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Sun|+ The Sun |+
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| colspan="2" align="center" | |-
! bgcolor="#ffffc0" colspan="2" align="center" | Observation data
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! align="left" | Mean distance from Earth
| 149.6 km
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! align="left" | Visual brightness
| −26.8m
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! align="left" | Absolute magnitude
| 4.8m
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! align="left" | Spectral classification
| G2V
|-
! bgcolor="#ffffc0" colspan="2" align="center" | Orbital characteristics
|-
! align="left" | Mean distance from Milky Way core
| ~2.5 km
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Timeline
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1275 Chinese astronomers observe a total eclipse of the Sun in China on March 4.
1735 Mathematical calculations suggest it was on this day that Pluto moved closer to the Sun than Neptune for the last time before 1979.
1769 James Cook arrives in Tahiti on the ship HM Bark ''Endeavour'', preparing to observe the solar eclipse of the planet Venus, which took place on June 3rd. After the voyage, the data was found to be inaccurate in determining the distance between the Sun and Earth.
1836 Francis Baily, during an eclipse of the sun, observes the phenomenon named after him as Baily's beads.
1859 Solar flares first observed on the Sun by English astronomer Richard Carrington.
1866 Calculations indicate Pluto reached its most recent aphelion (furthest point from Sun) on this day. The next aphelion will occur in ''August 2113''.
1982 Syzygy: all nine planets align on the same side of the Sun.
1994 An annular eclipse of the sun is visible across much of North America.
1999 Pluto, a dwarf planet with an eccentric orbit, moves further from the Sun than Neptune. It had been nearer than Neptune since 1979, and will become again in ''2231''.
2000 A rare conjunction occurs on the New Moon, including all seven of the traditional celestial bodies known from ancient times up until 1781 with the discovery of Uranus. The May 2000 conjunction consisted of: the Sun and Moon, Mercury, Venus, Mars, Jupiter, and Saturn.
More Events >>
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Encyclopedia
>|+
|+ The Sun
|+
|-
| colspan="2" align="center" |
|-
! bgcolor="#ffffc0" colspan="2" align="center" | Observation data
|-
! align="left" | Mean distance from
Earth
| 149.6 km
|-
! align="left" | Visual brightness
| −26.8m
|-
! align="left" | Absolute magnitude
| 4.8m
|-
! align="left" | Spectral classification
| G2V
|-
! bgcolor="#ffffc0" colspan="2" align="center" | Orbital characteristics
|-
! align="left" | Mean distance from
Milky Way core
| ~2.5 km
|-
! align="left" | Galactic period
| 2.25-2.50 a
|-
! align="left" | Velocity
| 217 km/s orbit around the center of the Galaxy, 20 km/s relative to average velocity of other stars in stellar neighborhood
|-
! bgcolor="#ffffc0" colspan="2" align="center" | Physical characteristics
|-
! align="left" | Mean diameter
| 1.392 km
|-
! align="left" | Circumference
| 4.373 km
|-
! align="left" | Oblateness
| 9
|-
! align="left" | Surface area
| 6.09 km²
|-
! align="left" | Volume
| 1.41 km³
|-
! align="left" | Mass
| 1.988 435 kg
|-
! align="left" | Density
| 1.408 g/cm³
|-
! align="left" | Surface gravity
| 273.95 m s-2
|-
! align="left" | Escape velocity
from the surface
| 617.54 km/s
|-
! align="left" | Surface temperature
| 5785 K
|-
! align="left" | Temperature of corona
| 5 MK
|-
! align="left" | Core temperature
| ~13.6 MK
|-
! align="left" | Luminosity
| 3.827 W
~9 cd
~3 cd
|-
! align="left" | Mean Intensity
| 2.009 W m-2 sr-1
|-
! bgcolor="#ffffc0" colspan="2" align="center" | Rotation characteristics
|-
! align="left" | Obliquity
| 7.25°
67.23°
|-
! align="left" | Right ascension
of North pole
| 286.13°
|-
! align="left" | Declination
of North pole
| +63.87°
|-
! align="left" | Rotation period
at equator
| 25.3800 days
The Sun orbits the center of the Milky Way galaxy at a distance of about 25,000 to 28,000 light-years from the galactic center, completing one revolution in about 225–250 million years. The orbital speed is 217 km/s, equivalent to one light-year every 1,400 years, and one AU every 8 days.
The Sun is a third generation star, whose formation may have been triggered by shockwaves from a nearby supernova. 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 main source of energy near the surface of 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 . 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 , 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.
Sunlight has several interesting biological properties. Ultraviolet light from the Sun has antiseptic properties and can be used to sterilize tools. It also causes sunburn, and has other medical effects such as the production of Vitamin D. Ultraviolet light is strongly attenuated by Earth's atmosphere, so that the amount of UV varies greatly with latitude because of the longer passage of sunlight through the atmosphere at high latitudes. This variation is responsible for many biological adaptations, including variations in human skin color in different regions of the globe.
Observed from Earth, the path of the Sun 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 , there is an East/West component as well. The North/South swing in apparent angle is the main source of seasons on Earth.
Life cycleThe Sun will spend a total of approximately 10 billion years as a main sequence star. Its current age, determined using computer models of stellar evolution and nucleocosmochronology, is thought to be about 4.57 billion years.
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 about 3 K. 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 and most of the atmosphere will be boiled away.
Following the red giant phase, intense thermal pulsations will cause the Sun to throw off its outer layers, forming a planetary nebula. The Sun will then evolve into a white dwarf, slowly cooling over eons. This stellar evolution scenario is typical of low- to medium-mass stars.
Structure
While 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, which means that its polar diameter differs from its equatorial diameter by only 10 km. While the Sun does not rotate as a solid body , it takes approximately 28 days to complete one full rotation; the centrifugal effect of this slow rotation is 18 million times weaker than the surface gravity at the Sun's equator. Tidal effects from the planets do not significantly affect the shape of the Sun, although the Sun itself orbits the center of mass of the solar system, which is located nearly a solar radius away from the center of the Sun mostly because of the large mass of Jupiter.
The Sun does not have a definite boundary as rocky planets do; 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 below which the gases are thick enough to be opaque but above which they are transparent; the photosphere is the surface most readily visible to the naked eye. Most of the Sun's mass lies within about 0.7 radii of the center.
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 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.
CoreThe 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 and a temperature of close to 15,000,000 Kelvins . Energy is produced by exothermic thermonuclear reactions that mainly convert hydrogen into helium, helium into carbon, carbon into iron. 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 8.9 protons are converted into helium nuclei every second, releasing energy at the matter-energy conversion rate of 4.26 million tonnes per second, 383 yottawatts or 9.15 megatons of TNT per second. The rate of nuclear fusion depends strongly on density, 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 released in fusion reactions take a long time to reach the Sun's surface, slowed down by the indirect path taken, as well as by constant absorption and reemission at lower energies in the solar mantle. Estimates of the "photon travel time" range from as much as 50 million years to as little as 17,000 years. After a final trip through the convective outer layer to the transparent "surface" of the photosphere, the photons escape as visible light. Each gamma ray in the Sun's core is converted into several million visible light photons before escaping into space. Neutrinos are also released by the fusion reactions in the core, but unlike photons they 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 much lower than theories predicted, a problem which was recently resolved through a better understanding of the effects of neutrino oscillation.
Radiation zoneFrom 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.
Convection zone
From about 0.7 solar radii to the Sun's visible surface, the material in the Sun 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 of the Sun. Once the material cools off at the surface, it plunges back downward to the base of the convection zone, to receive more heat from the top of the radiative zone. Convective overshoot is thought to occur at the base of the convection zone, carrying turbulent downflows into the outer layers of the radiative 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.
PhotosphereThe 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 because of the decreasing overall particle density: the photosphere is actually tens to hundreds of kilometers thick, being slightly less opaque than air on Earth. Sunlight has approximately a black-body spectrum that indicates its temperature is about 6,000 K , interspersed with atomic absorption lines from the tenuous layers above the photosphere. The photosphere has a particle density of about 1023 m−3 .
During early studies of the optical spectrum of the photosphere, some absorption lines were found that did not correspond to any chemical elements then known on Earth. In 1868, Norman Lockyer hypothesized that these absorption lines were because of a new element which he dubbed "helium", after the Greek Sun god Helios. It was not until 25 years later that helium was isolated on Earth.
Atmosphere
The parts of the Sun above the photosphere are referred to collectively as the solar atmosphere. They can be viewed with telescopes operating across the electromagnetic spectrum, from radio through visible light to gamma rays, and comprise five principal zones: the temperature minimum, the chromosphere, the transition region, the corona, and the heliosphere. The heliosphere, which may be considered the tenuous outer atmosphere of the Sun, extends outward past the orbit of Pluto to the heliopause, where it forms a sharp shock front boundary with the interstellar medium. The chromosphere, transition region, and corona are much hotter than the surface of the Sun; the reason why is not yet known.
The coolest layer of the Sun is a temperature minimum region about 500 km above the photosphere, with a temperature of about 4,000 K. This part of the Sun is cool enough to support simple molecules such as carbon monoxide and water, which can be detected by their absorption spectra.
Above the temperature minimum layer is a thin layer about 2,000 km thick, dominated by a spectrum of emission and absorption lines. It is called the chromosphere from the Greek root chroma, meaning color, because the chromosphere is visible as a colored flash at the beginning and end of total eclipses of the Sun. The temperature in the chromosphere increases gradually with altitude, ranging up to around 100,000 K near the top.
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 1014 m−3–1016 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 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.
Solar activity
Sunspots and the solar 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 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 10-12 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.
The solar cycle has a great influence on space weather, and seems also to have a strong influence on the Earth's climate. Solar minima tend to be correlated with colder temperatures, and longer than average solar cycles tend to be correlated with hotter temperatures. In the 17th century, the solar cycle appears to have stopped entirely for several decades; very few sunspots were observed during this period. During this era, which is known as the Maunder minimum or Little Ice Age, Europe experienced very cold temperatures. Earlier extended minima have been discovered through analysis of tree rings and also appear to have coincided with lower-than-average global temperatures.
Effects on EarthSolar activity has several effects on the Earth and its surroundings. Because the Earth has a magnetic field, charged particles from the solar wind cannot impact the atmosphere directly, but are instead deflected by the magnetic field and aggregate to form the Van Allen belts. The Van Allen belts consist of an inner belt composed primarily of protons and an outer belt composed mostly of electrons. Radiation within the Van Allen belts can occasionally damage satellites passing through them.
The Van Allen belts form arcs around the Earth with their tips near the north and south poles. The most energetic particles can 'leak out' of the belts and strike the Earth's upper atmosphere, causing auroras, known as aurorae borealis in the northern hemisphere and aurorae australis in the southern hemisphere. In periods of normal solar activity, aurorae can be seen in oval-shaped regions centered on the magnetic poles and lying roughly at a geomagnetic latitude of 65°, but at times of high solar activity the auroral oval can expand greatly, moving towards the equator. Aurorae borealis have been observed from locales as far south as Mexico.
Theoretical problems
Solar neutrino problem
For many years the number of solar electron neutrinos detected on Earth was only a third of the number expected, according to theories describing the nuclear reactions in the Sun. This anomalous result was termed the solar neutrino problem. Theories proposed to resolve the problem either tried to reduce the temperature of the Sun's interior to explain the lower neutrino flux, or posited that electron neutrinos could oscillate, that is, change into undetectable tau and muon neutrinos as they traveled between the Sun and the Earth. Several neutrino observatories were built in the 1980s to measure the solar neutrino flux as accurately as possible, including the Sudbury Neutrino Observatory and Kamiokande. Results from these observatories eventually led to the discovery that neutrinos have a very small rest mass and can indeed oscillate.. Moreover, 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 only one-third of the neutrinos seen at Earth were of the electron type.
Coronal heating problemThe optical surface of the Sun 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.
Currently, it is unclear whether waves are an efficient heating mechanism. All waves except Alfven 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. One possible candidate to explain coronal heating is continuous flaring at small scales, but this remains an open topic of investigation.
Faint young sun problem
Theoretical models of the sun's development suggest that 3.8 to 2.5 billion years ago, during the Archean period, the Sun was only about 75% as bright as it is today. Such a weak star would not have been able to sustain liquid water on the Earth's surface, and thus life should not have been able to develop. However, the geological record demonstrates that the Earth has remained at a fairly constant temperature throughout its history, and in fact that the young Earth was somewhat warmer than it is today. The general consensus among scientists is that the young Earth's atmosphere contained much larger quantities of greenhouse gases than are present today, which trapped enough heat to compensate for the lesser amount of solar energy reaching the planet.
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 than it does at higher latitudes . The differential rotation of the Sun's latitudes causes its magnetic field lines to become twisted together over time, causing magnetic field lo |
