The Sun 
 |
| Observation data |
Mean distance
from Earth |
1.496×1011 m
8.31 min at light speed |
| Visual brightness (V) |
−26.74m [54] |
| Absolute magnitude |
4.83m [54] |
| Spectral classification |
G2V |
| Angular size |
31.6' - 32.7' [55] |
| Adjectives |
solar |
| Orbital characteristics |
Mean distance
from Milky Way core |
~2.5×1020 m
26,000 light-years |
| Galactic period |
2.25–2.50×108 a |
| Velocity |
2.17×105 m/s
(orbit around the center of the Galaxy)
2×104 m/s
(relative to average velocity of other stars in stellar neighborhood) |
| Physical characteristics |
| Mean diameter |
1.392×109 m [54]
109 Earths |
| Equatorial radius |
6.955×108 m [56] |
| Equatorial circumference |
4.379×109 m [56] |
| Flattening |
9×10−6 |
| Surface area |
[[1 E+18 m²|6.088]]×1018 m² [56]
11,900 Earths |
| Volume |
[[1 E+27 m³|1.4122]]×1027 m³ [56]
1,300,000 Earths |
| Mass |
1.9891 ×1030 kg[54]
332,946 Earths |
| Average density |
1.409 ×103 kg/m³ [56] |
| Equatorial surface gravity |
274.0 m/s2 [54]
27.94 g |
Escape velocity
(from the surface) |
617.7 km/s [56]
55 Earths |
Temperature
of surface (effective) |
5,778 K [54] |
Temperature
of corona |
~5,000,000 K
|
Temperature
of core |
~15,710,000 K [54] |
| Luminosity (Lsol) |
3.846×1026 W [54]
~3.75×1028 lm
~98 lm/W efficacy |
| Mean Intensity (Isol) |
2.009×107 W m-2 sr-1 |
| Rotation characteristics |
| Obliquity |
7.25° [54]
(to the ecliptic)
67.23°
(to the galactic plane) |
Right ascension
of North pole[57] |
286.13°
19 h 4 min 30 s |
Declination
of North pole |
+63.87°
63°52' North |
Sidereal Rotation period
(at 16° latitude) |
25.38 days [54]
25 d 9 h 7 min 13 s[57] |
| (at equator) |
25.05 days [54] |
| (at poles) |
34.3 days [54] |
Rotation velocity
(at equator) |
7.284 ×103 km/h |
| Photospheric composition (by mass) |
| Hydrogen |
73.46 % |
| Helium |
24.85 % |
| Oxygen |
0.77 % |
| Carbon |
0.29 % |
| Iron |
0.16 % |
| Sulfur |
0.12 % |
| Neon |
0.12 % |
| Nitrogen |
0.09 % |
| Silicon |
0.07 % |
| Magnesium |
0.05 % |
|
The Sun (Latin: Sol) is the star at the center of the Solar System. The Earth and other matter (including other planets, asteroids, meteoroids, comets
and dust) orbit the Sun, which by itself accounts for about
99.8% of the solar system's mass. Energy from the Sun, in the form of sunlight, supports almost all life on Earth via photosynthesis, and drives the Earth's climate and weather.
The Sun is composed of hydrogen (about 74% of its mass, or 92% of its volume),
helium (about 25% of mass, 7% of volume), and trace quantities of other elements. The Sun has a spectral class of G2V. G2
implies that it has a surface temperature of approximately 5,780 K, giving it a
white color which, because of atmospheric scattering, appears yellow as seen from the surface of the Earth. This is a subtractive effect, as the
preferential scattering of blue photons (causing the sky color) removes enough blue
light to leave a residual reddishness that is perceived as yellow. (When low enough in the sky, the Sun appears orange or red,
due to this scattering.)
Its spectrum contains lines of ionized and neutral metals as well as very weak hydrogen
lines. The V (Roman five) suffix indicates that the Sun, like most stars, is a
main sequence star. This means that it generates its energy by nuclear fusion of hydrogen nuclei into helium and is in a state of hydrostatic equilibrium, neither
contracting nor expanding over time. There are more than 100 million G2 class stars in our galaxy. Because of logarithmic size
distribution, the Sun is actually brighter than 85% of the stars in the galaxy, most of which
are red dwarfs.[1]
The Sun orbits the center of the Milky Way galaxy at a
distance of approximately 26,000 light-years from the galactic center, completing one revolution in about 225–250 million years. The orbital speed is 217 km/s, equivalent to one light-year every 1,400 years, and one AU every 8 days.[2]
It is currently traveling through the Local Interstellar Cloud in the
low-density Local Bubble zone of diffuse high-temperature gas, in the inner rim of the
Orion Arm of the Milky Way Galaxy, between the larger
Perseus and Sagittarius arms of the galaxy. Of the
50 nearest stellar systems within 17 light years from the Earth, the sun ranks 4th
in absolute magnitude as a fourth magnitude star (M=4.83).
Overview
The Sun is a Population I, or third generation, star whose formation may have been
triggered by shockwaves from one or more nearby supernovae.[3] This is suggested by a high abundance of heavy elements such as
gold and uranium in the solar system. These elements could most
plausibly have been produced by endergonic nuclear reactions during a supernova, or by
transmutation via neutron absorption
inside a massive second-generation star.
Sunlight is 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
1,370 watts per square meter of area at a distance of one AU from the Sun (that is, on or near Earth). Sunlight on the surface of Earth is attenuated by the Earth's atmosphere so that less power arrives at the surface—closer to 1,000 watts per
directly exposed square meter in clear conditions when the Sun is near the zenith. This energy
can be harnessed via a variety of natural and synthetic processes—photosynthesis by
plants captures the energy of sunlight and converts it to chemical form (oxygen and reduced carbon compounds), while direct
heating or electrical conversion by solar cells are used by solar power equipment to generate electricity or to do other useful work. The energy stored in
petroleum and other fossil fuels was originally converted
from sunlight by photosynthesis in the distant past.
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. The angle that the Sun makes with Zenith at noon has been responsible for many biological
adaptations, including variations in human skin color in different regions of the
globe.[4]
Observed from Earth, the Sun's path across the sky varies throughout the year. The shape described by the Sun's position,
considered at the same time each day for a complete year, is called the analemma and resembles
a figure 8 aligned along a north/south axis. While the most obvious variation in the Sun's apparent position through the year is
a north/south swing over 47 degrees of angle (because of the 23.5-degree tilt of the Earth with respect to the Sun), there is an
east/west component as well, caused by the acceleration of the Earth as it approaches its perihelion with the sun, and the reduction in the Earth's speed as it moves away to approach its aphelion. The north/south swing in apparent angle is the main source of seasons on
Earth.
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. 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 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. Solar activity changes the structure of Earth's outer atmosphere.
Although it is the nearest star to Earth and has been intensively studied by scientists, many questions about the Sun remain
unanswered, such as why its outer atmosphere has a temperature of over 1 million K while
its visible surface (the photosphere) has a temperature of less than 6,000 K. Current topics
of scientific inquiry include the Sun's regular cycle of sunspot activity, the physics and
origin of flares and prominences, the magnetic
interaction between the chromosphere and the corona, and
the origin (propulsion source) of solar wind.
Life cycle
-
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.[5]
It is thought that about 4592.1 million years ago, the rapid collapse of a hydrogen
molecular cloud led to the formation of a third generation T Tauri Population I star, the Sun, in a region of the
Galactic Habitable Zone (GHZ). The nascent star assumed a nearly circular orbit about
26,000 light-years from the centre of the Milky Way Galaxy.[6]
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 4–5 billion years,
it will enter a red giant phase, its outer layers expanding as the hydrogen fuel in the core
is consumed and the core contracts and heats up. Helium fusion will begin when the core temperature reaches around 100 MK, and
will produce carbon and oxygen, entering the asymptotic giant branch of a
planetary nebula phase in about 7.8 billion years, during which instabilities in
interior temperature lead the surface of the sun to shed mass. While it is likely that the expansion of the outer layers of the
Sun will reach the current position of Earth's orbit, recent research suggests that mass lost from the Sun earlier in its red
giant phase will cause the Earth's orbit to move further out, preventing it from being engulfed.[7] However, Earth's water will be boiled away and most of its
atmosphere will escape into space. The increase in solar temperatures over this period is sufficient that by about 500-700
million years into the future, the surface of the Earth will become too hot for the survival of life as we know it.
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.[7][8]
Structure
An illustration of the structure of the Sun
The Sun is a yellow dwarf star. It comprises approximately 99% of the total mass of the solar system. The Sun is a
near-perfect sphere, with an oblateness estimated at about 9
millionths,[9] 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
undergoes differential rotation as it spins on its axis
(i.e. it rotates faster at the equator than at the poles). The
period of this actual rotation is approximately 25 days at the equator and 35 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 Sun at its equator is about 28 days. The centrifugal effect of this slow rotation is 18 million
times weaker than the surface gravity at the Sun's equator. Also, the tidal effect from the planets does not significantly affect
the shape of the Sun.
The Sun does not have a definite boundary as rocky planets do; in its outer parts the density of its gases drops approximately
exponentially with increasing distance from the center of the Sun.
Nevertheless, the Sun has a well-defined interior structure, described below. The Sun's radius is measured from its center to the
edge of the photosphere. This is simply the layer above which the gases are too cool or too
thin to radiate a significant amount of light; the photosphere is the surface most readily visible to the naked eye. The solar core comprises 10 percent of its total volume, but 40 percent of its total mass.[10]
The solar interior is not directly observable, and the Sun itself is opaque to electromagnetic radiation. However, just as seismology
uses waves generated by earthquakes to reveal the interior structure of the Earth, the
discipline of helioseismology makes use of pressure waves (infrasound) traversing the Sun's interior to measure and visualize the Sun's inner structure.
Computer modeling of the Sun is also used as a theoretical tool to investigate its
deeper layers.
Core
Cross-section of a solar-type star. (NASA)
The core of the Sun is considered to extend from the center to about 0.2 solar radii. It
has a density of up to 150,000 kg/m³ (150 times the density of water on Earth) and a temperature of close to 13,600,000 kelvins
(by contrast, the surface of the Sun is close to 5,785 kelvins (1/2350th of the core). Recent analysis of
SOHO mission data favors a faster rotation rate in the core than in
the rest of the radiative zone.[11] Through most of the
Sun's life, energy is produced by nuclear fusion through a series of steps called the
p–p (proton–proton) chain; this process converts hydrogen into helium. 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.
About 3.4×1038 protons (hydrogen nuclei) are converted into helium nuclei every
second (out of ~8.9×1056 total amount of free protons in the Sun), releasing energy at the matter–energy conversion
rate of 4.26 million tonnes per second, 383 yottawatts (3.83×1026 W) or
9.15×1010 megatons of TNT per second.
This actually corresponds to a surprisingly low rate of energy production in the Sun's core—about 0.3 µW/cm³ (microwatts per
cubic cm), or about 6 µW/kg of matter. For comparison, a candela of light (roughly one candle)
produces heat at the rate 1 W/cm³, and the human body at approximately the rate 1.2 W/kg—millions of times more heat production.
The use of plasma with similar parameters for energy production on Earth would be completely impractical—even a modest 1 GW
fusion power plant would require about 170 billion tonnes of plasma occupying almost one cubic mile. Thus, terrestrial fusion
reactors utilize far higher plasma temperatures than those in Sun's interior.
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 (cosmic, gamma and X-rays) released in fusion reactions are absorbed in only few millimetres 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 from as much as 50 million years[12] to as little as 17,000 years.[13] 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.
Radiation zone
From about 0.2 to about 0.7 solar radii, solar material is hot and dense enough that thermal radiation is sufficient to transfer the intense heat of the core outward. In this zone there
is no thermal convection; while the material grows cooler as altitude increases, 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 a brief distance before being reabsorbed by
other ions. In this way energy makes its way very slowly (see above) outward.
Convection zone
In the Sun's outer layer (down to approximately 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. 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 radiant zone.
Convective overshoot is thought to occur at the base of the convection zone,
carrying turbulent downflows into the outer layers of the radiant zone.
The thermal columns in the convection zone form an imprint on the surface of the Sun, in the form of the solar granulation and supergranulation. The turbulent
convection of this outer part of the solar interior gives rise to a "small-scale" dynamo that produces magnetic north and south
poles all over the surface of the Sun.
Photosphere
The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes opaque to visible light. Above the
photosphere visible sunlight is free to propagate into space, and its energy escapes the Sun entirely. The change in opacity is
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.[14][15] 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 about 1023 m−3 (this is about 1% of the particle density of
Earth's atmosphere at sea level).
During early studies of the optical spectrum of the photosphere, some absorption
lines were found that did not correspond to any chemical elements then known on Earth.
In 1868, Norman Lockyer hypothesized that these absorption lines were because of a
new element which he dubbed "helium", after the Greek Sun god Helios. It was not until 25 years later that helium was isolated on Earth.[16]
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 is not yet known.
The coolest layer of the Sun is a temperature minimum region about 500 km above the photosphere, with a temperature of
about 4,000 K. 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 thin layer about 2,000 km thick, dominated by a spectrum of emission and absorption
lines. It is called the chromosphere from the Greek root chroma, meaning color, because the chromosphere is visible
as a colored flash at the beginning and end of total eclipses of the Sun. The temperature
in the chromosphere increases gradually with altitude, ranging up to around 100,000 K near the top.
Taken by
Hinode's Solar Optical Telescope on
January 12,
2007, this image of the Sun reveals the filamentary nature of the plasma connecting regions of
different magnetic polarity.
Above the chromosph
