The 'Sun' () 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 (or approximately 5,515 degrees Celsius / 9,940 Fahrenheit), 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 (135 mi/s), equivalent to one light-year every 1,400 years, and one
AU every 8 days.
[2]
It is currently travelling 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 the primary source of energy to Earth. 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 varied 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
flaress and
prominences, the magnetic interaction between the
chromosphere and the
corona, and the origin (propulsion source) of
solar wind.
Life cycle
Main articles: Formation and evolution of the solar system,
Stellar evolution
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.
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.
Life-cycle of the Sun
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 an averaged-sized star. It contains 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
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 (1/2350
th 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
protons (hydrogen nuclei) are converted into helium nuclei every second (out of about ~8.9 total amount of free protons in Sun), releasing energy at the matter-energy conversion rate of 4.26 million tonnes per second, 383
yottawatts (383 W) or 9.15
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 (micro-watts 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 very rarely interact with matter, so almost all are able to escape the Sun immediately. For many years measurements of the number of neutrinos produced in the Sun were
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 slower than the
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
Structure of the Sun
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 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.
[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 (300 mi) 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 (1,000 mi) 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
chromosphere is a
transition region in which the temperature rises rapidly from around 100,000
K to coronal temperatures closer to one million K. The increase is because of a
phase transition as
helium within the region becomes fully
ionized by the high temperatures. The transition region does not occur at a well-defined altitude. Rather, it forms a kind of
nimbus around chromospheric features such as
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
far 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 merges smoothly with the
solar wind that fills the
solar system and
heliosphere. The low corona, which is very near the surface of the Sun, has a particle density of 10
14 m
−3–10
16 m
−3. (Earth's atmosphere near sea level has a particle density of about 2 m
−3.) The temperature of the corona is several million kelvin. While no complete theory yet exists to account for the temperature of the corona, at least some of its heat is known to be from
magnetic reconnection.
The
heliosphere 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.
[17]
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 a 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 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. 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.
[18] 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 which 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. Like many theories in astrophysics, this theory cannot be tested directly.
[19][20]
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.
[21] 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.
[22] 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. Hence, the problem is now resolved.
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 at a temperature of 1,000,000 K. The high temperature of the corona shows that it is heated by something other than direct heat
conduction from the photosphere.
It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere, and two main mechanisms have been proposed to explain coronal heating. The first is
wave heating, in which sound, gravitational and magnetohydrodynamic waves are produced by turbulence in the convection zone. These waves travel upward and dissipate in the corona, depositing their energy in the ambient gas in the form of heat. The other is
magnetic 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.
[23]
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.
[24] In addition, Alfvén waves do not easily dissipate in the corona. Current research focus has therefore shifted towards flare heating mechanisms. One possible candidate to explain coronal heating is continuous flaring at small scales,
[25] but this remains an open topic of investigation.
Faint young Sun problem
Main articles: Faint young Sun paradox
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 lesser amount of solar energy reaching the planet.
[26]
Magnetic field
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 influence of the Sun's rotating magnetic field on the plasma in the
interplanetary medium creates the
heliospheric current sheet, which separates regions with magnetic fields pointing in different directions. The plasma in the interplanetary medium is also responsible for the strength of the Sun's magnetic field at the orbit of the Earth. If space were a vacuum, then the Sun's 10
-4 tesla magnetic dipole field would reduce with the cube of the distance to about 10
-11 tesla. But satellite observations show that it is about 100 times greater at around 10
-9 tesla.
Magnetohydrodynamic (MHD) theory predicts that the motion of a conducting fluid (e.g., the interplanetary medium) in a magnetic field, induces electric currents which in turn generates magnetic fields, and in this respect it behaves like an
MHD dynamo.
History of solar observation
Early understanding of the Sun
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, and
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
solstice (some of the most prominent megaliths are located in
Nabta Playa,
Egypt, and at
Stonehenge in
England); 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. With respect to the
fixed stars, the Sun appears from Earth to revolve once a year along the
ecliptic through the
zodiac, and so the Sun was considered by Greek astronomers 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 modern scientific understanding
One of the first people to offer a scientific 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 later released through the intervention of
Pericles).
Eratosthenes might have been the first person to have accurately calculated the distance from the Earth to the Sun, in the 3rd century
BCE, as 149 million kilometers, roughly the same as the modern accepted figure.
The theory that the Sun is the center around which the planets move was apparently proposed by the ancient Greek
Aristarchus and Indians (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 and other astronomers. Galileo made some of the first known 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.
[28]
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,
[29] while in 1800
William Herschel discovered
infrared radiation beyond the red part of the solar spectrum.
[30] 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.
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.
[31] 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 several billion years suggested by geology. In 1890
Joseph Lockyer, who discovered helium in the solar spectrum, proposed a meteoritic hypothesis for the formation and evolution of the Sun.
[32]
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.
[33] 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'' = ''mc''².
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.
[34] 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.
[35][36]
Finally, a seminal paper was published in 1957 by
Margaret Burbidge, entitled "Synthesis of the Elements in Stars".
[37] The paper demonstrated convincingly that most of the elements in the universe had been
synthesized by nuclear reactions inside stars, some like our Sun. This revelation stands today as one of the great achievements of science.
Solar space missions
Solar "
fireworks" in sequence as recorded in November 2000 by four instruments onboard the
SOHO spacecraft.
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 period of time, transmitting data until
1987.
[38]
In the 1970s,
Helios 1 and the
Skylab Apollo Telescope Mount provided scientists with significant new data on solar wind and the solar corona. The Helios 1 satellite 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. 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.
[39]
Japan's
Yohkoh (''Sunbeam'') satellite, launched in 1991, 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.
[40]
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
December 2,
1995. Originally a two-year mission, SOHO has now operated for over ten years (
as of 2007). It has proved so useful that a follow-on mission, the
Solar Dynamics Observatory, is planned for launch in 2008. 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.
[41]
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 (450 mi/s—slower than expected), and that there were large magnetic waves emerging from high latitudes which scattered galactic
cosmic rays.
[42]
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.
If one were to observe it from
Alpha Centauri, the closest star system, the Sun would appear to be in the constellation
Cassiopeia.
Sun observation and eye damage
The Sun as it appears through a camera
lens from the surface of Earth
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
[43][44]. 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
[45][46].
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.
[47] 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
[48][49]; 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
attenuating (ND) filter to dim the sunlight. 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.
[50] One way to view the Sun safely is by projecting its image onto a screen using telescope and eyepiece without cemented elements. This should only be done with a small refracting telescope (or binoculars) with a clean eyepiece. Other kinds of telescope can be damaged by this procedure.
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.
[51] 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 direct 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.
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. Filters on telescopes or binoculars should be on the
objective lens or
aperture, ''never'' on the
eyepiece, because eyepiece filters can suddenly crack or shatter due to high heat loads from the absorbed sunlight. Welding glass #14 is an acceptable solar filter, but "black" exposed photographic film is not (it passes too much infrared).
Solar cultural history
Like other natural phenomena, the Sun has been an object of veneration in many cultures throughout human history. 'Sol' () was the name of the Sun in
Latin. The Latin name is widely known, but not common in general English language usage, although the related adjective ''solar'' is more common. 'Sol' is more frequently used in
science fiction writing (''
Star Trek'' in particular) as a formal name for the specific
star, since in many stories the local sun is a different star and thus the generic term "the sun" would be ambiguous. By extension, the
Solar System is often referred to in science fiction as the "Sol System".
The term ''sol'' is used by planetary astronomers to refer to the duration of a
solar day on
Mars.
[52] A mean Earth solar day is approximately 24 hours. A mean Martian solar day, or "sol", is 24 hours, 39 minutes, and 35.244 seconds.
[53] See also
Timekeeping on Mars.
''Sol'' is also the modern word for "Sun" in
Portuguese,
Spanish,
Icelandic,
Danish,
Norwegian,
Swedish,
Catalan and
Galician. 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).
Also "Sol" in
Persian means a
solar year.
See also
★
List of Solar Cycles
★
List of Solar System bodies formerly regarded as planets
★
Formation and evolution of the Solar System
★
The Sun in human culture
★
Ecliptic
★
Solar deity
★
Stellar classification
★
Solar power
★
Energy Independence
★
Advanced Composition Explorer
References
1. Astronomers Had it Wrong: Most Stars are Single Ker Than
2. Review of galactic constants, , F. J., Kerr, Monthly Notices of the Royal Astronomical Society, 1986
3. Are supernovae sources of presolar grains?, , S. W., Falk, Nature, 1977
4. Barsh G.S., 2003, ''What Controls Variation in Human Skin Color?'', PLoS Biology, v. 1, p. 19
5. The age of the Sun and the relativistic corrections in the EOS, , A., Bonanno, Astronomy and Astrophysics, 2002
6. According to isotopicAges, the Ca-Al-I's (= Ca-Al-rich inclusions) here formed in a proplyd (= protoplanetary disk]). The page Protoplanetary disk says that proplyds are never older than 25 Ma. If 4567 Ma is given for the age of the Earth, then 4567 + 25 = 4592. But 25 Ma is the "maximum age" of proplyds. If proplyds slowly decay from the influence of the Sun and from planetesimal formation, then most Ca-Al-I's must have been formed some time within the range of 0 Ma and 25 Ma after the formation of the proplyd. If the median of Ca-Al-I ages are about 10 Ma after the proplyd formation, then we get 4565 + 10 = 4575, but this figure is created by speculating twice. Since it is assumed that planetary formation occurs over a period of about 100,000 years, that is the date given here
7. The Once and Future Sun Pogge, Richard W.
8. Our Sun. III. Present and Future, , I.-Juliana, Sackmann, Astrophysical Journal, 1993
9. The solar oblateness and its relationship with the structure of the tachocline and of the Sun's subsurface, , S., Godier, Astronomy and Astrophysics, 2000
10. From Core to Corona: Layers of the Sun Hannah Cohen
11. Garcia R. A. et al. "Tracking Solar Gravity Modes: The Dynamics of the Solar Core", ''Science'', '316', 5831, 1591 - 1593 (2007)
12. The Illustrated Encyclopedia of the Universe, , Richard, Lewis, Harmony Books, New York, 1983,
13. Bitesize Tour of the Solar System: The Long Climb from the Sun's Core
14. The Quiet Sun, , Edward G., Gibson, NASA, 1973,
15. The Physics of Astrophysics, , Frank H., Shu, University Science Books, 1991,
16. Discovery of Helium
17. The Distortion of the Heliosphere: our Interstellar Magnetic Compass European Space Agency
18. Estimating the Sun's radiative output during the Maunder Minimum, , J., Lean, Geophysical Research Letters, 1992
19.
Solar Resonant Diffusion Waves as a Driver of Terrestrial Climate Change, , Robert, Ehrlich, Journal of Atmospheric and Solar-Terrestrial Physics, 2007
20. Sun's fickle heart may leave us cold, , , , New Scientist, 2007
21. The Solar Neutrino Problem, , W. C., Haxton, Annual Review of Astronomy and Astrophysics, 1995
22. Three-flavor oscillation solutions for the solar neutrino problem, , H., Schlattl, Physical Review D, 2001
23. Magneto-hydrodynamic waves, and the heating of the solar corona, , H., Alfvén, Monthly Notices of the Royal Astronomical Society, 1947
24. Coronal heating by stochastic magnetic pumping, , P. A., Sturrock, Astrophysical Journal, 1981
25. Nanoflares and the solar X-ray corona, , E. N., Parker, Astrophysical Journal, 1988
26. Climatic Consequences of Very High Carbon Dioxide Levels in the Earth’s Early Atmosphere, , J. F., Kasting, Science, 1986
27. The Mean Magnetic Field of the Sun
28. Galileo Galilei (1564–1642)
29. Sir Isaac Newton (1643–1727)
30. Herschel Discovers Infrared Light
31. On the Age of the Sun’s Heat, , Sir William, Thomson, Macmillan's Magazine, 1862
32. The meteoritic hypothesis; a statement of the results of a spectroscopic inquiry into the origin of cosmical systems, , Joseph Norman, Lockyer, Macmillan and Co., 1890,
33. The Nature of Scientific Inquiry
34. Studying the stars, testing relativity: Sir Arthur Eddington
35. On the Formation of Deuterons by Proton Combination, , H., Bethe, Physical Review, 1938
36. Energy Production in Stars, , H., Bethe, Physical Review, 1939
37. Synthesis of the Elements in Stars, E. Margaret Burbidge; G. R. Burbidge; William A. Fowler; F. Hoyle, , , Reviews of Modern Physics, 1957
38. Pioneer 6-7-8-9-E
39. Solar Maximum Mission Overview
40. Result of Re-entry of the Solar X-ray Observatory "Yohkoh" (SOLAR-A) to the Earth's Atmosphere Japan Aerospace Exploration Agency
41. SOHO Comets
42. Ulysses - Science - Primary Mission Results
43. Chorioretinal temperature increases from solar observation, T.J. White, M.A. Mainster, P.W. Wilson, and J.H.Tips, , , Bulletin of Mathematical Biophysics, 1971
44. The Human Fovea After Sungazing, "M.O.M. Tso and F.G. La Piana, , , Transactions of the American Academy of Ophthalmology & Otolaryngology, 1975
45. Ultrastructural findings in solar retinopathy, Hopeross, M. W., , , , 1993
46. Solar Retinopathy from Sun-Gazing Under Influence of LSD, Schatz, H. & Mendelbl, F., , , , 1973
47. Eye Safety During Solar Eclipses, Chou, B. Ralph, MSc, OD, , , , 1997 "''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.''"
48. Retinal sensitivity to damage from short wavelength light, W.T. Ham Jr., H.A. Mueller, and D.H. Sliney, , , Nature,
49. Solar Retinopathy as a function of Wavelength: its Significance for Protective Eyewear, W.T. Ham Jr., H.A. Mueller, J.J. Ruffolo Jr., and D. Guerry III, , , "The Effects of Constant Light on Visual Processes", edited by T.P. Williams and B.N. Baker, 1980
50. Observing the Sun in Safety, , J. C. D., Marsh, J. Brit. Ast. Assoc., 1982
51. Eye Safety During Solar Eclipses - adapted from NASA RP 1383 Total Solar Eclipse of 1998 February 26, April 1996, p. 17
52. Opportunity's View, Sol 959 (Vertical)
53. Technical Notes on Mars Solar Time as Adopted by the Mars24 Sunclock
★ Thompson, M. J. (2004), ''Solar interior: Helioseismology and the Sun's interior'', Astronomy & Geophysics, v. 45, p. 4.21-4.25
★ T. J. White; M. A. Mainster; P. W. Wilson; and J. H. Tips, ''Chorioretinal temperature increases from solar observation'', Bulletin of Mathematical Biophysics 33, 1–17 (1971)
External links
★
Current SOHO snapshots
★
Far-Side Helioseismic Holography from
Stanford
★
NASA Eclipse homepage
★
Nasa SOHO (Solar & Heliospheric Observatory) satellite FAQ
★
Sun Profile by
NASA's Solar System Exploration
★
Solar Sounds from
Stanford
★
Spaceweather.com
★
Eric Weisstein's World of Astronomy - Sun
★
The Position of the Sun
★
A collection of solar movies
★
The Institute for Solar Physics- Movies of Sunspots and spicules
★
NASA/Marshall Solar Physics website
★
Solar Position Algorithm and from the
National Renewable Energy Laboratory
★
libnova — a celestial mechanics and astronomical calculation library
★
MySolarSystem.com: Information and Pictures of the Sun
★
NASA Podcast
★
National Solar Observatory
★
Illustration comparing the size of the Sun with the solar system planets and with other stars
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