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UPDATE 09-12-06

NASA Aids in Resolving Long Standing Solar Cycle Mystery

Scientists predict the next solar activity cycle will be 30 to 50 percent stronger than the previous one and up to a year late. Accurately predicting the sun's cycles will help plan for the effects of solar storms. The storms can disrupt satellite orbits and electronics; interfere with radio communication; damage power systems; and can be hazardous to unprotected astronauts.

The breakthrough "solar climate" forecast by Mausumi Dikpati and colleagues at the National Center for Atmospheric Research in Boulder, Colo. was made with a combination of computer simulation and groundbreaking observations of the solar interior from space using NASA's Solar and Heliospheric Observatory (SOHO). NASA's Living With a Star program and the National Science Foundation funded the research.

The sun goes through a roughly 11-year cycle of activity, from stormy to quiet and back again. Solar storms begin with tangled magnetic fields generated by the sun's churning electrically charged gas (plasma). Like a rubber band twisted too far, solar magnetic fields can suddenly snap to a new shape, releasing tremendous energy as a flare or a coronal mass ejection (CME). This violent solar activity often occurs near sunspots, dark regions on the sun caused by concentrated magnetic fields.

Understanding plasma flows in the sun's interior is essential to predicting the solar activity cycle. Plasma currents within the sun transport, concentrate, and help dissipate solar magnetic fields. "We understood these flows in a general way, but the details were unclear, so we could not use them to make predictions before," Dikpati said. Her paper about this research was published in the March 3 online edition of Geophysical Research Letters.

The new technique of "helioseismology" revealed these details by allowing researchers to see inside the sun. Helioseismology traces sound waves reverberating inside the sun to build up a picture of the interior, similar to the way an ultrasound scan is used to create a picture of an unborn baby.

Two major plasma flows govern the cycle. The first acts like a conveyor belt. Deep beneath the surface, plasma flows from the poles to the equator. At the equator, the plasma rises and flows back to the poles, where it sinks and repeats. The second flow acts like a taffy pull. The surface layer of the sun rotates faster at the equator than it does near the poles. Since the large-scale solar magnetic field crosses the equator as it goes from pole to pole, it gets wrapped around the equator, over and over again, by the faster rotation there. This is what periodically concentrates the solar magnetic field, leading to peaks in solar storm activity.

"Precise helioseismic observations of the 'conveyor belt' flow speed by the Michelson Doppler Imager (MDI) instrument on board SOHO gave us a breakthrough," Dikpati said. "We now know it takes two cycles to fill half the belt with magnetic field and another two cycles to fill the other half. Because of this, the next solar cycle depends on characteristics from as far back as 40 years previously - the sun has a magnetic 'memory'."

The magnetic data input comes from the SOHO/MDI instrument and historical records. Computer analysis of the past eight years' magnetic data matched actual observations over the last 80 years. The team added magnetic data and ran the model ahead 10 years to get their prediction for the next cycle. The sun is in the quiet period for the current cycle (cycle 23).

The team predicts the next cycle will begin with an increase in solar activity in late 2007 or early 2008, and there will be 30 to 50 percent more sunspots, flares, and CMEs in cycle 24. This is about one year later than the prediction using previous methods, which rely on such statistics as the strength of the large-scale solar magnetic field and the number of sunspots to make estimates for the next cycle. This work will be advanced by more detail observations from the Solar Dynamics Observatory, scheduled to launch in August 2008. more

The Sun is the star of our solar system. The Earth and other matter (including other planets, asteroids, meteoroids, comets and dust) orbit the Sun, which by itself accounts for more than 99% of the solar system's mass. Energy from the Sun—in the form of insolation from sunlight—supports almost all life on Earth via photosynthesis, and drives the Earth's climate and weather.

The Sun is sometimes referred to by its Latin name Sol or by its Greek name Helios. Its astrological and astronomical symbol is a circle with a point at its center: \bigodot. Some ancient peoples of the world considered it a planet.

One of the first people in the Western world 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 3th century BCE, as 149 million kilometers, roughly the same as the modern accepted figure.

Another scientist to challenge the accepted view was Nicolaus Copernicus, who in the 16th century developed the theory that the Earth orbited the Sun, rather than the other way around. In the early 17th century, Galileo pioneered telescopic observations of the Sun, making some of the first known observations of sunspots and positing that they were on the surface of the Sun rather than small objects passing between the Earth and the Sun. Isaac Newton observed the Sun's light using a prism, and showed that it was made up of light of many colors, while in 1800 William Herschel discovered infrared radiation beyond the red part of the solar spectrum. The 1800s saw spectroscopic studies of the Sun advance, and Joseph von Fraunhofer made the first observations of absorption lines in the spectrum, the strongest of which are still often referred to as Fraunhofer lines.

In the early years of the modern scientific era, the source of the Sun's energy was a significant puzzle. Lord Kelvin suggested that the Sun was a gradually cooling liquid body that was radiating an internal store of heat. Kelvin and Hermann von Helmholtz then proposed the Kelvin-Helmholtz mechanism to explain the energy output. Unfortunately the resulting age estimate was only 20 million years, well short of the time span of several billion years suggested by geology. In 1890 Joseph Lockyer, the discoverer of helium in the solar spectrum, proposed a meteoritic hypothesis for the formation and evolution of the sun. Another proposal was that the Sun extracted its energy from friction of its gas masses.

It would be 1904 before a potential solution was offered. Ernest Rutherford suggested that the energy 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 a Sun's energy with his mass-energy 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 into helium, resulting in a production of energy from the net change in mass. This theoretical concept 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, in 1957, a paper titled Synthesis of the Elements in Stars was published that demonstrated convincingly that most of the elements in the universe had been created by nuclear reactions inside stars like the Sun.

The Sun (and therefore the Earth and Solar System) may be found close to the inner rim of the Orion Arm, in the Local Fluff, at a distance of 7.94±0.42 kpc from the Galactic Center. The distance between the local arm and the next arm out, the Perseus Arm, is about 6,500 light-years.Our Sun, and thus the solar system, is found in what scientists call the galactic habitable zone.

The Apex of the Sun's Way, or the solar apex, refers to the direction that the Sun travels through space in the Milky Way. The general direction of the sun's galactic motion is towards the star Vega near the constellation of Hercules, at an angle of roughly 86 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.

It takes the solar system about 225-250 million years to complete one orbit (a galactic year), and so is thought to have completed about 20-25 orbits during its lifetime or .0008 orbit since the origin of man. The orbital speed of the solar system is 217 km/s, i.e. 1 light-year in ca. 1400 years, and 1 AU in 8 days. The Sun is about 4.6 billion years old and 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 tons of matter are converted into energy within the Sun's core, producing neutrinos and solar radiation.

In about 5 billion years, the Sun will evolve into a red giant and then a white dwarf, creating a planetary nebula in the process.

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. 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 the solar wind that carry material through the solar system. The 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, and strongly affects 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 a million K while its visible surface (the photosphere) has a temperature of just 6,000 K. Current topics of scientific enquiry include the sun's regular cycle of sunspot activity, the physics and origin of solar flares and prominences, the magnetic interaction between the chromosphere and the corona, and the origin of the solar wind.

Our Sun is a normal main-sequence G2 star, one of more than 100 billion stars in our galaxy.

diameter: 1,390,000 km.
mass: 1.989e30 kg
temperature: 5800 K (surface)
15,600,000 K (core)

The Sun is by far the largest object in the solar system. It contains more than 99.8% of the total mass of the Solar System (Jupiter contains most of the rest).

It is often said that the Sun is an "ordinary" star. That's true in the sense that there are many others similar to it. But there are many more smaller stars than larger ones; the Sun is in the top 10% by mass. The median size of stars in our galaxy is probably less than half the mass of the Sun.

The Sun is personified in many mythologies: the Greeks called it Helios and the Romans called it Sol.

The Sun is, at present, about 70% hydrogen and 28% helium by mass everything else ("metals") amounts to less than 2%. This changes slowly over time as the Sun converts hydrogen to helium in its core.

The outer layers of the Sun exhibit differential rotation: at the equator the surface rotates once every 25.4 days; near the poles it's as much as 36 days. This odd behavior is due to the fact that the Sun is not a solid body like the Earth. Similar effects are seen in the gas planets. The differential rotation extends considerably down into the interior of the Sun but the core of the Sun rotates as a solid body.

Conditions at the Sun's core (approximately the inner 25% of its radius) are extreme. The temperature is 15.6 million Kelvin and the pressure is 250 billion atmospheres. At the center of the core the Sun's density is more than 150 times that of water.

The Sun's energy output (3.86e33 ergs/second or 386 billion billion megawatts) is produced by nuclear fusion reactions. Each second about 700,000,000 tons of hydrogen are converted to about 695,000,000 tons of helium and 5,000,000 tons (=3.86e33 ergs) of energy in the form of gamma rays. As it travels out toward the surface, the energy is continuously absorbed and re-emitted at lower and lower temperatures so that by the time it reaches the surface, it is primarily visible light. For the last 20% of the way to the surface the energy is carried more by convection than by radiation.

The surface of the Sun, called the photosphere, is at a temperature of about 5800 K. Sunspots are "cool" regions, only 3800 K (they look dark only by comparison with the surrounding regions). Sunspots can be very large, as much as 50,000 km in diameter. Sunspots are caused by complicated and not very well understood interactions with the Sun's magnetic field.

A small region known as the chromosphere lies above the photosphere.

The highly rarefied region above the chromosphere, called the corona, extends millions of kilometers into space but is visible only during a total solar eclipse (left). Temperatures in the corona are over 1,000,000 K.

It just happens that the Moon and the Sun appear the same size in the sky as viewed from the Earth. And since the Moon orbits the Earth in approximately the same plane as the Earth's orbit around the Sun sometimes the Moon comes directly between the Earth and the Sun. This is called a solar eclipse; if the alignment is slighly imperfect then the Moon covers only part of the Sun's disk and the event is called a partial eclipse. When it lines up perfectly the entire solar disk is blocked and it is called a total eclipse of the Sun. Partial eclipses are visible over a wide area of the Earth but the region from which a total eclipse is visible, called the path of totality, is very narrow, just a few kilometers (though it is usually thousands of kilometers long). Eclipses of the Sun happen once or twice a year. If you stay home, you're likely to see a partial eclipse several times per decade. But since the path of totality is so small it is very unlikely that it will cross you home. So people often travel half way around the world just to see a total solar eclipse. To stand in the shadow of the Moon is an awesome experience. For a few precious minutes it gets dark in the middle of the day. The stars come out. The animals and birds think it's time to sleep. And you can see the solar corona. It is well worth a major journey.

The Sun's magnetic field is very strong (by terrestrial standards) and very complicated. Its magnetosphere (also known as the heliosphere) extends well beyond Pluto.

In addition to heat and light, the Sun also emits a low density stream of charged particles (mostly electrons and protons) known as the solar wind which propagates throughout the solar system at about 450 km/sec. The solar wind and the much higher energy particles ejected by solar flares can have dramatic effects on the Earth ranging from power line surges to radio interference to the beautiful aurora borealis.

Recent data from the spacecraft Ulysses show that during the minimum of the solar cycle the solar wind emanating from the polar regions flows at nearly double the rate, 750 kilometers per second, that it does at lower latitudes. The composition of the solar wind also appears to differ in the polar regions. During the solar maximum, however, the solar wind moves at an intermediate speed.

Further study of the solar wind will be done by the recently launched Wind, ACE and SOHO spacecraft from the dynamically stable vantage point directly between the Earth and the Sun about 1.6 million km from Earth.

The solar wind has large effects on the tails of comets and even has measurable effects on the trajectories of spacecraft.

Spectacular loops and prominences are often visible on the Sun's limb (left).

The Sun's output is not entirely constant. Nor is the amount of sunspot activity. There was a period of very low sunspot activity in the latter half of the 17th century called the Maunder Minimum. It coincides with an abnormally cold period in northern Europe sometimes known as the Little Ice Age. Since the formation of the solar system the Sun's output has increased by about 40%.

The Sun is about 4.5 billion years old. Since its birth it has used up about half of the hydrogen in its core. It will continue to radiate "peacefully" for another 5 billion years or so (although its luminosity will approximately double in that time). But eventually it will run out of hydrogen fuel. It will then be forced into radical changes which, though commonplace by stellar standards, will result in the total destruction of the Earth (and probably the creation of a planetary nebula).

The Sun's satellites
There are eight planets and a large number of smaller objects orbiting the Sun. (Exactly which bodies should be classified as planets and which as "smaller objects" has been the source of some controversy, but in the end it is really only a matter of definition. Pluto is no longer officially a planet but we'll keep it here for history's sake.)

Distance Radius Mass
Planet (000 km) (km) (kg) Discoverer Date
--------- --------- ------ ------- ---------- -----
Mercury 57,910 2439 3.30e23
Venus 108,200 6052 4.87e24
Earth 149,600 6378 5.98e24
Mars 227,940 3397 6.42e23
Jupiter 778,330 71492 1.90e27
Saturn 1,426,940 60268 5.69e26
Uranus 2,870,990 25559 8.69e25 Herschel 1781
Neptune 4,497,070 24764 1.02e26 Galle 1846
Pluto 5,913,520 1160 1.31e22 Tombaugh 1930

About 74% of the Sun's mass is hydrogen, 25% is helium, and the rest is made up of trace quantities of heavier elements. Because of this, there are no craters on the sun, as it is entirely made up of gas. The Sun has a spectral class of G2V. "G2" means that it has a surface temperature of approximately 5,500 K, giving it a white color, which because of atmospheric scattering appears yellow. Its spectrum contains lines of ionized and neutral metals as well as very weak hydrogen lines. The "V" 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 balance, 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.[2] The 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 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 220 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.

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 3×108 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.

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 (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.

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 (because of the 23.5-degree tilt of the Earth with respect to the Sun), there is an East/West component as well. The North/South swing in apparent angle is the main source of seasons on Earth.

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 (the rotational period is 25 days at the equator and about 35 days at the poles), 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 (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.


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 15,000,000 Kelvins (by contrast, the surface of the Sun is close to 6,000 Kelvins). Energy is produced by exothermic thermonuclear reactions (nuclear fusion) that mainly convert 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 8.9 ×1037 protons (hydrogen nuclei) are converted into helium nuclei every second, releasing energy at the matter-energy conversion rate of 4.26 million tonnes per second, 383 yottawatts (383 ×1024 W) or 9.15 ×1010 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 (gamma and X-rays) 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[8] to as little as 17,000 years.[9] 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 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.

Structure of the Sun

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 (photosphere) of the Sun. Once the material cools off at the surface, it plunges back downward to the base of the convection zone, to receive more heat from the top of the radiative zone. 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.


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 because of 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. Sunlight has approximately a black-body spectrum that indicates its temperature is about 6,000 K (10,340°F / 5,727°C), interspersed with atomic absorption lines from the tenuous layers above the photosphere. The photosphere has a particle density of about 1023/m3 (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.


During a total solar eclipse, the sun's atmosphere is more apparent to the eye.
During a total solar eclipse, the sun's atmosphere is more apparent to the eye.

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/m3–1016/m3. (Earth's atmosphere near sea level has a particle density of about 2x1025/m3.) 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.

Stars are classified based on their position on the Hertzsprung-Russell diagram, a graph which plots the brightness of stars against their surface temperature. Generally speaking, the hotter a star is, the brighter it is. Stars which follow this pattern are said to be on the main sequence, and the Sun lies right in the middle of it. This has led many astronomy textbooks to label the Sun as "average;" however, stars brighter and hotter than it are rare, whereas stars dimmer and cooler than it are common. The vast majority of stars are dim red dwarfs, though they are under-represented in star catalogues as we can observe only those few that are very near the Sun in space.

The Sun's position on the main sequence means, according to current theories of stellar evolution, that it is in the "prime of life" for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion, and been forced, as older red giants must, to fuse more inefficient elements such as helium and carbon. The Sun is growing increasingly bright as it ages. Early in its history, it was roughly 75 percent as bright as it is today. Calculations of the ratios of hydrogen and helium within the Sun suggest it is roughly halfway through its life cycle, and will eventually begin moving off the main sequence, becoming larger, brighter and redder, until, about five billion years from now, it too will become a red giant.

The Sun is a population I star, meaning that it is fairly new in galactic terms, having been born in the later stages of the universe's evolution. As such, it contains far more elements heavier than hydrogen and helium ("metals" in astronomical parlance) than older population II stars such as those found in globular clusters. Since elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, the first generation of stars had to die before the universe could be enriched with them. For this reason, the very oldest stars contain very little "metal", while stars born later have more. This high "metallicity" is thought to have been crucial in the Sun's developing a planetary system, because planets form from accretion of metals.

The Sun radiates a continuous stream of charged particles, a plasma known as solar wind, ejecting it outwards at speeds greater than 2 million kilometres per hour, creating a very tenuous "atmosphere" (the heliosphere), that permeates the solar system for at least 100 AU. This environment is known as the interplanetary medium. Small quantities of cosmic dust (some of it arguably interstellar in origin) are also present in the interplanetary medium and are responsible for the phenomenon of zodiacal light. The influence of the Sun's rotating magnetic field on the interplanetary medium creates the largest structure in the solar system, the heliospheric current sheet.

Earth's magnetic field protects its atmosphere from interacting with the solar wind. However, Venus and Mars do not have magnetic fields, and the solar wind causes their atmospheres to gradually bleed away into space.

(Roman - Sol)

Helios is the young Greek god of the sun, often confused with Apollo. He is the son of the Titans Hyperion and Theia and the brother of Eos (Dawn) and Selene (Moon). By the Oceanid Perse, he became the father of Aeetes, Circe, and Pasiphae. His other two daughters are Phaethusa ("radiant") and Lampetia ("shining").

At the end of each night his sister, rosy-fingered Eos (Dawn), rises from her couch in the east and, mounted on a chariot pulled by the horses Lampus and Phaethon, she rides to Olympus to announce the approach of her brother, Helios. Once Helios appears Eos becomes Hemera (Day) and escorts him on his travels across the sky until, becoming Hespera, she announces their safe arrival on the western shores of Oceanus.

Roused by the rooster, his sacred animal, he leaves his splendid palace in the far east and daily travels his four-horse chariot across the Heavens, until finally he reaches an equally-magnificent palace in the far west. The palaces were built by Hephaestus in gratitude for being rescued by Helios when the Giants overwhelmed him during their attack on Olympus. His chariot is pulled by four horses - Pyrois, Eos, Aethon and Phlegon - and often others (see bottom of page).

At the end of the day Helios lets his horses graze and rest in the Islands of the Blessed. Afterwards he sails home along the great stream called Oceanus, which flows around the entire world. To get back to his far east palace, Hephaestus crafted a golden ferry-boat for the sun god and his chariot and horse team to use, and Helios sleeps comfortably in his royal cabin on their nightly voyage on Oceanus.

Helios sees and knows all that happens on earth, and was often called upon as witness, but is not always very observant - some of the companions of the Trojan War hero, Odysseus, once had the nerve to steal some of his sacred cattle, and Helios actually failed to notice it! Probably because he had several herds of such cattle, each herd numbering three hundred and fifty head.

Represented as a handsome youth with a halo standing in a chariot, his usual attributes are the rooster, the whip and the globe. His island is beautiful Rhodes, where the worshipful natives in his honor built the Colossus of Rhodes, an awesome statue that straddled the harbor entrance and under whose legs all ships, even the tallest, easily passed. It was the sixth of the seven wonders of the ancient world. Some have said that the Colossus of Rhodes was dedicated in honor of Apollo, god of light, with whom Helios was often confused.

In addition to Rhodes, Zeus also added the island of Sicily to the dominion of Helios. This island was a missile that was tossed in the battle with the Giants and had formed Sicily upon landing.

Phaethon was the son of Helios and Clymene (or some say Rhode), but didn't know the identity of his real father since his mother now lived with Merops, who was King of Ethiopia. He did know that he was adopted by the King, and he badgered his mother until she revealed to him that his father truly was the sun god. She told Phaethon that he could verify this if he were to visit his father's nearby palace and pose the question to him directly.

The youth did just that and once he told Helios who he was, the sun god stood and lovingly embraced him. Feeling guilty because he had ignored his young son all those years, he swore an oath by the sacred river Styx that Phaethon could have whatever he wanted. Name your gift, son.

Phaethon impulsively replied that what he wanted most was to drive his father's golden chariot across the sky, much to the dismay of Helios. He knew that the inexperienced boy wouldn't be able to control the frisky team and he took his job very seriously. Not wanting to jeopardize the safety of both the earth and his son, Helios begged his boy to name another wish.

To no avail. Phaethon wanted to impress his sisters and show them what a high stature he had attained. He insisted that his father honor the oath, and since a god cannot break an oath sworn by Styx without suffering terrible consequences, Helios reluctantly agreed to his son's request.

Phaethon's sisters helped him yoke his father's white horses and offered encouraging words to their beloved brother. But the youth was inexperienced and in his excitement at this incredible ride soon lost control of the strong steeds. They sensed that there was a stranger at the reins and felt free to go wherever they wished, seeing as their new driver didn't follow the usual path.

Flying too far from the earth, they caused the inhabitants to freeze and shiver as the sun chariot flew higher and higher, and all the plants began to shrivel and die. But at once plunging down and flying too close to the earth, they scorched and burned the people and landscape, causing terrible grief and hysteria.

Zeus heard the anguished cries of the people and, seeing the runaway team of horses pulling the sun, with a terrified Phaethon clinging on for dear life, became very angry. Gaea (Mother Earth), distressed at the danger her realm was in, beseeched Zeus to do something to stop it. Knowing that if he didn't act quickly all life on earth was in serious peril, Zeus chased after the sun chariot on his eagle and, quickly catching up to it, hurled one of his fearsome thunderbolts at Phaethon, striking the foolish young man dead.

The incompetent Phaethon tumbled from the sky into either the river Po or the river Eridanus, and there the Naiads, who were Nymphs of the water, carved his epitaph:

"Here lies Phaethon: In Helios' car he fared
And though he greatly failed, more greatly dared."
(Ovid, Metamorphoses)

Because of this catastrophe it have been said that one entire day went without the sun, but still light was not lacking because the world was burning so brightly from the close fly-by of Phaethon. His mother Clymene wandered the entire earth looking for his limbs and bones and his best friend, King Cycnus of Liguria, mourned so much for his dead friend that he abandoned his kingdom and went weeping along the river Eridanus until finally he was turned into a swan.

Phaeton's grieving sisters, the Heliades, were changed into either poplar trees or alder trees and they reside on the banks of the river, where they endlessly weep amber tears.

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