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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
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
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
"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
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 (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
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
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
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. 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
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
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 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.
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
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
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 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
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
THE MYTH :: HELIOS (Helius)
(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
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."
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.