The
Sun is the
star at the
center of the
Solar System. The Sun has
a diameter of about (about 109
Earths), and by
itself accounts for about 99.86% of the Solar System's
mass; the remainder consists of the
planets (including
Earth),
asteroids,
meteoroids,
comets, and
dust in
orbit.About
three-fourths of the Sun's mass consists of
hydrogen, most of the rest is
helium. Less than 2% consists of other elements,
including
iron,
oxygen,
carbon,
neon, and
others.
The Sun's color is white, although from the surface of the Earth it
may appear yellow because of atmospheric
scattering. It has a
spectral class of
G2V, informally designated a
"yellow star" because the majority of its radiation is in the
yellow-green portion of the visible spectrum. The
G2
indicates its
surface
temperature of approximately 5,780 K (5,510 °C.) The
V
(
Roman five) in the spectral class
indicates that the Sun, like most stars, is a
main sequence star, and thus generates its
energy by
nuclear fusion of hydrogen
nuclei into helium. Once regarded as a
small and relatively insignificant star, the Sun is now presumed to
be brighter than 85% of the stars in the
galaxy, most of which are
red
dwarfs. (Estimates for its
magnitude are around 4.8) The Sun's hot
corona continuously expands in space creating
the
solar wind, a hypersonic stream of
charged particles that extends to the
heliopause at roughly 100
AU. The bubble in the
interstellar medium formed by the solar
wind, the
heliosphere, is the largest
continuous structure in the Solar System.
The Sun is currently traveling through the
Local Interstellar Cloud in the
Local Bubble zone, within the inner rim
of the
Orion Arm of the Milky Way Galaxy.
Of the 50
nearest stellar systems
within 17 light-years from the Earth, the Sun ranks 4th
in mass.
The Sun orbits the center of the
Milky Way
galaxy at a distance of approximately –
light years from the
galactic center, completing one clockwise
orbit, as viewed from the
galactic
north pole, in about 225–250 million years.
The mean distance of the Sun from the Earth is approximately
149.6 million kilometers (1 AU), though this varies as the
Earth moves from
perihelion in January to
aphelion in July. At this average distance,
light travels from the Sun to the earth in
about 8 minutes 19 seconds. The
energy from
this
sunlight supports
almost all life on Earth via
photosynthesis,
and drives the Earth's climate and weather. The enormous impact of the Sun on the Earth has been recognized since pre-historic times, and the Sun has been regarded by some cultures as a deity. An accurate scientific understanding of the Sun developed slowly, and as recently as the 19th century prominent scientists had little notion of the Sun's physical composition and source of energy. This understanding is still developing; there are a number of present-day anomalies in the Sun's behavior that remain unexplained.
Characteristics

An illustration of the structure of
the Sun:
1.
The Sun is a G-type main sequence star comprising about 99.86% of
the total mass of the
Solar System. It
is a near-perfect sphere, with an
oblateness estimated at about 9 millionths, which
means that its polar diameter differs from its equatorial diameter
by only 10 km (6 mi). As the Sun exists in a
plasmatic state and is not solid, it
rotates faster at its
equator than at its
poles. This behavior is known as differential
rotation, and is caused by convection
in the Sun and the movement of mass, due to steep temperature
gradients from the core outwards. This mass carries a portion of
the Sun’s counter-clockwise angular momentum, as viewed from the
ecliptic north pole, thus redistributing the angular velocity. The
period of this
actual rotation is approximately 25.6 days
at the equator and 33.5 days at the poles. However, due to our
constantly changing vantage point from the Earth as it orbits the
Sun, the
apparent rotation of the star at its equator is
about 28 days.Phillips, 1995, pp. 78–79 The centrifugal effect of
this slow rotation is 18 million times weaker than the surface
gravity at the Sun's equator. The tidal effect of the planets is
even weaker, and does not significantly affect the shape of the
Sun.
The Sun is a
Population
I, or heavy element-rich, 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. These elements could most plausibly have been
produced by
endergonic nuclear reactions
during a supernova, or by
transmutation via
neutron absorption inside a massive
second-generation star.
The Sun does not have a definite boundary as rocky planets do, and
in its outer parts the density of its gases drops approximately
exponentially with
increasing distance from its center.Zirker, 2002, p. 11
Nevertheless, it has a well-defined interior structure, described
below. The Sun's radius is measured from its center to the edge of
the
photosphere. This is simply the
layer above which the gases are too cool or too thin to radiate a
significant amount of light, and is therefore the surface most
readily visible to the
naked eye.Phillips,
1995, p. 73
The solar interior is not directly observable, and the Sun itself
is opaque to
electromagnetic
radiation. However, just as
seismology uses waves generated by earthquakes to
reveal the interior structure of the Earth, the discipline of
helioseismology makes use of
pressure waves (
infrasound) traversing
the Sun's interior to measure and visualize the star's inner
structure.Phillips, 1995, pp. 58–67
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 to 0.25 solar radii. It has a
density of up to (150 times the density of water on Earth) and a
temperature of close to 13,600,000
Kelvin (by
contrast, the surface of the Sun is around 5,800 Kelvin). Recent
analysis of
SOHO
mission data favors a faster rotation rate in the core than in the
rest of the radiative zone. Through most of the Sun's life, energy
is produced by
nuclear fusion through
a series of steps called the
p–p chain; this process
converts hydrogen into helium. Less than 2% of the helium generated
in the Sun comes from the
CNO cycle. The
core is the only location in the Sun that produces an appreciable
amount of heat 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 or
kinetic energy of
particles.Zirker, 2002, pp. 15–34Phillips, 1995, pp. 47–53
The
proton-proton chain occurs
around 9.2 times each second in the core of the Sun. Since this
reaction uses four protons, it converts about 3.7
protons (hydrogen nuclei) to helium nuclei every
second (out of a total of ~8.9 free protons in the Sun), or about
6.2 kg per second. Since fusing hydrogen into helium releases
around 0.7% of the fused mass as energy, the Sun releases energy at
the matter–energy conversion rate of 4.26 million metric tons per
second, 383
yottawatts (
), or 9.15
megatons of
TNT per second. Power density is about of
matter, though since most fusion occurs in the relatively small
core the plasma power density there is about 150 times bigger. For
comparison, the human body produces heat at approximately the rate
, roughly 600 times greater per unit mass. Assuming core density
150 times higher than average, this corresponds to a surprisingly
low rate of energy production in the Sun's core—about . This power
is much less than generated by a single candle. The use of plasma
with similar parameters for energy production on Earth would be
completely impractical—even a modest fusion power plant would
require about 5 billion metric tons of plasma.
The rate of nuclear fusion depends strongly on density and
temperature, so the fusion rate in the core is in a self-correcting
equilibrium: a slightly higher rate of fusion would cause the core
to heat up more and
expand
slightly against the
weight of the outer
layers, reducing the fusion rate and correcting the
perturbation; and a slightly lower
rate would cause the core to cool and shrink slightly, increasing
the fusion rate and again reverting it to its present level.
The high-energy
photons (gamma rays) released
in
fusion reactions are absorbed in
only a few millimeters of solar plasma and then re-emitted again in
random direction (and at slightly lower energy)—so 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.
After a final trip through the convective outer layer to the
transparent "surface" of the photosphere, the photons escape as
visible light. Each gamma ray in the
Sun's core is converted into several million visible light photons
before escaping into space.
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 recently resolved through the
discovery of the effects of
neutrino oscillation: the Sun in fact
emits the number of neutrinos predicted by the
theory, but neutrino detectors were missing 2/3 of
them because the neutrinos had changed
flavor.
Radiative zone
From about 0.25 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; while the material grows cooler as
altitude increases (from 7,000,000 °C to about 2,000,000 °C) this
temperature gradient is less
than the value of
adiabatic lapse
rate and hence cannot drive convection. Heat is transferred by
radiation—
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/cm
³ to only 0.2 g/cm
³)
from the bottom to the top of the radiative zone.
Between the radiative zone and the convection zone is a transition
layer called 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—a condition where
successive horizontal layers slide past one another. The fluid
motions found in the convection zone above, slowly disappear from
the top of this layer to its bottom, matching the calm
characteristics of the radiative zone on the bottom. Presently, it
is hypothesized (see
Solar dynamo),
that a magnetic dynamo within this layer generates the Sun's
magnetic field.
Convective zone
In the Sun's outer layer, from its surface down to approximately
200,000 km (or 70% of the solar radius), the solar plasma is
not dense enough or hot enough to transfer the heat energy of the
interior outward via radiation (in other words it is opaque
enough). As a result, thermal convection occurs as
thermal columns carry hot material to the surface
(photosphere) 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. At
the visible surface of the Sun, the temperature has dropped to
5,700° K and the density to only 0.2 g/m
³ (about
1/10,000th the density of air at sea level).
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. 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. The Sun's thermal columns are
Bénard cells and therefore tend to
be hexagonal prisms.
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 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 actually tens to hundreds of kilometers thick, being
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. Sunlight has approximately a
black-body spectrum that indicates its
temperature is about 6,000
K, interspersed
with atomic
absorption lines from
the tenuous layers above the photosphere. The photosphere has a
particle density of ~10
23 m
−3 (this is
about 1% of the particle density of
Earth's atmosphere at sea level).
During early studies of the
optical
spectrum of the photosphere, some absorption lines were found
that did not correspond to any
chemical
elements then known on Earth. In 1868,
Norman Lockyer hypothesized that these
absorption lines were because of a new element which he dubbed
"
helium", after the Greek Sun god
Helios. 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, from
radio through visible light to
gamma
rays, and comprise five principal zones: the
temperature
minimum, the
chromosphere, the
transition region, the
corona, and the
heliosphere. The heliosphere, which may be
considered the tenuous outer atmosphere of the Sun, extends outward
past the orbit of
Pluto to the
heliopause, where it forms a sharp
shock front boundary with the
interstellar medium. The chromosphere,
transition region, and corona are much hotter than the surface of
the Sun. The reason why has not been conclusively proven; evidence
suggests that
Alfvén waves may have
enough energy to heat the corona.
The coolest layer of the Sun is a temperature minimum region about
above the photosphere, with a temperature of about . This part of
the Sun is cool enough to support simple molecules such as
carbon monoxide and water, which can be
detected by their absorption spectra.
Above the temperature minimum layer is a layer about 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. The temperature in
the chromosphere increases gradually with altitude, ranging up to
around near the top. In the upper part of chromosphere
helium becomes partially
ionized.
Above the chromosphere there is a thin (about 200 km)
transition region in which the
temperature rises rapidly from around 20,000
K in the upper chromosphere to coronal temperatures
closer to one million 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
spicule and
filament, 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
spectrum.
The
corona is the extended outer atmosphere
of the Sun, which is much larger in volume than the Sun itself. The
corona continuously expands into the space forming the
solar wind, which fills all the
Solar System. The low corona, which is very
near the surface of the Sun, has a particle density around
10
15–10
16 m
−3. The average
temperature of the corona and solar wind is about 1–2 million
kelvins, however, in the hottest regions it is 8–20 million
kelvins. While no complete theory yet exists to account for the
temperature of the corona, at least some of its heat is known to be
from
magnetic
reconnection.
The
heliosphere, which is the cavity
around the Sun filled with the solar wind plasma, extends from
approximately 20 solar radii (0.1 AU) to the outer fringes of the
Solar System. Its inner boundary is defined as the layer in which
the flow of the
solar wind becomes
superalfvénic—that is, where the flow becomes faster than
the speed of
Alfvén waves.
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 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. Both of the Voyager
probes have recorded higher levels of energetic particles as they
approach the boundary.
Magnetic field
The Sun is a magnetically active star. It supports a strong,
changing
magnetic field that varies
year-to-year and reverses direction about every eleven years around
solar maximum.Zirker, 2002, pp. 119–120 The Sun's magnetic field
gives rise to many effects that are collectively called
solar activity, including
sunspots on the surface of the Sun,
solar flares, and variations in
solar wind that carry material through the Solar
System. Effects of solar activity on Earth include
aurora 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. Solar activity changes the
structure of Earth's
outer
atmosphere.
All matter in the Sun is in the form of
gas and
plasma because of its high
temperatures. This makes it possible for the Sun to rotate faster
at its equator (about 25 days) than it does at higher latitudes
(about 35 days near its poles). The
differential rotation of the Sun's latitudes
causes its
magnetic field lines to
become twisted together over time, causing
magnetic field loops to erupt from the Sun's
surface and trigger the formation of the Sun's dramatic
sunspots and
solar
prominences (see
magnetic
reconnection). This twisting action gives rise to the
solar dynamo and an 11-year
solar cycle of magnetic activity as the Sun's
magnetic field reverses itself about every 11 years.
The solar magnetic field extends well beyond the Sun itself. The
magnetized solar wind plasma carries Sun's magnetic field into the
space forming what is called the
interplanetary magnetic field.
Since the plasma can only move along the magnetic field lines, the
interplanetary magnetic field is initially stretched radially away
from the Sun. Because the fields above and below the solar equator
have different polarities pointing towards and away from the Sun,
there exists a thin current layer in the solar equatorial plane,
which is called the
heliospheric current sheet. At
the large distances the rotation of the Sun twists the magnetic
field and the current sheet into the
Archimedean spiral like structure called
the
Parker spiral. The interplanetary
magnetic field is much stronger than the dipole component of the
solar magnetic field. The Sun's 50–400
μT (in the photosphere) magnetic dipole field
reduces with the cube of the distance to about 0.1 nT at the
distance of the Earth. However, according to spacecraft
observations the interplanetary field at the Earth's location is
about 100 times greater at around 5 nT.
Chemical composition
The 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 percent of the mass. The most abundant metals
are oxygen (roughly 1% of the Sun's mass), carbon (0.3%), neon
(0.2%), and iron (0.2%).
The Sun inherited its chemical composition from the
interstellar medium out of which it
formed: the hydrogen and helium in the Sun were produced by
Big Bang nucleosynthesis.
The metals were produced by
stellar nucleosynthesis in
generations of stars which completed their
stellar evolution and returned their
material to the interstellar medium prior to 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 the Sun formed, the helium and heavy
elements have settled out of the photosphere. Therefore, the
photosphere now contains slightly less helium and only 84% of the
heavy elements than the protostellar Sun did; the protostellar Sun
was 71.1% hydrogen, 27.4% helium, and 1.5% metals.
In the inner portions of the Sun, nuclear fusion has modified the
composition by converting hydrogen into helium, so the innermost
portion of the Sun is now roughly 60% helium, with the metal
abundance unchanged. Because the interior of the Sun is radiative,
not convective (see
Structure above),
none of the fusion products from the core have risen to the
photosphere.
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 thus not affected by
settling of heavy elements. The two methods generally agree
well.
Singly ionized iron group elements
In 1970s, much research focused on the abundances of
iron group elements in the Sun. Although
significant research was done, the abundance determination of some
iron group elements (e.g.,
cobalt and
manganese) was still difficult at least as
far as 1978 because of their hyperfine structures.
The first largely complete set of
oscillator strengths of singly ionized
iron group elements were made available first in the 1960s, and
improved oscillator strengths were computed in 1976. In 1978 the
abundances of
singly ionized elements
of the iron group were derived.
Solar and planetary mass fractionation relationship
Various authors have considered the existence of a mass
fractionation relationship between the
isotopic compositions of solar and planetary
noble gases, for example correlations between
isotopic compositions of planetary and solar
Ne
and
Xe. Nevertheless, the belief that the whole
Sun has the same composition as the solar atmosphere was still
widespread, at least until 1983.
In 1983, it was claimed that it was the fractionation in the Sun
itself that caused the fractionation relationship between the
isotopic compositions of planetary and solar wind implanted noble
gases.
Solar cycles
Sunspots and the sunspot cycle

Measurements of solar cycle variation
during the last 30 years
When observing the Sun with appropriate filtration, the most
immediately visible features are usually its
sunspots, which are well-defined surface areas that
appear darker than their surroundings because of lower
temperatures. Sunspots are regions of intense magnetic activity
where
convection 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 flares
and
coronal mass ejections.
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 an 11-year cycle known as the
solar cycle. At a typical solar minimum, few
sunspots are visible, and occasionally none at all can be seen.
Those that do appear are at high solar latitudes. As the sunspot
cycle progresses, the number of sunspots increases and they move
closer to the equator of the Sun, a phenomenon described by
Spörer's law. Sunspots usually
exist as pairs with opposite magnetic polarity. The magnetic
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.

History of the number of observed
sunspots during the last 250 years, which shows the ~11-year solar
cycle
The solar cycle has a great influence on
space weather, and is a significant influence
on the Earth's climate
since luminosity has
a direct relationship with magnetic activity. Solar activity
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 or
Little Ice Age, Europe experienced very cold
temperatures. Earlier extended minima have been discovered through
analysis of
tree rings and also appear to
have coincided with lower-than-average global temperatures.
Possible long-term cycle
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.
Life cycle
The Sun was formed about 4.57 billion years ago when a hydrogen
molecular cloud collapsed.Zirker,
2002, pp. 7–8 Solar formation is dated in two ways: the Sun's
current
main sequence age, determined
using
computer models of
stellar evolution and
nucleocosmochronology, is thought to
be about 4.57 billion years. This is in close accord with the
radiometric date of the oldest
Solar System material, at 4.567 billion years ago.
The Sun is about halfway through its
main-sequence evolution, during which
nuclear fusion reactions in its core
fuse hydrogen into helium. Each second, more than 4 million
tonnes of matter are converted into energy
within the Sun's core, producing
neutrinos
and
solar radiation; at this rate,
the Sun will have so far converted around 100 Earth-masses of
matter into energy. The Sun will spend a total of approximately 10
billion years as a main sequence
star.
The Sun does not have enough mass to explode as a
supernova. Instead, in about 5 billion years, it
will enter a
red giant phase, its outer
layers expanding as the hydrogen fuel in the core is consumed and
the core contracts and heats up.
Helium
fusion will begin when the core temperature reaches around 100
million kelvins and will produce carbon, entering the
asymptotic giant branch phase.

Life-cycle of the Sun; sizes are not
drawn to scale.
Earth's fate is precarious. As a red giant, the Sun will have a
maximum radius beyond the Earth's current orbit, , 250 times the
present radius of the Sun. However, by the time it is an asymptotic
giant branch star, the Sun will have lost roughly 30% of its
present mass due to a stellar wind, so the orbits of the planets
will move outward. If it were only for this, Earth would probably
be spared, but new research suggests that Earth will be swallowed
by the Sun owing to tidal interactions. Even if Earth would escape
incineration in the Sun, still all its water will be boiled away
and most of its atmosphere would escape into space. In fact, even
during its current life in the main sequence, the Sun is gradually
becoming more luminous (about 10% every 1 billion years), and its
surface temperature is slowly rising. The Sun used to be fainter in
the past, which is possibly the reason why life on Earth has only
existed for about 1 billion years on land. The increase in solar
temperatures is such that already in about a billion years, the
surface of the Earth will become too hot for liquid water to exist,
ending all terrestrial life.
Following the red giant phase, intense thermal pulsations will
cause the Sun to throw off its outer layers, forming a
planetary nebula. The only object that will
remain after the outer layers are ejected is the extremely hot
stellar core, which will slowly cool and fade as a
white dwarf over many billions of years. This
stellar evolution scenario is
typical of low- to medium-mass stars.
Sunlight
Sunlight is Earth's primary source of energy. 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 (
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 the
Earth's atmosphere so that less power arrives at the surface—closer
to in clear conditions when the Sun is near the
zenith.
Solar energy can be harnessed via a variety of natural and
synthetic processes—
photosynthesis by
plants captures the energy of sunlight and converts it to chemical
form (oxygen and reduced carbon compounds), while direct heating or
electrical conversion by
solar cells are
used by
solar power equipment to
generate electricity or to do other useful work. The energy stored
in
petroleum and other fossil fuels was
originally converted from sunlight by
photosynthesis in the distant past.Phillips,
1995, pp. 319–321
Motion and location within the galaxy
The Sun's motion about the
centre
of mass of the Solar System is complicated by perturbations
from the planets. Every few hundred years this motion switches
between
prograde and
retrograde.The Sun lies close to the inner rim of the
Milky Way Galaxy's Orion Arm, in the
Local
Fluff or the
Gould Belt, at a
hypothesized distance of 7.5–8.5
kpc
(25,000–28,000 lightyears) from the
Galactic Center,contained within the Local
Bubble, a space of rarefied hot gas, possibly produced by the
supernova remnant,
Geminga. 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 through space in
the Milky Way. The general direction of the Sun's galactic motion
is towards the star
Vega near the constellation
of
Hercules, at an angle of
roughly 60 sky degrees to the direction of the
Galactic Center. If one were to observe it
from
Alpha Centauri, the closest star
system, the Sun would appear to be in the constellation
Cassiopeia.
The Sun's orbit around the Galaxy is expected to be roughly
elliptical with the addition of perturbations due to the galactic
spiral arms and non-uniform mass distributions. In addition the Sun
oscillates up and down relative to the galactic plane approximately
2.7 times per orbit. This is very similar to how a
simple harmonic oscillator works
with no drag force (damping) term. 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 of the galaxy (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 Galaxy is approximately 251 km/s. At this speed,
it takes around 1,400 years for the Solar System to travel a
distance of 1 light-year, or 8 days to travel 1
AU.
Theoretical problems
Solar neutrino problem
For many years the number of solar
electron neutrinos detected on Earth was
one third to one half of the number predicted by the
standard solar model. This anomalous
result was termed the
solar
neutrino problem. Theories proposed to resolve the problem
either tried to reduce the temperature of the Sun's interior to
explain the lower neutrino flux, or posited that electron neutrinos
could
oscillate—that is, change
into undetectable
tau and
muon neutrinos as they traveled between the
Sun and the Earth.
Several neutrino observatories were built in
the 1980s to measure the solar neutrino flux as accurately as
possible, including the Sudbury Neutrino Observatory
and Kamiokande.
Results from these observatories eventually led to the discovery
that neutrinos have a very small
rest mass
and do indeed oscillate. Moreover, in 2001 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 depending on
the neutrino energy as few as one-third of the neutrinos seen at
Earth are of the electron type. This proportion agrees with that
predicted by the
Mikheyev-Smirnov-Wolfenstein
effect (also known as the matter effect), which describes
neutrino oscillation in matter, and it is now considered a solved
problem.
Coronal heating problem
The optical surface of the Sun (the
photosphere) is known to have a temperature of
approximately 6,000
K. Above it lies the
solar corona, rising to a temperature of 1–2 million 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 gas in the form of heat. The other is
magnetic heating, in which magnetic energy is
continuously built up by photospheric motion and released through
magnetic reconnection in the
form of large
solar flares and myriad
similar but smaller events—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.
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 consensus among scientists is that the young
Earth's atmosphere contained much larger quantities of
greenhouse gases (such as
carbon dioxide,
methane and/or
ammonia) than
are present today, which trapped enough heat to compensate for the
smaller amount of
solar energy reaching
the planet.
Present anomalies
The Sun is presently behaving unexpectedly in a number of ways.
- It is in the midst of an unusual sunspot minimum, lasting far
longer and with a higher percentage of spotless days than normal;
since May 2008, predictions of an imminent rise in activity have
been regularly made and as regularly confuted.
- It is measurably dimming; its output has dropped 0.02% at
visible wavelengths and 6% at EUV wavelengths in
comparison with the levels at the last solar minimum.
- Over the last two decades, the solar
wind's speed has dropped 3%, its temperature 13%, and its
density 20%.
- Its magnetic field is at less than half strength compared to
the minimum of 22 years ago. The entire heliosphere, which fills the Solar System, has shrunk as a result, resulting
in an increase in the level of cosmic
radiation striking the Earth and its atmosphere.
History of observation
Early understanding
Humanity's most fundamental understanding of the Sun is 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 phenomenon.
Worship of the Sun was central to civilizations
such as the Inca of South America and the Aztecs of what is now Mexico
.
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
Castillo
at Chichén Itzá
in Mexico is designed to cast shadows in the shape
of serpents climbing the pyramid at the
vernal and autumn equinoxes. During
the Roman era the Sun's birthday was a holiday celebrated as
Sol Invictus (literally "unconquered
sun") soon after the winter solstice which may have been an
antecedent to
Christmas. With respect to
the
fixed stars, the Sun appears from
Earth to revolve once a year along the
ecliptic through the
zodiac,
and so Greek astronomers considered it to be one of the seven
planets (Greek
planetes,
"wanderer"), after which the seven days of the
week are named in some languages.
Development of scientific understanding
One of the
first people to offer a scientific or philosophical explanation for
the Sun, was the Greek philosopher Anaxagoras, who reasoned that it was a giant
flaming ball of metal even larger than the Peloponnesus
, and not the chariot of
Helios. For teaching this
heresy, he was imprisoned by the authorities and
sentenced to death, though he was
later released through the intervention of
Pericles.
Eratosthenes
estimated the
distance between the
Earth and the Sun in the 3rd century BCE 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 (148–153 million km); the latter value is
correct to within a few percent. In the 1st century CE,
Ptolemy estimated the distance as 1,210 times the
Earth radius.
Medieval Arabic
contributions include
Albatenius discovering that the direction of the Sun's
eccentric is changing, and
Ibn Yunus observing more than 10,000 entries for
the Sun's position for many years using a large
astrolabe.
The theory that the Sun is the center around which the planets move
was apparently proposed by the ancient Greek
Aristarchus as well as several
ancient Babylonian,
ancient Indian and medieval Arabic
astronomers (see
Heliocentrism). This
view was revived in the 16th century by
Nicolaus Copernicus. In the early 17th
century, the invention of the
telescope
permitted detailed observations of sunspots by
Thomas Harriot,
Galileo Galilei and other astronomers.
Galileo made some of the first known Western observations of
sunspots and posited that they were on the surface of the Sun
rather than small objects passing between the Earth and the Sun.
Sunspots were also observed since the
Han
dynasty and Chinese astronomers maintained records of these
observations for centuries.
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 was made up of
light of many colors, while in 1800
William Herschel discovered
infrared radiation beyond the red part of the solar
spectrum. The 1800s saw spectroscopic studies of the Sun advance,
and
Joseph von Fraunhofer made
the first observations of
absorption
lines in the spectrum, the strongest of which are still often
referred to as Fraunhofer lines. When expanding the spectrum of
light from the Sun, there are large number of missing colors can be
found.
In the early years of the modern scientific era, the source of the
Sun's energy was a significant puzzle.
Lord
Kelvin suggested that the Sun was a gradually cooling liquid
body that was radiating an internal store of heat. Kelvin and
Hermann von Helmholtz then
proposed the
Kelvin-Helmholtz
mechanism to explain the energy output. Unfortunately 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 substantiated 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
.
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
Sun was confirmed in 1925 by
Cecilia Payne. 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.
Finally, a seminal paper was published in 1957 by
Margaret Burbidge, entitled "Synthesis of
the Elements in Stars". The paper demonstrated convincingly that
most of the elements in the universe had been
synthesized by nuclear reactions inside
stars, some like our Sun.
Solar space missions
The first
satellites designed to observe the Sun were NASA
's Pioneer 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 the
Earth,
and made the first detailed measurements of the solar wind and the
solar magnetic field. Pioneer 9 operated for a particularly long
period of time, transmitting data until 1987.
In the 1970s, two
Helios spacecraft
and the
Skylab Apollo Telescope Mount provided
scientists with significant new data on solar wind and the solar
corona. The Helios 1 and 2 probes was a joint U.S.–German probe
that studied the solar wind from an orbit carrying the spacecraft
inside
Mercury's orbit at
perihelion. The Skylab space station, launched by
NASA in 1973, included a solar
observatory module called the Apollo 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.
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 Challenger
mission STS-41C retrieved the satellite and repaired its
electronics before re-releasing it into orbit. The Solar Maximum
Mission subsequently acquired thousands of images of the solar
corona before
re-entering the
Earth's atmosphere in June 1989.
Launched in 1991, Japan's
Yohkoh
(
Sunbeam) satellite observed solar flares at X-ray
wavelengths. Mission data allowed scientists to identify several
different types of flares, and also 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 reentry in
2005.
One of the
most important solar missions to date has been the Solar and Heliospheric
Observatory, jointly built by the European Space
Agency
and NASA
and launched
on 2 December 1995. Originally intended to serve a two-year
mission, SOHO is still in operation as of 2009. It has proven so
useful that a follow-on mission, the
Solar Dynamics Observatory, is
planned for launch in November 2009. Situated at the
Lagrangian point between the Earth and the
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. In addition to its direct solar observation, SOHO has
enabled the discovery of large numbers of comets, mostly very tiny
sungrazing comets which incinerate
as they pass the Sun.
All these satellites have observed the Sun from the plane of the
ecliptic, and so have only observed its equatorial regions in
detail. The
Ulysses probe was launched
in 1990 to study the Sun's polar regions. It first traveled to
Jupiter, to "slingshot" past the planet into
an orbit which would take it far above the plane of the ecliptic.
Serendipitously, it was well-placed to observe the collision of
Comet Shoemaker-Levy 9 with
Jupiter in 1994. 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
which scattered galactic
cosmic
rays.
Elemental abundances in the photosphere are well known from
spectroscopic studies, but
the composition of the interior of the Sun is more poorly
understood. A
solar wind sample return
mission,
Genesis, was designed
to allow astronomers to directly measure the composition of solar
material. Genesis returned to Earth in 2004 but was damaged by a
crash landing after its
parachute failed
to deploy on reentry into Earth's atmosphere. Despite severe
damage, some usable samples have been recovered from the
spacecraft's sample return module and are undergoing
analysis.
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 the
Earth. This enables
stereoscopic
imaging of the Sun and solar phenomena, such as
coronal mass ejections.
Observation and effects
Sunlight is very bright, and looking directly at the Sun with the
naked eye for brief periods can be
painful, but is not particularly hazardous for normal, non-dilated
eyes. Looking directly at the Sun causes
phosphene visual artifacts and temporary partial
blindness. It also delivers about 4 milliwatts of sunlight to
the retina, slightly heating it and potentially 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, not on 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.
Viewing the Sun through light-concentrating
optics such as
binoculars
is very hazardous without an appropriate filter that blocks UV and
substantially dims the sunlight. An
attenuating filter might not filter
UV and so is still dangerous. Attenuating filters to view the Sun
should be specifically designed for that use: some improvised
filters pass UV or IR rays that can harm the eye at high brightness
levels.Unfiltered binoculars can deliver over 500 times as much
energy to the retina as using the naked eye, killing retinal cells
almost instantly (even though the power per unit area of image on
the retina is the same, the heat cannot dissipate fast enough
because the image is larger). Even brief glances at the midday Sun
through unfiltered binoculars can cause permanent blindness.
Partial
solar eclipses are hazardous
to view because the eye's
pupil is not adapted
to the unusually high visual contrast: the pupil dilates according
to the total amount of light in the field of view,
not by
the brightest object in the field. During partial eclipses most
sunlight is blocked by the
Moon passing in
front of the Sun, but the uncovered parts of the photosphere have
the same
surface brightness as
during a normal day. In the overall gloom, the pupil expands from
~2 mm to ~6 mm, and each retinal cell exposed to the
solar image receives about ten times more light than it would
looking at the non-eclipsed Sun. This can damage or kill those
cells, resulting in small permanent blind spots for the viewer. The
hazard is insidious for inexperienced observers and for children,
because there is no perception of pain: it is not immediately
obvious that one's vision is being destroyed.
During
sunrise and
sunset sunlight is attenuated due to
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.
A rare
optical phenomenon may
occur shortly after sunset or before sunrise, known as a
green flash. The flash is caused by light from
the 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
medical effects such as the production of
vitamin D. 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.
Terminology
Like other natural phenomena, the Sun has been an object of
veneration in many cultures throughout human history, and was the
source of the word
Sunday.
The Sun has no official name according to the
International Astronomical
Union, the body responsible for naming celestial objects. The
name
Sol ( , from
Latin Sol,
the
Sun god), is accepted but not
commonly used; the adjectival form is the related word
solar. "Sol" is the modern word for "Sun" in many other
languages.
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, while a mean Martian sol, is
24 hours, 39 minutes, and 35.244 seconds. See also
Timekeeping on Mars.
In
East Asia the Sun is represented by the
symbol 日 (Chinese
pinyin rì or
Japanese nichi) or
太阳(simplified)/太陽(traditional) (pinyin
tài yáng or
Japanese
taiyō). In
Vietnamese these
Han words are called
nhật and
thái dương respectively, while the native Vietnamese word
mặt trời literally means "face of the heavens". The
Moon and the Sun are associated with the
yin and yang where the Moon represents
yin and the Sun
yang as dynamic opposites.Osgood,
Charles E. "From Yang and Yin to and or but." Language 49.2 (1973):
380–412 . JSTOR. 16 Nov. 2008 /www.jstor.org/search>.
See also
Notes
- A Star with two North Poles, April 22, 2003,
Science @ NASA
- Riley, Pete; Linker, J. A.; Mikić, Z., " Modeling the heliospheric current sheet: Solar cycle
variations", (2002) Journal of Geophysical Research
(Space Physics), Volume 107, Issue A7, pp. SSH 8-1, CiteID 1136,
DOI 10.1029/2001JA000299. ( Full text)
- In astronomical
sciences, the term heavy elements (or metals)
refers to all elements except hydrogen and helium.
- p. 102, The physical universe: an introduction to
astronomy, Frank H. Shu, University Science Books, 1982, ISBN
0935702059.
- * *
- Ross and Aller 1976, Withbroe 1976, Hauge and Engvold 1977,
cited in Biemont 1978.
- Corliss and Bozman (1962 cited in Biemont 1978) and Warner
(1967 cited in Biemont 1978)
- Smith (1976 cited in Biemont 1978)
- Signer and Suess 1963; Manuel 1967; Marti 1969; Kuroda and
Manuel 1970; Srinivasan and Manuel 1971, all cited in Manuel and
Hwaung 1983
- Kuroda and Manuel 1970 cited in Manuel and Hwaung 1983:7
- See also
- Sun's retrograde motion and violation of even-odd cycle
rule in sunspot activity, J. Javaraiah, 2005
- Robert Zimmerman, "What's Wrong with Our Sun?", Sky and
Telescope August 2009
-
http://science.nasa.gov/headlines/y2009/01apr_deepsolarminimum.htm
- NASA, "The Sun's Sneaky Variability", October 27, 2009
- Note: select the Etymology tab
- A short History of scientific ideas to 1900, C.
Singer, Oxford University Press, 1959, p. 151.
- The Arabian Science, C. Ronan, pp. 201–244 in The Cambridge
Illustrated History of the World's Science, Cambridge
University Press, 1983; at pp. 213–214.
- "While environmental exposure to UV radiation is known to
contribute to the accelerated aging of the outer layers of the eye
and the development of cataracts, the concern over improper viewing
of the Sun during an eclipse is for the development of "eclipse
blindness" or retinal burns."
- William Little (ed.) Oxford Universal Dictionary,
1955. See entry on "Sol".
- "Sol", Merriam-Webster online, accessed July 19,
2009
- This includes Portuguese, Spanish,
Icelandic, Danish, Norwegian,
Swedish, Leonese, Catalan and
Galician. In addition, the Peruvian currency nuevo sol is named after the Sun (in
Spanish), like its successor (and predecessor, in use 1985–1991)
the Inti (in
Quechua). In
Persian, sol means "solar year".
References
- A Star with two North Poles, April 22, 2003,
Science @ NASA
- Riley, Pete; Linker, J. A.; Mikić, Z., " Modeling the heliospheric current sheet: Solar cycle
variations", (2002) Journal of Geophysical Research
(Space Physics), Volume 107, Issue A7, pp. SSH 8-1, CiteID 1136,
DOI 10.1029/2001JA000299. ( Full text)
- In astronomical
sciences, the term heavy elements (or metals)
refers to all elements except hydrogen and helium.
- p. 102, The physical universe: an introduction to
astronomy, Frank H. Shu, University Science Books, 1982, ISBN
0935702059.
- * *
- Ross and Aller 1976, Withbroe 1976, Hauge and Engvold 1977,
cited in Biemont 1978.
- Corliss and Bozman (1962 cited in Biemont 1978) and Warner
(1967 cited in Biemont 1978)
- Smith (1976 cited in Biemont 1978)
- Signer and Suess 1963; Manuel 1967; Marti 1969; Kuroda and
Manuel 1970; Srinivasan and Manuel 1971, all cited in Manuel and
Hwaung 1983
- Kuroda and Manuel 1970 cited in Manuel and Hwaung 1983:7
- See also
- Sun's retrograde motion and violation of even-odd cycle
rule in sunspot activity, J. Javaraiah, 2005
- Robert Zimmerman, "What's Wrong with Our Sun?", Sky and
Telescope August 2009
-
http://science.nasa.gov/headlines/y2009/01apr_deepsolarminimum.htm
- NASA, "The Sun's Sneaky Variability", October 27, 2009
- Note: select the Etymology tab
- A short History of scientific ideas to 1900, C.
Singer, Oxford University Press, 1959, p. 151.
- The Arabian Science, C. Ronan, pp. 201–244 in The Cambridge
Illustrated History of the World's Science, Cambridge
University Press, 1983; at pp. 213–214.
- "While environmental exposure to UV radiation is known to
contribute to the accelerated aging of the outer layers of the eye
and the development of cataracts, the concern over improper viewing
of the Sun during an eclipse is for the development of "eclipse
blindness" or retinal burns."
- William Little (ed.) Oxford Universal Dictionary,
1955. See entry on "Sol".
- "Sol", Merriam-Webster online, accessed July 19,
2009
- This includes Portuguese, Spanish,
Icelandic, Danish, Norwegian,
Swedish, Leonese, Catalan and
Galician. In addition, the Peruvian currency nuevo sol is named after the Sun (in
Spanish), like its successor (and predecessor, in use 1985–1991)
the Inti (in
Quechua). In
Persian, sol means "solar year".
Further reading
External links