THE SUN: The star at the center of the Solar System
Introduction
General
The Sun is a huge
ball of incandescent plasma at the center of our Solar System.
Ancient civilizations, such as the Romans worshipped the Sun
because they saw it as something that brought life. It was given various names
such as Sol by the Romans and Helios by the Greeks. And perhaps that worship
was reasonable, for without the Sun, life on Earth just wouldn’t be possible.
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields. It
has a diameter of about 1,392,000 km, about 109 times that of Earth, and its mass (about 2×1030 kilograms,
330,000 times that of Earth) accounts for about 99.86% of the total mass
of the Solar System and provides all the energy
we need for life here on Earth.
Chemically, about three quarters of the
Sun's mass consists of hydrogen, while the
rest is mostlyhelium. The
remainder (1.69%, which nonetheless equals 5,628 times the mass of Earth)
consists of heavier elements, including oxygen, carbon, neonand iron, among others.
The Sun's stellar classification, based on spectral class, is G2V, and is informally designated as a yellow dwarf, because its
visible radiation is most intense in the yellow-green portion of the spectrum and although its color is white, from
the surface of the Earth it may appear yellow because of atmospheric scattering of
blue light.
In the spectral class label, G2 indicates its surface temperature of
approximately 5778 K (5505 °C), and V 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.
In its core, the Sun fuses
620 million metric
tons of hydrogen each
second. Once regarded by astronomers as a small and relatively insignificant
star, the Sun is now thought to be brighter than about 85% of the stars in the Milky Way galaxy,
most of which are red dwarfs. The absolute magnitude of
the Sun is +4.83; however, as the star closest to Earth, the Sun is the
brightest object in the sky with an apparent magnitude of
-26.74.
The Sun's hot corona continuously expands in space
creating the solar wind, a
stream of charged particles that extends to the heliopause at roughly 100 astronomical units. 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 Earth (the closest being a red dwarf named Proxima Centauri at approximately 4.2 light-years
away), the Sun ranks fourth in mass.[22] The Sun orbits the center of the Milky
Way at a distance of approximately 24,000-26,000 light-years from thegalactic center,
completing one clockwise orbit,
as viewed from the galactic north pole, in about 225–250 million years.
Since our galaxy 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, the Sun's resultant velocity with respect to the CMB
is about 370 km/s in the direction of Crater orLeo.
The mean distance of the Sun from the
Earth is approximately 149.6 million kilometers (1 AU), though the
distance varies as the Earth moves from perihelion in January to aphelion in July. At this average distance, light travels from the Sun to Earth in
about 8 minutes and 19 seconds. The energy of this sunlight supports almost all life on
Earth by photosynthesis, and
drives Earth's climate and weather.
The enormous effect of the Sun on the
Earth has been recognized since prehistoric 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 knowledge 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.
Name and etymology
The English proper noun Sun developed from Old English sunne (around 725, attested in Beowulf), and
may be related to south. Cognates to English sun
appear in other Germanic languages, including Old Frisian sunne, sonne ("sun"), Old Saxon sunna, Middle Dutch sonne, modern Dutch zon, Old High German sunna, modern German Sonne, Old Norse sunna, and Gothic sunnō. All Germanic terms for
the Sun stem from Proto-Germanic * sunnōn. In
relation, the Sun is personified as a goddess in Germanic paganism; Sól/Sunna. Scholars
theorize that the Sun, as Germanic goddess, may represent an extension of
an earlier Proto-Indo-European sun
deity due to Indo-European linguistic connections
between Old Norse Sól, Sanskrit Surya,Gaulish Sulis, Lithuanian Saulė, and Slavic Solnitse.
The English weekday name Sunday is attested in 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 heméra helíou. The Latin name
for the star, Sol, is
widely known but 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, while a mean Martian 'sol' is 24 hours, 39 minutes, and
35.244 seconds.
Characteristics
The structure
1. Core
4. Photosphere
5. Chromosphere
6. Corona
7. Sunspot
8. Granules
9. Prominence.
The Sun is a G-type main-sequence starcomprising 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. As the Sun consists of a plasmaand is not
solid, it rotates faster at its equatorthan at its poles. This behavior is known asdifferential rotation,
and is caused byconvection 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. 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 nearbysupernovae. 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 through 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 exponentially with increasing distance from its center. 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.
The solar interior is not directly
observable, and the Sun itself is opaque to electromagnetic radiation. However, just as seismologyuses
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. 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 20–25% of the solar radius. It has a density of up to 150 g/cm3 (about 150
times the density of water) and a temperature of close to 15.7 million kelvin (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 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 (proton–proton) chain; this process
convertshydrogen into helium. Only
0.8% of the energy generated in the Sun comes from the CNO cycle.
The
core is the only region in the Sun that produces an appreciable amount of
thermal energy through fusion; inside 24% of the Sun's radius, 99% of the power
has been generated, and by 30% of the radius, fusion has stopped nearly
entirely. The rest of the star is heated by energy that is transferred outward
from the core and the layers just outside. The energy produced by fusion in the
core must then travel through many successive layers to the solar photosphere
before it escapes into space as sunlight or kinetic energy of particles.
The proton–proton chain occurs
around 9.2×1037 times each second in the core of the
Sun. Since this reaction uses four free protons (hydrogen
nuclei), it converts about 3.7×1038 protons to 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 per second. Since fusing hydrogen
into helium releases around 0.7% of the fused mass as energy, the Sun releases
energy at the mass-energy conversion rate of 4.26 million metric tons per
second, 384.6 yotta watts
(3.846×1026 W), or 9.192×1010 megatons of TNT per second. This mass is not destroyed
to create the energy, rather, the mass is carried away in the radiated energy, as described by
the concept of mass-energy equivalence.
The power production by fusion in the
core varies with distance from the solar center. At the center of the Sun,
theoretical models estimate it to be approximately 276.5 watts/m3, a
power production density that more nearly approximates reptile metabolism than
a thermonuclear bomb. Peak power
production in the Sun has been compared to the volumetric heats generated in an
active compost heap. The
tremendous power output of the Sun is not due to its high power per volume, but
instead due to its large size.
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 gamma rays (high-energy photons) 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.
Therefore 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. Since energy transport in the Sun is a process which 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
take the Sun to return to a stable state if the rate of energy generation in
its core were suddenly to be changed.
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 photons of
visible light 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
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.
Radiative zone
Below 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. This zone is free of thermal convection; while
the material gets cooler from 7 to about 2 million kelvin with increasing
altitude, this temperature gradient is
less than the value of the adiabatic lapse rate and
hence cannot drive convection. Energy 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/cm3 to
only 0.2 g/cm3) from 0.25 solar radii to the top of the radiative
zone.
The radiative zone and the convection
form 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 - 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, 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 thermal energy
of the interior outward through 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 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/m3 (about 1/6,000th the density of air at
sea level).
The thermal columns in the convection
zone form an imprint on the surface of the Sun as the solar granulation and supergranulation.
The turbulent convection of this outer part of the solar interior causes 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 becomesopaque 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 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 ~1023 m−3 (this is about 0.37% of the particle
number per volume of Earth's atmosphere at
sea level; however, photosphere particles are electrons and protons, so the
average particle in air is 58 times as heavy).
During early studies of the optical spectrum of the photosphere, some absorption
lines were found that did not correspond to anychemical 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 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 500 km above the photosphere, with a
temperature of about 4,100 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 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 20,000 K near the top. In the upper part of
chromosphere helium becomes
partially ionized.
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 thespectrum.
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 space forming the solar wind, which
fills all the Solar System. The low corona, near the surface of the Sun, has a
particle density around 1015–1016 m−3. 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. While no complete theory yet exists to
account for the temperature of the corona, at least some of its heat is known
to be frommagnetic 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. The Sun's magnetic field leads to many
effects that are collectively calledsolar activity,
including sunspots on the surface of the Sun, solar flares, and
variations insolar
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.
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 creates 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.
The following table shows the main data
of the Sun:
Observation data
|
|
Mean distance
from Earth |
1.496×108 km
8 min 19 s at light speed |
−26.74
|
|
4.83
|
|
G2V
|
|
Z = 0.0122
|
|
31.6′ – 32.7′
|
|
Adjectives
|
Solar
|
Orbital characteristics
|
|
Mean
distance from Milky Waycore
|
~2.5×1017 km
26,000 light-years |
Galactic period
|
(2.25–2.50)×108 a
|
~220 km/s (orbit around the center of the
Galaxy)
~20 km/s (relative to average velocity of other stars in stellar neighborhood) ~370 km/s (relative to the cosmic microwave background) |
|
Physical characteristics
|
|
Mean
diameter
|
1.392×106 km
109 × Earth |
Equatorialradius
|
6.955×105 km
109 × Earth |
Equatorialcircumference
|
4.379×106 km
109 × Earth |
9×10−6
|
|
6.0877×1012 km2
11,990 × Earth |
|
1.412×1018 km3
1,300,000 × Earth |
|
1.9891×1030 kg
333,000 × Earth |
|
1.408×103 kg/m3
|
|
Center (model): 1.622×105 kg/m3
Lower photosphere: 2×10−4 kg/m3 Lower chromosphere: 5×10−6 kg/m3 |
|
Equatorialsurface
gravity
|
274.0 m/s2
27.94 g 28 × Earth |
Escape
velocity (from the surface)
|
617.7 km/s
55 × Earth |
Luminosity(Lsol)
|
|
Meanintensity (Isol)
|
2.009×107 W·m−2·sr−1
|
Age
|
4.57 billion years
|
Rotation characteristics
|
|
7.25° (to
the ecliptic)
67.23° (to the galactic plane)
|
|
Right
ascension of North pole
|
286.13°
19 h 4 min 30 s |
Declination
of North pole
|
+63.87°
63° 52' North |
Sidereal rotation
period (at equator)
|
25.05 days
|
(at
16° latitude)
|
25.38 days
25 d 9 h 7 min 12 s |
(at
poles)
|
34.4 days
|
Rotation
velocity (at equator)
|
7.189×103 km/h
|
Photospheric composition
(by mass)
|
|
73.46%
|
|
24.85%
|
|
0.77%
|
|
0.29%
|
|
0.16%
|
|
0.12%
|
|
0.09%
|
|
0.07%
|
|
0.05%
|
|
0.04%
|
|
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%
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 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 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 Radiative zone 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 the 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 neon and xenon. 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
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 causes 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.
The solar cycle has a great influence
on space weather, and
a significant influence on the Earth's climate since the Sun's 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
appeared to have stopped entirely for several decades; few sunspots were
observed during this period. During this era, known as the Maunder minimum or Little Ice Age, 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.
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
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The Sun was formed about 4.57 billion years
ago from the collapse of part of a giant molecular cloud that consisted mostly of hydrogen and
helium and which 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 only
form 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 gases 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 due to conservation of angular momentum and heat up with the increasing
pressure. Much of the mass became concentrated in the center, while the rest
flattened out into a disk which 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 gas from the surrounding disk, eventually triggering nuclear fusion. Thus, our Sun was born.
The 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 neutrinosand solar radiation. At
this rate, the Sun has 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 will expand as
the hydrogen fuel at the core is consumed and the core will contract and heat
up. Hydrogen fusion will continue along a shell surrounding a helium core,
which will steadily expand as more helium is produced. Once the core
temperature reaches around 100 million kelvin, helium fusion at the core will begin producing
carbon, and the Sun will enter the asymptotic giant branch phase.
Following the red giant phase, intense thermal pulsations will cause the Sun to
throw off its outer layers, forming aplanetary 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.
Earth's fate
Earth's ultimate fate is precarious. As
a red giant, the Sun will have a maximum radius beyond the Earth's current
orbit, 1 AU (1.5×1011 m), 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 should escape incineration
in the Sun, still all its water will be boiled away and most of its atmosphere
will escape into space. 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 life on Earth has only existed for about 1
billion years on land. The increase in solar temperatures is such that in about
another billion years the surface of the Earth will likely become too hot for
liquid water to exist, ending all terrestrial life.
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 1,368 W/m2 (watts per
square meter) at a distance of oneastronomical 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 1,000 W/m2 in clear conditions when the Sun is
near the zenith.
Solar energy can be harnessed by 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 bysolar cells are used by solar power equipment to generate electricity or
to do other useful work, sometimes employing concentrating solar power (that
it is measured in suns). The energy stored in petroleum and other fossil fuels was originally
converted from sunlight by photosynthesis in the distant past.
Motion and location within the galaxy
The Sun lies close to the inner rim of
the Milky Way Galaxy's Orion Arm, in the Local Fluffor 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 thegalactic
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, relative to
other nearby stars. The general direction of the Sun's galactic motion is
towards the star Vega in
the constellation of Lyra at
an angle of roughly 60 sky degrees to the direction of the Galactic Center.
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. 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.[115] 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
1,190 years for the Solar System to travel a distance of 1 light-year, or 7
days to travel 1 AU.
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.
Theoretical problems
Solar neutrino problem
For many years the number of solar electron neutrinos detected on Earth was 1⁄3 to 1⁄2 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 in Canada
and the Kamiokande laboratory in Japan
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 1,000,000-2,000,000 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 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 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 currently 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.
-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 by 3%, its temperature by 13%, and its density by 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, thereby increasing the level of cosmic radiation striking the Earth and its atmosphere.
History of observation
Early understanding
Like other natural phenomena, the Sun
has been an object of veneration in many cultures throughout human history.
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
atStonehenge,
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.
In the late Roman Empire 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.
Regarding 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
In the early first millennium BCE, Babylonian astronomers observed
that the Sun's motion along the ecliptic was not uniform, though they were
unaware of why this was; it is today known that this is due to the Earth moving
in an elliptic orbit around the Sun, with the 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 theGreek philosopher Anaxagoras, who
reasoned that it was a giant flaming ball of metal even larger than the Peloponnesus rather than the chariot of Helios, 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 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 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 CE, Ptolemy estimated the distance as 1,210 times
the Earth radius,
approximately 7.71 million kilometers (0.0515 AU).
The theory that the Sun is the center
around which the planets move was first proposed by the ancient Greek Aristarchus of Samosin the 3rd century BCE, and later adopted
by Seleucus of Seleucia (see Heliocentrism).
This largely philosophical view was developed into fully predictive mathematical model of
a heliocentric system 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 telescopic 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.[138] Sunspots were also observed since the Han Dynasty(206
BCE – 220 CE) by Chinese astronomers who maintained records of these
observations for centuries. Averroes also provided a description of
sunspots in the 12th century.
Arabic astronomical 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 transit of Venus was first observed in 1032 by Persian
astronomer and polymathAvicenna,
who concluded that Venus is closer to the Earth than the Sun,[142] while one of the first observations of
the transit of Mercury was
conducted by Ibn Bajjah in the 12th century.
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 1800William 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 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 was a gradually
cooling liquid body that was radiating an internal store of heat. Kelvin and Hermann von Helmholtz then
proposed a gravitational contraction 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 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 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 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 the Earth, and made the first detailed
measurements of the solar wind and the solar magnetic field. Pioneer 9 operated
for a particularly long time, transmitting data until May 1983.
In the 1970s, two Helios spacecraft and theSkylab Apollo Telescope 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
Skylab space station, launched by NASA in 1973, included a solarobservatory 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 UVradiation 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 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 anannular 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 Agency and 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, was launched in
February 2010. 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. Besides
its direct solar observation, SOHO has enabled
the discovery of a large number of comets, mostly 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 travelled 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 ofComet 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 re-entry 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.
The Indian Space Research Organisation has scheduled launch of a 100 kg
satellite named Aditya. The satellite will be launched in 2012, and will study
the dynamic Solar corona.
Observation and effects
The brightness of the sun can cause
pain from looking at it with the naked eye, although
doing so for brief periods is not 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 may result in permanent damage to the
retina without an appropriate filter that blocks UV and substantially dims the
sunlight. An attenuating (ND) 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 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 isscattered 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.
References
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