun is high in the sky. When
measuring all the photons emitted, the
Sun is actually emitting more
photons in the green portion of the spectrum than any other.
Sun is low in the sky, atmospheric scattering renders the Sun
yellow, red, orange, or magenta. Despite its typical whiteness, most
people mentally picture the
Sun as yellow; the reasons for this are
the subject of debate. The
Sun is a G2V star, with G2 indicating
its surface temperature of approximately 5,778 K (5,505 °C,
9,941 °F), and V that it, like most stars, is a main-sequence
star. The average luminance of the
Sun is about
1.88 giga candela per square metre, but as viewed through
Earth's atmosphere, this is lowered to about 1.44 Gcd/m2.[d]
However, the luminance is not constant across the disk of the Sun
See also: Molecules in stars
Sun is composed primarily of the chemical elements hydrogen and
helium; they account for 74.9% and 23.8% of the mass of the
Sun in the
photosphere, respectively. All heavier elements, called metals in
astronomy, account for less than 2% of the mass, with oxygen (roughly
1% of the Sun's mass), carbon (0.3%), neon (0.2%), and iron (0.2%)
being the most abundant.
Sun inherited its chemical composition from the interstellar
medium out of which it formed. The hydrogen and helium in the
produced by Big Bang nucleosynthesis, and the heavier elements were
produced by stellar nucleosynthesis in generations of stars that
completed their stellar evolution and returned their material to the
interstellar medium before the formation of the Sun. The chemical
composition of the photosphere is normally considered representative
of the composition of the primordial Solar System. However, since
Sun formed, some of the helium and heavy elements have
gravitationally settled from the photosphere. Therefore, in today's
photosphere the helium fraction is reduced, and the metallicity is
only 84% of what it was in the protostellar phase (before nuclear
fusion in the core started). The protostellar Sun's composition is
believed to have been 71.1% hydrogen, 27.4% helium, and 1.5% heavier
Today, nuclear fusion in the Sun's core has modified the composition
by converting hydrogen into helium, so the innermost portion of the
Sun is now roughly 60% helium, with the abundance of heavier elements
unchanged. Because heat is transferred from the Sun's core by
radiation rather than by convection (see
Radiative zone below), none
of the fusion products from the core have risen to the
The reactive core zone of "hydrogen burning", where hydrogen is
converted into helium, is starting to surround an inner core of
"helium ash". This development will continue and will eventually cause
Sun to leave the main sequence, to become a red giant.
The solar heavy-element abundances described above are typically
measured both using spectroscopy of the Sun's photosphere and by
measuring abundances in meteorites that have never been heated to
melting temperatures. These meteorites are thought to retain the
composition of the protostellar
Sun and are thus not affected by
settling of heavy elements. The two methods generally agree well.
Singly ionized iron-group elements
In the 1970s, much research focused on the abundances of iron-group
elements in the Sun. Although significant research was done,
until 1978 it was difficult to determine the abundances of some
iron-group elements (e.g. cobalt and manganese) via spectrography
because of their hyperfine structures.
The first largely complete set of oscillator strengths of singly
ionized iron-group elements were made available in the 1960s, and
these were subsequently improved. In 1978, the abundances of
singly ionized elements of the iron group were derived.
Various authors have considered the existence of a gradient in the
isotopic compositions of solar and planetary noble gases, e.g.
correlations between isotopic compositions of neon and xenon in the
Sun and on the planets.
Prior to 1983, it was thought that the whole
Sun has the same
composition as the solar atmosphere. In 1983, it was claimed that
it was fractionation in the
Sun itself that caused the
isotopic-composition relationship between the planetary and
solar-wind-implanted noble gases.
Structure and energy production
Main article: Solar core
The structure of the Sun
The core of the
Sun extends from the center to about 20–25% of the
solar radius. It has a density of up to
7005150000000000000♠150 g/cm3 (about 150 times the
density of water) and a temperature of close to 15.7 million kelvins
(K). By contrast, the Sun's surface temperature is approximately
5,800 K. Recent analysis of SOHO mission data favors a faster
rotation rate in the core than in the radiative zone above.
Through most of the Sun's life, energy has been produced by nuclear
fusion in the core region through a series of steps called the p–p
(proton–proton) chain; this process converts hydrogen into
helium. Only 0.8% of the energy generated in the
Sun comes from
the CNO cycle, though this proportion is expected to increase as the
Sun becomes older.
The core is the only region in the
Sun that produces an appreciable
amount of thermal energy through fusion; 99% of the power is generated
within 24% of the Sun's radius, and by 30% of the radius, fusion has
stopped nearly entirely. The remainder of the
Sun is heated by this
energy as it is transferred outwards through many successive layers,
finally to the solar photosphere where it escapes into space as
sunlight or the kinetic energy of particles.
The proton–proton chain occurs around
7037919999999999999♠9.2×1037 times each second in the core,
converting about 3.7×1038 protons into alpha particles (helium
nuclei) every second (out of a total of ~8.9×1056 free protons in the
Sun), or about 6.2×1011 kg/s. Fusing four free protons (hydrogen
nuclei) into a single alpha particle (helium nucleus) releases around
0.7% of the fused mass as energy, so the
Sun releases energy at
the mass–energy conversion rate of 4.26 million metric tons per
second (which requires 600 metric megatons of hydrogen ), for
384.6 yottawatts (7026384600000000000♠3.846×1026 W),
or 9.192×1010 megatons of TNT per second. Theoretical models of
the Sun's interior indicate a power density of approximately 276.5
W/m3, a value that more nearly approximates that of reptile
metabolism or a compost pile than of a thermonuclear bomb.[e]
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 density and hence the fusion rate and correcting the perturbation;
and a slightly lower rate would cause the core to cool and shrink
slightly, increasing the density and increasing the fusion rate and
again reverting it to its present rate.
Main article: Radiative zone
From the core out to about 0.7 solar radii, thermal radiation is the
primary means of energy transfer. The temperature drops from
approximately 7 million to 2 million kelvins with increasing distance
from the core. This temperature gradient is less than the value of
the adiabatic lapse rate and hence cannot drive convection, which
explains why the transfer of energy through this zone is by radiation
instead of thermal convection.
Ions of hydrogen and helium emit
photons, which travel only a brief distance before being reabsorbed by
other ions. The density drops a hundredfold (from 20 g/cm3 to 0.2
g/cm3) from 0.25 solar radii to the 0.7 radii, the top of the
Main article: Tachocline
The radiative zone and the convective zone are separated by a
transition layer, the tachocline. This is a region where the sharp
regime change between the uniform rotation of the radiative zone and
the differential rotation of the convection zone results in a large
shear between the two—a condition where successive horizontal layers
slide past one another. Presently, it is hypothesized (see Solar
dynamo) that a magnetic dynamo within this layer generates the Sun's
The Sun's convection zone extends from 0.7 solar radii
(500,000 km) to near the surface. In this layer, the solar plasma
is not dense enough or hot enough to transfer the heat energy of the
interior outward via radiation. Instead, the density of the plasma is
low enough to allow convective currents to develop and move the Sun's
energy outward towards its surface. Material heated at the tachocline
picks up heat and expands, thereby reducing its density and allowing
it to rise. As a result, an orderly motion of the mass develops into
thermal cells that carry the majority of the heat outward to the Sun's
photosphere above. Once the material diffusively and radiatively cools
just beneath the photospheric surface, its density increases, and it
sinks to the base of the convection zone, where it again picks up heat
from the top of the radiative zone and the convective cycle continues.
At the photosphere, the temperature has dropped to 5,700 K and the
density to only 0.2 g/m3 (about 1/6,000 the density of air at sea
The thermal columns of the convection zone form an imprint on the
surface of the
Sun giving it a granular appearance called the solar
granulation at the smallest scale and supergranulation at larger
scales. Turbulent convection in this outer part of the solar interior
sustains "small-scale" dynamo action over the near-surface volume of
the Sun. The Sun's thermal columns are
Bénard cells and take the
shape of hexagonal prisms.
The effective temperature, or black body temperature, of the Sun
(5,777 K) is the temperature a black body of the same size must have
to yield the same total emissive power.
Main article: Photosphere
The visible surface of the Sun, the photosphere, is the layer below
Sun becomes opaque to visible light. Above the
photosphere visible sunlight is free to propagate into space, and
almost all of its energy escapes the
Sun entirely. The change in
opacity is due to the decreasing amount of H− ions, which absorb
visible light easily. Conversely, the visible light we see is
produced as electrons react with hydrogen atoms to produce H−
ions. The photosphere is tens to hundreds of kilometers thick,
and is slightly less opaque than air on Earth. Because the upper part
of the photosphere is cooler than the lower part, an image of the Sun
appears brighter in the center than on the edge or limb of the solar
disk, in a phenomenon known as limb darkening. The spectrum of
sunlight has approximately the spectrum of a black-body radiating at
about 6,000 K, interspersed with atomic absorption lines from the
tenuous layers above the photosphere. The photosphere has a particle
density of ~1023 m−3 (about 0.37% of the particle number per
Earth's atmosphere at sea level). The photosphere is not
fully ionized—the extent of ionization is about 3%, leaving almost
all of the hydrogen in atomic form.
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 caused by a new element that he
dubbed helium, after the Greek
Sun god Helios. Twenty-five years
later, helium was isolated on Earth.
Corona and Coronal loop
During a total solar eclipse, the solar corona can be seen with the
naked eye, during the brief period of totality.
During a total solar eclipse, when the disk of the
Sun is covered by
that of the Moon, parts of the Sun's surrounding atmosphere can be
seen. It is composed of four distinct parts: the chromosphere, the
transition region, the corona and the heliosphere.
The coolest layer of the
Sun is a temperature minimum region extending
to about 7005500000000000000♠500 km above the photosphere, and
has a temperature of about 7003410000000000000♠4,100 K.
This part of the
Sun is cool enough to allow the existence of simple
molecules such as carbon monoxide and water, which can be detected via
their absorption spectra.
The chromosphere, transition region, and corona are much hotter than
the surface of the Sun. The reason is not well understood, but
evidence suggests that Alfvén waves may have enough energy to heat
Above the temperature minimum layer is a layer about
7006200000000000000♠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 solar
eclipses. The temperature of the chromosphere increases gradually
with altitude, ranging up to around
7004200000000000000♠20,000 K near the top. In the upper
part of the chromosphere helium becomes partially ionized.
Taken by Hinode's Solar Optical
Telescope on 12 January 2007, this
image of the
Sun reveals the filamentary nature of the plasma
connecting regions of different magnetic polarity.
Above the chromosphere, in a thin (about 200 km) transition
region, the temperature rises rapidly from around 20,000 K in the
upper chromosphere to coronal temperatures closer to 1,000,000 K.
The temperature increase is facilitated by the full ionization of
helium in the transition region, which significantly reduces radiative
cooling of the plasma. The transition region does not occur at a
well-defined altitude. 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 extreme ultraviolet portion of the
The corona is the next layer of the Sun. The low corona, near the
surface of the Sun, has a particle density around 1015 m−3 to
1016 m−3.[f] The average temperature of the corona and
solar wind is about 1,000,000–2,000,000 K; however, in the hottest
regions it is 8,000,000–20,000,000 K. Although 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 corona is the extended atmosphere of the
Sun, which has a volume much larger than the volume enclosed by the
Sun's photosphere. A flow of plasma outward from the
interplanetary space is the solar wind.
The heliosphere, the tenuous outermost atmosphere of the Sun, is
filled with the solar wind plasma. This outermost layer of the
defined to begin at the distance where the flow of the solar wind
becomes superalfvénic—that is, where the flow becomes faster than
the speed of Alfvén waves, at approximately 20 solar radii (0.1
AU). Turbulence and dynamic forces in the heliosphere 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 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. In late 2012
Voyager 1 recorded a marked increase in
cosmic ray collisions and a sharp drop in lower energy particles from
the solar wind, which suggested that the probe had passed through the
heliopause and entered the interstellar medium.
Photons and neutrinos
See also: solar irradiance
High-energy gamma-ray photons initially released with fusion reactions
in the core are almost immediately absorbed by the solar plasma of the
radiative zone, usually after traveling only a few millimeters.
Re-emission happens in a random direction and usually at a slightly
lower energy. With this sequence of emissions and absorptions, it
takes a long time for radiation to reach the Sun's surface. Estimates
of the photon travel time range between 10,000 and
170,000 years. In contrast, it takes only 2.3 seconds for
the neutrinos, which account for about 2% of the total energy
production of the Sun, to reach the surface. Because energy transport
Sun is a process that involves photons in thermodynamic
equilibrium with matter, the time scale of energy transport in the Sun
is longer, on the order of 30,000,000 years. This is the time it would
Sun to return to a stable state, if the rate of energy
generation in its core were suddenly changed.
Neutrinos are also released by the fusion reactions in the core, but,
unlike photons, they 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 lower than theories
predicted by a factor of 3. This discrepancy was resolved in 2001
through the discovery of the effects of neutrino oscillation: the Sun
emits the number of neutrinos predicted by the theory, but neutrino
detectors were missing 2⁄3 of them because the neutrinos had
changed flavor by the time they were detected.
Magnetism and activity
See also: Stellar magnetic field, Sunspots, List of solar cycles, and
Visible light photograph of sunspot, 13 December 2006
Butterfly diagram showing paired sunspot pattern. Graph is of sunspot
In this false-color ultraviolet image, the
Sun shows a C3-class solar
flare (white area on upper left), a solar tsunami (wave-like
structure, upper right) and multiple filaments of plasma following a
magnetic field, rising from the stellar surface.
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.
Sun has a magnetic field that varies across the surface of the
Sun. Its polar field is 1–2 gauss (0.0001–0.0002 T), whereas
the field is typically 3,000 gauss (0.3 T) in features on the Sun
called sunspots and 10–100 gauss (0.001–0.01 T) in solar
The magnetic field also varies in time and location. The
quasi-periodic 11-year solar cycle is the most prominent variation in
which the number and size of sunspots waxes and wanes.
Sunspots are visible as dark patches on the Sun's photosphere, and
correspond to concentrations of magnetic field where the convective
transport of heat is inhibited from the solar interior to the surface.
As a result, sunspots are slightly cooler than the surrounding
photosphere, and, so, they appear dark. At a typical solar minimum,
few sunspots are visible, and occasionally none can be seen at all.
Those that do appear are at high solar latitudes. As the solar cycle
progresses towards its maximum, sunspots tend form closer to the solar
equator, a phenomenon known as Spörer's law. The largest sunspots can
be tens of thousands of kilometers across.
An 11-year sunspot cycle is half of a 22-year Babcock–Leighton
dynamo cycle, which corresponds to an oscillatory exchange of energy
between toroidal and poloidal solar magnetic fields. At solar-cycle
maximum, the external poloidal dipolar magnetic field is near its
dynamo-cycle minimum strength, but an internal toroidal quadrupolar
field, generated through differential rotation within the tachocline,
is near its maximum strength. At this point in the dynamo cycle,
buoyant upwelling within the convective zone forces emergence of
toroidal magnetic field through the photosphere, giving rise to pairs
of sunspots, roughly aligned east–west and having footprints with
opposite magnetic polarities. The magnetic polarity of sunspot pairs
alternates every solar cycle, a phenomenon known as the Hale
During the solar cycle's declining phase, energy shifts from the
internal toroidal magnetic field to the external poloidal field, and
sunspots diminish in number and size. At solar-cycle minimum, the
toroidal field is, correspondingly, at minimum strength, sunspots are
relatively rare, and the poloidal field is at its maximum strength.
With the rise of the next 11-year sunspot cycle, differential rotation
shifts magnetic energy back from the poloidal to the toroidal field,
but with a polarity that is opposite to the previous cycle. The
process carries on continuously, and in an idealized, simplified
scenario, each 11-year sunspot cycle corresponds to a change, then, in
the overall polarity of the Sun's large-scale magnetic
The solar magnetic field extends well beyond the
Sun itself. The
electrically conducting solar wind plasma carries the Sun's magnetic
field into space, forming what is called the interplanetary magnetic
field. In an approximation known as ideal magnetohydrodynamics,
plasma particles only move along the magnetic field lines. As a
result, the outward-flowing solar wind stretches the interplanetary
magnetic field outward, forcing it into a roughly radial structure.
For a simple dipolar solar magnetic field, with opposite hemispherical
polarities on either side of the solar magnetic equator, a thin
current sheet is formed in the solar wind. At great distances,
the rotation of the
Sun twists the dipolar magnetic field and
corresponding current sheet into an
Archimedean spiral structure
called the Parker spiral. The interplanetary magnetic field is
much stronger than the dipole component of the solar magnetic field.
The Sun's dipole magnetic field of 50–400 μT (at the
photosphere) reduces with the inverse-cube of the distance to about
0.1 nT at the distance of Earth. However, according to spacecraft
observations the interplanetary field at Earth's location is around
5 nT, about a hundred times greater. The difference is
due to magnetic fields generated by electrical currents in the plasma
surrounding the Sun.
Variation in activity
Measurements from 2005 of solar cycle variation during the last 30
The Sun's magnetic field leads to many effects that are collectively
called solar activity.
Solar flares and coronal-mass ejections tend to
occur at sunspot groups. Slowly changing high-speed streams of solar
wind are emitted from coronal holes at the photospheric surface. Both
coronal-mass ejections and high-speed streams of solar wind carry
plasma and interplanetary magnetic field outward into 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.
With solar-cycle modulation of sunspot number comes a corresponding
modulation of space weather conditions, including those surrounding
Earth where technological systems can be affected.
Long-term secular change in sunspot number is thought, by some
scientists, to be correlated with long-term change in solar
irradiance, which, in turn, might influence Earth's long-term
climate. For example, in the 17th century, the solar cycle
appeared to have stopped entirely for several decades; few sunspots
were observed during a period known as the Maunder minimum. This
coincided in time with the era of the Little Ice Age, when Europe
experienced unusually cold temperatures. Earlier extended minima
have been discovered through analysis of tree rings and appear to have
coincided with lower-than-average global temperatures.
A recent theory claims that there are magnetic instabilities in the
core of the
Sun that cause fluctuations with periods of either 41,000
or 100,000 years. These could provide a better explanation of the ice
ages than the Milankovitch cycles.
Main articles: Formation and evolution of the
Solar System and Stellar
Sun today is roughly halfway through the most stable part of its
life. It has not changed dramatically for over four billion[a] years,
and will remain fairly stable for more than five billion more.
However, after hydrogen fusion in its core has stopped, the
undergo severe changes, both internally and externally.
Sun formed about 4.6 billion years ago from the collapse of part
of a giant molecular cloud that consisted mostly of hydrogen and
helium and that probably gave birth to many other stars. This age
is estimated using computer models of stellar evolution and through
nucleocosmochronology. The result is consistent with the
radiometric date of the oldest
Solar System material, at 4.567 billion
years ago. Studies of ancient meteorites reveal traces of
stable daughter nuclei of short-lived isotopes, such as iron-60, that
form only in exploding, short-lived stars. This indicates that one or
more supernovae must have occurred near the location where the Sun
formed. A shock wave from a nearby supernova would have triggered the
formation of the
Sun by compressing the matter within the molecular
cloud and causing certain regions to collapse under their own
gravity. As one fragment of the cloud collapsed it also began to
rotate because of conservation of angular momentum and heat up with
the increasing pressure. Much of the mass became concentrated in the
center, whereas the rest flattened out into a disk that would become
the planets and other
Solar System bodies. Gravity and pressure within
the core of the cloud generated a lot of heat as it accreted more
matter from the surrounding disk, eventually triggering nuclear
fusion. Thus, the
Sun was born.
Evolution of the Sun's luminosity, radius and effective temperature
compared to the present Sun. After Ribas (2010)
Sun is about halfway through its main-sequence stage, during which
nuclear fusion reactions in its core fuse hydrogen into helium. Each
second, more than four million tonnes of matter are converted into
energy within the Sun's core, producing neutrinos and solar radiation.
At this rate, the
Sun has so far converted around 100 times the mass
Earth into energy, about 0.03% of the total mass of the Sun. The
Sun will spend a total of approximately 10 billion years as a
main-sequence star. The
Sun is gradually becoming hotter during
its time on the main sequence, because the helium atoms in the core
occupy less volume than the hydrogen atoms that were fused. The core
is therefore shrinking, allowing the outer layers of the
Sun to move
closer to the centre and experience a stronger gravitational force,
according to the inverse-square law. This stronger force increases the
pressure on the core, which is resisted by a gradual increase in the
rate at which fusion occurs. This process speeds up as the core
gradually becomes denser. It is estimated that the
Sun has become 30%
brighter in the last 4.5 billion years. At present, it is
increasing in brightness by about 1% every 100 million years.
After core hydrogen exhaustion
The size of the current
Sun (now in the main sequence) compared to its
estimated size during its red-giant phase in the future
Sun does not have enough mass to explode as a supernova. Instead
it will exit the main sequence in approximately 5 billion years and
start to turn into a red giant. As a red giant, the
grow so large that it will engulf Mercury, Venus, and probably
Even before it becomes a red giant, the luminosity of the
have nearly doubled, and
Earth will receive as much sunlight as Venus
receives today. Once the core hydrogen is exhausted in 5.4 billion
Sun will expand into a subgiant phase and slowly double in
size over about half a billion years. It will then expand more rapidly
over about half a billion years until it is over two hundred times
larger than today and a couple of thousand times more luminous. This
then starts the red-giant-branch phase where the
Sun will spend around
a billion years and lose around a third of its mass.
Evolution of a Sun-like star. The track of a one solar mass star on
Hertzsprung–Russell diagram is shown from the main sequence to
the post-asymptotic-giant-branch stage.
After the red-giant branch the
Sun has approximately 120 million years
of active life left, but much happens. First, the core, full of
degenerate helium ignites violently in the helium flash, where it is
estimated that 6% of the core, itself 40% of the Sun's mass, will be
converted into carbon within a matter of minutes through the
triple-alpha process. The
Sun then shrinks to around 10 times its
current size and 50 times the luminosity, with a temperature a little
lower than today. It will then have reached the red clump or
horizontal branch, but a star of the Sun's mass does not evolve
blueward along the horizontal branch. Instead, it just becomes
moderately larger and more luminous over about 100 million years as it
continues to burn helium in the core.
When the helium is exhausted, the
Sun will repeat the expansion it
followed when the hydrogen in the core was exhausted, except that this
time it all happens faster, and the
Sun becomes larger and more
luminous. This is the asymptotic-giant-branch phase, and the
alternately burning hydrogen in a shell or helium in a deeper shell.
After about 20 million years on the early asymptotic giant branch, the
Sun becomes increasingly unstable, with rapid mass loss and thermal
pulses that increase the size and luminosity for a few hundred years
every 100,000 years or so. The thermal pulses become larger each time,
with the later pulses pushing the luminosity to as much as 5,000 times
the current level and the radius to over 1 AU. According to a
2008 model, Earth's orbit is shrinking due to tidal forces (and,
eventually, drag from the lower chromosphere), so that it will be
engulfed by the
Sun near the tip of the red giant branch phase, 1 and
3.8 million years after Mercury and
Venus have respectively suffered
the same fate. Models vary depending on the rate and timing of mass
loss. Models that have higher mass loss on the red-giant branch
produce smaller, less luminous stars at the tip of the asymptotic
giant branch, perhaps only 2,000 times the luminosity and less than
200 times the radius. For the Sun, four thermal pulses are
predicted before it completely loses its outer envelope and starts to
make a planetary nebula. By the end of that phase – lasting
approximately 500,000 years – the
Sun will only have about half of
its current mass.
The post-asymptotic-giant-branch evolution is even faster. The
luminosity stays approximately constant as the temperature increases,
with the ejected half of the Sun's mass becoming ionised into a
planetary nebula as the exposed core reaches 30,000 K. The final naked
core, a white dwarf, will have a temperature of over 100,000 K, and
contain an estimated 54.05% of the Sun's present day mass. The
planetary nebula will disperse in about 10,000 years, but the white
dwarf will survive for trillions of years before fading to a
hypothetical black dwarf.
Motion and location
Illustration of the Milky Way, showing the location of the Sun
Sun lies close to the inner rim of the Milky Way's Orion Arm, in
Local Interstellar Cloud
Local Interstellar Cloud or the Gould Belt, at a distance of
7.5–8.5 kpc (25,000–28,000 light-years) from the Galactic
Center.  The
Sun is contained within the
Local Bubble, a space of rarefied hot gas, possibly produced by the
supernova remnant Geminga, or multiple supernovae in subgroup B1
of the Pleiades moving group. The distance between the local arm
and the next arm out, the Perseus Arm, is about 6,500
light-years. The Sun, and thus the Solar System, is found in what
scientists call the galactic habitable zone. The Apex of the Sun's
Way, or the solar apex, is the direction that the
Sun travels relative
to other nearby stars. This motion is towards a point in the
constellation Hercules, near the star Vega. Of the 50 nearest stellar
systems within 17 light-years from
Earth (the closest being the red
Proxima Centauri at approximately 4.2 light-years), the Sun
ranks fourth in mass.
Orbit in Milky Way
Sun orbits the center of the Milky Way, and it is presently moving
in the direction of the constellation of Cygnus. A simple model of the
motion of a star in the galaxy gives the galactic coordinates X, Y,
and Z as:
displaystyle X(t)=X(0)+ frac U(0) kappa sin(kappa t)+ frac
V(0) 2B (1-cos(kappa t))
displaystyle Y(t)=Y(0)+2Aleft(X(0)+ frac V(0) 2B right)t- frac
Omega _ 0 Bkappa V(0)sin(kappa t)+ frac 2Omega _ 0 kappa ^ 2
displaystyle Z(t)= frac W(0) nu sin(nu t)+Z(0)cos(nu t)
where U, V, and W are the respective velocities with respect to the
local standard of rest, A and B are the Oort constants,
displaystyle Omega _ 0 =A-B
is the angular velocity of galactic rotation for the local standard
displaystyle kappa = sqrt -4Omega _ 0 B
is the "epicyclic frequency", and ν is the vertical oscillation
frequency. For the sun, the present values of U, V, and W are
km/s, and estimates for the other constants are
A = 15.5 km/s/kpc, B = −12.2 km/s/kpc,
κ = 37 km/s/kpc, and ν=74 km/s/kpc. We take X(0)
and Y(0) to be zero and Z(0) is estimated to be 17 parsecs. This
model implies that the sun circulates around a point that is itself
going around the galaxy. The period of the sun's circulation around
the point is
displaystyle 2pi /kappa
. which, using the equivalence that a parsec equals 1 km/s times
0.978 million years, comes to 166 million years, shorter than the time
it takes for the point to go around the galaxy. In the (X, Y)
coordinates, the sun describes an ellipse around the point, whose
length in the Y direction is
displaystyle 2times sqrt left( frac 2Omega _ 0 kappa ^ 2
U(0)right)^ 2 +left( frac Omega _ 0 Bkappa V(0)right)^ 2 =1035
text parsec .
and whose width in the X direction is
displaystyle 2times sqrt left( frac U(0) kappa right)^ 2
+left( frac V(0) 2B right)^ 2 =691 text parsec
The ratio of length to width of this ellipse, the same for all stars
in our neighborhood, is
displaystyle 2Omega /kappa approx 1.50.
The moving point is presently at
displaystyle X= frac V(0) 2B =-215 text parsec
displaystyle Y= frac 2Omega _ 0 kappa ^ 2 U(0)=405 text
The oscillation in the Z direction takes the sun
displaystyle sqrt left( frac W(0) nu right)^ 2 +Z(0)^ 2
=98 text parsec
above the galactic plane and the same distance below it, with a period
displaystyle 2pi /nu
or 83 million years, approximately 2.7 times per orbit. Although
displaystyle 2pi /Omega _ 0
is 222 million years, the value of
at the point around which the sun circulates is
displaystyle Omega approx Omega _ 0 - frac 2A R_ 0 Delta
Xapprox 26.1 text km/s/kpc
(see Oort constants), corresponding to 235 million years, and this is
the time that the point takes to go once around the galaxy. Other
stars with the same value of
have take the same amount of time to go around the galaxy as the sun
and thus remain in the same general vicinity as the sun.
The Sun's orbit around the
Milky Way is perturbed due to the
non-uniform mass distribution in Milky Way, such as that in and
between the galactic spiral arms. It has been argued that the Sun's
passage through the higher density spiral arms often coincides with
mass extinctions on Earth, perhaps due to increased impact
events. It takes the
Solar System about 225–250 million years
to complete one orbit through the
Milky Way (a galactic year), so
it is thought to have completed 20–25 orbits during the lifetime of
the Sun. The orbital speed of the
Solar System about the center of the
Milky Way is approximately 251 km/s (156 mi/s). At this
speed, it takes around 1,190 years for the
Solar System to travel a
distance of 1 light-year, or 7 days to travel 1 AU.
Milky Way is moving with respect to the cosmic microwave
background radiation (CMB) in the direction of the constellation Hydra
with a speed of 550 km/s, and the Sun's resultant velocity with
respect to the CMB is about 370 km/s in the direction of Crater
Map of the full
STEREO and SDO spacecraft
Coronal heating problem
Main article: Corona
The temperature of the photosphere is approximately 6,000 K,
whereas the temperature of the corona reaches
1,000,000–2,000,000 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 heating, in which sound, gravitational
or 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 matter 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—nanoflares.
Currently, it is unclear whether waves are an efficient heating
mechanism. All waves except Alfvén 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.
Main article: Faint young
Theoretical models of the Sun's development suggest that 3.8 to 2.5
billion years ago, during the
Archean eon, 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 Earth's surface, and thus life should not have
been able to develop. However, the geological record demonstrates that
Earth has remained at a fairly constant temperature throughout its
history, and that the young
Earth was somewhat warmer than it is
today. One theory among scientists is that the atmosphere of the young
Earth contained much larger quantities of greenhouse gases (such as
carbon dioxide, methane) than are present today, which trapped enough
heat to compensate for the smaller amount of solar energy reaching
However, examination of Archaean sediments appears inconsistent with
the hypothesis of high greenhouse concentrations. Instead, the
moderate temperature range may be explained by a lower surface albedo
brought about by less continental area and the "lack of biologically
induced cloud condensation nuclei". This would have led to increased
absorption of solar energy, thereby compensating for the lower solar
History of observation
The enormous effect of the
Earth has been recognized since
prehistoric times, and the
Sun has been regarded by some cultures as a
Trundholm sun chariot
Trundholm sun chariot pulled by a horse is a sculpture believed to
be illustrating an important part of
Nordic Bronze Age
Nordic Bronze Age mythology. The
sculpture is probably from around 1350 BC. It is displayed at the
National Museum of Denmark.
See also: The
Sun in culture
Sun has been an object of veneration in many cultures throughout
human history. Humanity's most fundamental understanding of the
as the luminous disk in the sky, 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 entity. Worship of the
Sun was central to civilizations
such as the ancient Egyptians, the
Inca of South America and the
Aztecs of what is now Mexico. In religions such as Hinduism, the Sun
is still considered a god. Many ancient monuments were constructed
with solar phenomena in mind; for example, stone megaliths accurately
mark the summer or winter solstice (some of the most prominent
megaliths are located in Nabta Playa, Egypt; Mnajdra, Malta and at
Stonehenge, England); Newgrange, a prehistoric human-built mount in
Ireland, was designed to detect the winter solstice; the pyramid of El
Chichén Itzá in
Mexico is designed to cast shadows in
the shape of serpents climbing the pyramid at the vernal and autumnal
The Egyptians portrayed the god Ra as being carried across the sky in
a solar barque, accompanied by lesser gods, and to the Greeks, he was
Helios, carried by a chariot drawn by fiery horses. From the reign of
Elagabalus in the late
Roman Empire the Sun's birthday was a holiday
Sol Invictus (literally "Unconquered Sun") soon after
the winter solstice, which may have been an antecedent to Christmas.
Regarding the fixed stars, the
Sun appears from
Earth to revolve once
a year along the ecliptic through the zodiac, and so Greek astronomers
categorized it as one of the seven planets (Greek planetes,
"wanderer"); the naming of the days of the weeks after the seven
planets dates to the Roman era.
Development of scientific understanding
In the early first millennium BC, Babylonian astronomers observed that
the Sun's motion along the ecliptic is not uniform, though they did
not know why; it is today known that this is due to the movement of
Earth in an elliptic orbit around the Sun, with
Earth moving faster
when it is nearer to the
Sun at perihelion and moving slower when it
is farther away at aphelion.
One of the first people to offer a scientific or philosophical
explanation for the
Sun was the Greek philosopher Anaxagoras. He
reasoned that it was not the chariot of Helios, but instead a giant
flaming ball of metal even larger than the land of the Peloponnesus
and that the
Moon reflected the light of the Sun. For teaching
this heresy, he was imprisoned by the authorities and sentenced to
death, though he was later released through the intervention of
Eratosthenes estimated the distance between
Earth and the
Sun in the 3rd century BC as "of stadia myriads 400 and 80000", the
translation of which is ambiguous, implying either 4,080,000 stadia
(755,000 km) or 804,000,000 stadia (148 to 153 million kilometers
or 0.99 to 1.02 AU); the latter value is correct to within a few
percent. In the 1st century AD,
Ptolemy estimated the distance as
1,210 times the radius of Earth, approximately 7.71 million
kilometers (0.0515 AU).
The theory that the
Sun is the center around which the planets orbit
was first proposed by the ancient Greek
Aristarchus of Samos
Aristarchus of Samos in the
3rd century BC, and later adopted by
Seleucus of Seleucia (see
Heliocentrism). This view was developed in a more detailed
mathematical model of a heliocentric system in the 16th century by
Observations of sunspots were recorded during the
Han Dynasty (206
BC–AD 220) by Chinese astronomers, who maintained records of these
observations for centuries.
Averroes also provided a description of
sunspots in the 12th century. The invention of the telescope in
the early 17th century permitted detailed observations of sunspots by
Galileo Galilei and other astronomers. Galileo posited
that sunspots were on the surface of the
Sun rather than small objects
Earth and the Sun.
Arabic astronomical contributions include Albatenius' discovery that
the direction of the Sun's apogee (the place in the Sun's orbit
against the fixed stars where it seems to be moving slowest) is
changing. (In modern heliocentric terms, this is caused by a
gradual motion of the aphelion of the Earth's orbit). Ibn Yunus
observed more than 10,000 entries for the Sun's position for many
years using a large astrolabe.
Sol, the Sun, from a 1550 edition of Guido Bonatti's Liber
From an observation of a transit of
Venus in 1032, the Persian
astronomer and polymath
Avicenna concluded that
Venus is closer to
Earth than the Sun. In 1672
Giovanni Cassini and Jean Richer
determined the distance to
Mars and were thereby able to calculate the
distance to the Sun.
Isaac Newton observed the Sun's light using a prism, and
showed that it is made up of light of many colors. In 1800,
William Herschel discovered infrared radiation beyond the red part of
the solar spectrum. The 19th century saw advancement in
spectroscopic studies of the Sun;
Joseph von Fraunhofer
Joseph von Fraunhofer recorded more
than 600 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 is a gradually
cooling liquid body that is radiating an internal store of heat.
Hermann von Helmholtz
Hermann von Helmholtz then proposed a gravitational
contraction mechanism to explain the energy output, but the resulting
age estimate was only 20 million years, well short of the time span of
at least 300 million years suggested by some geological discoveries of
that time. In 1890 Joseph Lockyer, who discovered helium in
the solar spectrum, proposed a meteoritic hypothesis for the formation
and evolution of the Sun.
Not until 1904 was a documented solution offered. Ernest Rutherford
suggested that the Sun's output 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 the Sun's energy output with his mass-energy
equivalence 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 (protons)
into helium nuclei, resulting in a production of energy from the net
change in mass. The preponderance of hydrogen in the
confirmed in 1925 by Cecilia Payne using the ionization theory
developed by Meghnad Saha, an Indian physicist. The theoretical
concept of fusion was developed in the 1930s by the astrophysicists
Subrahmanyan Chandrasekhar and Hans Bethe.
Hans Bethe calculated the
details of the two main energy-producing nuclear reactions that power
the Sun. In 1957, Margaret Burbidge, Geoffrey Burbidge,
William Fowler and
Fred Hoyle showed that most of the elements in the
universe have been synthesized by nuclear reactions inside stars, some
like the Sun.
Solar space missions
See also: Solar observatory
Sun giving out a large geomagnetic storm on 1:29 pm, EST, 13 March
A lunar transit of the
Sun captured during calibration of
ultraviolet imaging cameras
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. These
probes orbited the
Sun at a distance similar to that of Earth, and
made the first detailed measurements of the solar wind and the solar
Pioneer 9 operated for a particularly long time,
transmitting data until May 1983.
In the 1970s, two
Helios spacecraft and the
Mount provided scientists with significant new data on solar wind and
the solar corona. The
Helios 1 and 2 probes were U.S.–German
collaborations that studied the solar wind from an orbit carrying the
spacecraft inside Mercury's orbit at perihelion. The
station, launched by
NASA in 1973, included a solar observatory module
Telescope Mount that was operated by astronauts
resident on the station.
Skylab made the first time-resolved
observations of the solar transition region and of ultraviolet
emissions from the solar corona. 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.
Coronal hole on the
Sun forms a question mark (22 December 2017)
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 and solar
luminosity. Just a few months after launch, however, an electronics
failure caused the probe to go into standby mode, and it spent the
next three years in this inactive state. In 1984 Space Shuttle
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
Earth's atmosphere in June 1989.
Launched in 1991, Japan's
Yohkoh (Sunbeam) satellite observed solar
X-ray wavelengths. Mission data allowed scientists to
identify several different types of flares, and demonstrated that the
corona away from regions of peak activity was much more dynamic and
active than had previously been supposed.
Yohkoh observed an entire
solar cycle but went into standby mode when an annular eclipse in 2001
caused it to lose its lock on the Sun. It was destroyed by atmospheric
re-entry in 2005.
One of the most important solar missions to date has been the Solar
and Heliospheric Observatory, jointly built by the European Space
NASA and launched on 2 December 1995. Originally
intended to serve a two-year mission, a mission extension through 2012
was approved in October 2009. It has proven so useful that a
follow-on mission, the
Solar Dynamics Observatory
Solar Dynamics Observatory (SDO), was launched
in February 2010. Situated at the
Lagrangian point between Earth
Sun (at which the gravitational pull from both is equal), SOHO
has provided a constant view of the
Sun at many wavelengths since its
launch. Besides its direct solar observation, SOHO has enabled
the discovery of a large number of comets, mostly tiny sungrazing
comets that incinerate as they pass the Sun.
A solar prominence erupts in August 2012, as captured by SDO
All these satellites have observed the
Sun from the plane of the
ecliptic, and so have only observed its equatorial regions in detail.
Ulysses probe was launched in 1990 to study the Sun's polar
regions. It first travelled to Jupiter, to "slingshot" into an orbit
that would take it far above the plane of the ecliptic. 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,
which was slower than expected, and that there were large magnetic
waves emerging from high latitudes that scattered galactic cosmic
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.
The Solar Terrestrial Relations
Observatory (STEREO) mission was
launched in October 2006. Two identical spacecraft were launched into
orbits that cause them to (respectively) pull further ahead of and
fall gradually behind Earth. This enables stereoscopic imaging of the
Sun and solar phenomena, such as coronal mass ejections.
Indian Space Research Organisation
Indian Space Research Organisation has scheduled the launch of a
100 kg satellite named Aditya for 2017–18. Its main instrument
will be a coronagraph for studying the dynamics of the Solar
Observation and effects
During certain atmospheric conditions, the
Sun becomes clearly visible
to the naked eye, and can be observed without stress to the eyes.
Click on this photo to see the full cycle of a sunset, as observed
from the high plains of the Mojave Desert.
The Sun, as seen from low
Earth orbit overlooking the International
Space Station. This sunlight is not filtered by the lower atmosphere,
which blocks much of the solar spectrum
The brightness of the
Sun can cause pain from looking at it with the
naked eye; however, doing so for brief periods is not hazardous for
normal non-dilated eyes. Looking directly at the
phosphene visual artifacts and temporary partial blindness. It also
delivers about 4 milliwatts of sunlight to the retina, slightly
heating it and potentially causing damage in eyes that cannot respond
properly to the brightness. UV exposure gradually yellows
the lens of the eye over a period of years, and is thought to
contribute to the formation of cataracts, but this depends on general
exposure to solar UV, and not whether one looks directly at the
Sun. Long-duration viewing of the direct
Sun with the naked eye
can begin to cause UV-induced, sunburn-like lesions on the retina
after about 100 seconds, particularly under conditions where the UV
light from the
Sun is intense and well focused; conditions
are worsened by young eyes or new lens implants (which admit more UV
than aging natural eyes),
Sun angles near the zenith, and observing
locations at high altitude.
Sun through light-concentrating optics such as binoculars
may result in permanent damage to the retina without an appropriate
filter that blocks UV and substantially dims the sunlight. When using
an attenuating filter to view the Sun, the viewer is cautioned to use
a filter specifically designed for that use. Some improvised filters
that pass UV or IR rays, can actually harm the eye at high brightness
levels. Herschel wedges, also called Solar Diagonals, are
effective and inexpensive for small telescopes. The sunlight that is
destined for the eyepiece is reflected from an unsilvered surface of a
piece of glass. Only a very small fraction of the incident light is
reflected. The rest passes through the glass and leaves the
instrument. If the glass breaks because of the heat, no light at all
is reflected, making the device fail-safe. Simple filters made of
darkened glass allow the full intensity of sunlight to pass through if
they break, endangering the observer's eyesight. Unfiltered binoculars
can deliver hundreds of times as much energy as using the naked eye,
possibly causing immediate damage. It is claimed that even brief
glances at the midday
Sun through an unfiltered telescope can cause
Halo with sun dogs
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 up to 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 because of Rayleigh
scattering and Mie scattering from a particularly long passage through
Earth's atmosphere, and the
Sun is sometimes faint enough to be
viewed comfortably with the naked eye or safely with optics (provided
there is no risk of bright sunlight suddenly appearing through a break
between clouds). Hazy conditions, atmospheric dust, and high humidity
contribute to this atmospheric attenuation.
An optical phenomenon, known as a green flash, can sometimes be seen
shortly after sunset or before sunrise. The flash is caused by light
Sun just below the horizon being bent (usually through a
temperature inversion) towards the observer. Light of shorter
wavelengths (violet, blue, green) is bent more than that of longer
wavelengths (yellow, orange, red) but the violet and blue light is
scattered more, leaving light that is perceived as green.
Ultraviolet light from the
Sun has antiseptic properties and can be
used to sanitize tools and water. It also causes sunburn, and has
other biological effects such as the production of vitamin D and sun
Ultraviolet light is strongly attenuated by Earth's ozone
layer, so that the amount of UV varies greatly with latitude and has
been partially responsible for many biological adaptations, including
variations in human skin color in different regions of the globe.
Main article: Solar System
Sun has eight known planets. This includes four terrestrial
planets (Mercury, Venus, Earth, and Mars), two gas giants (
Saturn), and two ice giants (
Uranus and Neptune). The Solar System
also has at least five dwarf planets, an asteroid belt, numerous
comets, and a large number of icy bodies which lie beyond the orbit of
Book: The Sun
Advanced Composition Explorer
List of brightest stars
Timeline of the far future
Solar System portal
^ a b c All numbers in this article are short scale. One billion is
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Hydrothermal vent communities
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7005128000000000000♠128000 lux (see sunlight) times the square
of the distance to the center of the Sun, divided by the cross
sectional area of the Sun. 1.44 Gcd/m2 is calculated using
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