The Sun is a medium size star known
as a yellow dwarf. The Sun is just one of about 100
billion stars in our galaxy, The Milky Way. 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 . The Sun is about 93 million miles away from the Earth. The distance
from the Earth to the Sun varies throughout the year. At its closest, the Sun is
91.1 million miles from Earth. At its farthest the distance between the Sun and
the Earth is 94.2 million miles. It takes light 8 min. 20 sec to travel from the
sun to the Earth.
The Sun Dwarfs The Planets In Size
The Sun is about 4.5 billion years
old. It will continue to radiate for another 5 billion years. It will
start to run out of core hydrogen in *less* than 4 billion years from now, and
will be expanding into a red giant from that point on. In fact, the energy
output of the sun will increase prior to the red giant phase to a point where
the Earth will probably become too hot to support life in only 1 or 2 billion
years. When the sun does reach the red giant phase, the core will finally become
hot enough for helium fusion to occur (the "Helium flash", followed by
core Helium fusion and continued Hydrogen fusion in a shell around the growing
Helium core). After this Helium in the core is exhausted, the sun will then
collapse into a white dwarf.
The nearest stars to our sun
Proxima Centauri (Alpha Cen C) 4.22
Light years away -39,900,000,000,000 km away
Rigil Kentaurus (Alpha Cen A) 4.35
Light years away
Alpha Centauri (B) 4.35 Light years
Barnard's Star 5.9 Light years
Wolf 359 7.6 Light years away
Comparative size of selected Stars
"Erde" means Earth, "Sonne" is "the Sun"
A light-year is a unit of
length used by astronomers to measure interstellar distance (the distance
between stars). A light-year is defined as the distance that light will
travel in a year. The speed of light is 186,000 miles per second (300,000 km
x 60 sec/min x 60 min/hr x 24 hr/day x 365 days/yr
is equal to 9,500,000,000,000 kilometers or 5,865,696,000,000 miles
The Sun is personified in many
cultures: the Greeks called it Helios, the Egyptians principal god was Ra the
sun god and the Romans called it Sol.
The Sun is an average star,
similar to millions of others in the Universe. It is classified as a yellow
dwarf of spectral class G2.
Even a relatively quiet day on the Sun is busy.
This ultraviolet image shows
bright, glowing arcs of gas flowing around the sunspots.
Image Credit: NASA
It is a prodigious energy
machine, manufacturing about 4.0E023 kilowatts of energy per second. In other
words, if the total output of the Sun was gathered for one second it would
provide the U.S. with enough energy, at its current usage rate, for the next
The basic energy source for the Sun is nuclear fusion, which
uses the high temperatures and densities within the core to fuse hydrogen,
creating energy and producing helium as a by-product. The core is so dense and
the size of the Sun so great that energy released at the center of the Sun takes
about 50,000,000 years to make its way to the surface, undergoing countless
absorptions and reemissions in the process. The Sun's visible surface, called
the photosphere, has a temperature of 5,700 C(10,900 Degrees F). The gases heat up and become more
compressed at deeper levels, until the temperature reaches 15 million C( 27
Million Degrees F) deep
within the Sun's energy producing core (If the Sun were to stop producing
energy today, it would take 50,000,000 years for the effects to be felt at
Layers of the Sun
The center of the Sun
The center of the
sun is very hot (about 15 million degrees Celsius) and the pressure is immense
(about 100 billion times the airpressure here on Earth). Because of that, atoms
come so close to each other that they fuse.
In every second, the
Sun spends 700 billion tons of protons (or: Hydrogen) in this way. And only a
small fraction (0.7 percent) is turned into light. Right now, about half of the
amount of Hydrogen in the core of the Sun has been fused into Helium.
The Sun, can be divided into six
layers. From the center out, the layers of the Sun are as follows: the solar
interior composed of the core (which occupies the innermost quarter or so of the
Sun's radius), the radiative zone, and the the convective zone, then there is
the visible surface known as the photosphere, the chromosphere, and finally the
outermost layer, the corona.
The innermost layer of the sun is the core. With a
density of 160 g/cm^3, 10 times that of lead, the core might be expected to be
solid. However, the core's temperature of 15 million kelvins (27 million degrees
Fahrenheit) keeps it in a gaseous state.
In the core, fusion reactions
produce energy in the form of gamma rays and neutrinos. Gamma rays are photons
with high energy and high frequency. The gamma rays are absorbed and re-emitted
by many atoms on their journey from the envelope to the outside of the sun. When
the gamma rays leave atoms, their average energy is reduced. However, the first
law of thermodynamics (which states that energy can neither be created nor be
destroyed) plays a role and the number of photons increases. Each
high-energy gamma ray that leaves the solar envelope will eventually become a
thousand low-energy photons.
The neutrinos are extremely
nonreactive. To stop a typical neutrino, one would have to send it through a
light-year of lead! Several experiments are being performed to measure the
neutrino output from the sun. Chemicals containing elements with which neutrinos
react are put in large pools in mines, and the neutrinos' passage through the
pools can be measured by the rare changes they cause in the nuclei in the pools.
For example, perchloroethane contains some isotopes of chlorine with 37
particles in the nucleus (17 protons, 20 neutrons). These Cl-37 molecules can
take in neutrinos and become radioactive Ar-37 (18 protons, 19 neutrons). From
the amount of argon present, the number of neutrinos can be calculated.
The radiative zone extends
outward from the outer edge of the core to the interface layer or tachocline at
the base of the convection zone (from 25% of the distance to the surface to 70%
of that distance). The radiative zone is characterized by the method of energy
transport - radiation. The energy generated in the core is carried by light
(photons) that bounces from particle to particle through the radiative zone.
Although the photons travel at the speed of light, they bounce so many times
through this dense material that an individual photon takes about a million
years to finally reach the interface layer. The density drops from 20 g/cm³
(about the density of gold) down to only 0.2 g/cm³ (less than the density of
water) from the bottom to the top of the radiative zone. The temperature falls
from 7,000,000°C to about 2,000,000°C over the same distance.
The interface layer lies between
the radiative zone and the convective zone. The fluid motions found in the
convection zone slowly disappear from the top of this layer to its bottom where
the conditions match those of the calm radiative zone. This thin layer has
become more interesting in recent years as more details have been discovered
It is now believed that the Sun's magnetic field is generated by a magnetic
dynamo in this layer. The changes in fluid flow velocities across the layer
(shear flows) can stretch magnetic field lines of force and make them stronger.
This change in flow velocity gives this layer its alternative name - the
tachocline. There also appears to be sudden changes in chemical composition
across this layer.
The convection zone is the
outer-most layer of the solar interior. It extends from a depth of about 200,000
km right up to the visible surface. At the base of the convection zone the
temperature is about 2,000,000°C. This is "cool" enough for the
heavier ions (such as carbon, nitrogen, oxygen, calcium, and iron) to hold onto
some of their electrons. This makes the material more opaque so that it is
harder for radiation to get through. This traps heat that ultimately makes the
fluid unstable and it starts to "boil" or convect.
Convection occurs when the
temperature gradient (the rate at which the temperature falls with height or
radius) gets larger than the adiabatic gradient (the rate at which the
temperature would fall if a volume of material were moved higher without adding
heat). Where this occurs a volume of material moved upward will be warmer than
its surroundings and will continue to rise further. These convective motions
carry heat quite rapidly to the surface. The fluid expands and cools as it
rises. At the visible surface the temperature has dropped to 5,700°K and the
density is only 0.0000002 gm/cm³ (about 1/10,000th the density of air at sea
level). The convective motions themselves are visible at the surface as granules
Outside of the core is the
radiative envelope, which is surrounded by the convective envelope. The
temperature is 4 million kelvins (7 million degrees F). The density of the solar
envelope is much less than that of the core. The core contains 40 percent of the
sun's mass in 10 percent of the volume, while the solar envelope has 60 percent
of the mass in 90 percent of the volume.
The solar envelope puts pressure
on the core and maintains the core's temperature.
The hotter a gas is, the more
transparent it is. The solar envelope is cooler and more opaque than the core.
It becomes less efficient for energy to move by radiation, and heat energy
starts to build up at the outside of the radiative zone. The energy begins to
move by convection, in huge cells of circulating gas several hundred kilometers
in diameter. Convection cells nearer to the outside are smaller than the inner
cells. The top of each cell is called a granule. Seen through a telescope,
granules look like tiny specks of light. Variations in the velocity of particles
in granules cause slight wavelength changes in the spectra emitted by the sun.
The photosphere is the zone
from which the sunlight we see is emitted. The photosphere is a comparatively
thin layer of low pressure gasses surrounding the envelope. It is only a few
hundred kilometers thick, with a temperature of 6000 K. The composition,
temperature, and pressure of the photosphere are revealed by the spectrum of
sunlight. In fact, helium was discovered in 1896 by William Ramsey, when in
analyzing the solar spectrum he found features that did not belong to any gas
known on earth. The newly-discovered gas was named helium in honor of Helios,
the mythological Greek god of the sun.
In an eclipse, a red circle
around the outside of the sun can sometimes can be seen. This is the
chromosphere. Its red coloring is caused by the abundance of hydrogen.
From the center of the sun to the
chromosphere, the temperature decreases proportionally as the distance from the
core increases. The chromosphere's temperature, however, is 7000 K, hotter than
that of the photosphere. Temperatures continue to increase through the corona.
The outermost layer of the
sun is the corona. Only visible during eclipses, it is a low density cloud of
plasma with higher transparency than the inner layers. The white corona is a
million times less bright than the inner layers of the sun, but is many times
The corona is hotter than some of
the inner layers. Its average temperature is 1 million K (2 million degrees F)
but in some places it can reach 3 million K (5 million degrees F).
Temperatures steadily decrease as
we move farther away from the core, but after the photosphere they begin to rise
again. There are several theories that explain this, but none have been proven.
sunspot group -- Active region 9169
Sunspots are dark spots on
the photosphere, typically with the same diameter as the Earth. They have cooler
temperatures than the photosphere. The center of a spot, the umbra, looks dark
gray if heavily filtered and is only 4500 K (as compared to the photosphere at
6000K). Around it is the penumbra, which looks lighter gray (if filtered).
Sunspots come in cycles, increasing sharply (in numbers) and then decreasing
sharply. The period of this solar cycle is about 11 years.
The sun has enormous organized
magnetic fields that reach from pole to pole. Loops of the magnetic field oppose
convection in the convective envelope and stop the flow of energy to the
surface. This results in cool spots at the surface which produce less light than
the warmer areas. These cool, dark spots are the sunspots.
The Great Conveyor
Belt is a massive circulating current of fire (hot plasma) within the sun. It
has two branches, north and south, each taking about 40 years to complete one
circuit. Researchers believe the turning of the belt controls the sunspot cycle.
This collage of solar images from NASA's
Solar Dynamics Observatory (SDO) shows how observations of the sun in
different wavelengths helps highlight different aspects of the sun's surface
and atmosphere. (The collage also includes images fr...om other SDO
instruments that display magnetic and Doppler information.) Credit: NASA/SDO/Goddard
Space Flight Center
Taking a photo of the sun with a standard camera will provide a familiar
image: a yellowish, featureless disk, perhaps colored a bit more red when
near the horizon since the light must travel through more of Earth's
atmosphere and consequently loses blue wavelengths before getting to the
camera's lens. The sun, in fact, emits light in all colors, but since yellow
is the brightest wavelength from the sun, that is the color we see with our
naked eye -- which the camera represents, since one should never look
directly at the sun. When all the visible colors are summed together,
scientists call this “white light.”
Specialized instruments, either in ground-based or space-based telescopes,
however, can observe light far beyond the ranges visible to the naked eye.
Different wavelengths convey information about different components of the
sun's surface and atmosphere, so scientists use them to paint a full picture
of our constantly changing and varying star.
Yellow-green light of 5500 Angstroms, for example, generally emanates from
material of about 10,000 degrees F (5700 degrees C), which represents the
surface of the sun. Extreme ultraviolet light of 94 Angstroms, on the other
hand, comes from atoms that are about 11 million degrees F (6,300,000
degrees C) and is a good wavelength for looking at solar flares, which can
reach such high temperatures. By examining pictures of the sun in a variety
of wavelengths – as is done through such telescopes as NASA's Solar
Dynamics Observatory (SDO), NASA's Solar Terrestrial Relations Observatory
(STEREO) and the ESA/NASA Solar and Heliospheric Observatory (SOHO) --
scientists can track how particles and heat move through the sun's
We see the visible spectrum of light simply because the sun is made up of a
hot gas – heat produces light just as it does in an incandescent light
bulb. But when it comes to the shorter wavelengths, the sun sends out
extreme ultraviolet light and x-rays because it is filled with many kinds of
atoms, each of which give off light of a certain wavelength when they reach
a certain temperature. Not only does the sun contain many different atoms
– helium, hydrogen, iron, for example -- but also different kinds of each
atom with different electrical charges, known as ions. Each ion can emit
light at specific wavelengths when it reaches a particular temperature.
Scientists have cataloged which atoms produce which wavelengths since the
early 1900s, and the associations are well documented in lists that can take
up hundreds of pages.
Solar telescopes make use of this wavelength information in two ways. For
one, certain instruments, known as spectrometers, observe many wavelengths
of light simultaneously and can measure how much of each wavelength of light
is present. This helps create a composite understanding of what temperature
ranges are exhibited in the material around the sun. Spectrographs don't
look like a typical picture, but instead are graphs that categorize the
amount of each kind of light.
On the other hand, instruments that produce conventional images of the sun
focus exclusively on light around one particular wavelength, sometimes not
one that is visible to the naked eye. SDO scientists, for example, chose 10
different wavelengths to observe for its Atmospheric Imaging Assembly (AIA)
instrument. Each wavelength is largely based on a single, or perhaps two
types of ions – though slightly longer and shorter wavelengths produced by
other ions are also invariably part of the picture. Each wavelength was
chosen to highlight a particular part of the sun's atmosphere.
From the sun's surface on out, the wavelengths SDO observes, measured in
4500: Showing the sun's surface or photosphere.
1700: Shows surface of the sun, as well as a layer of the sun's atmosphere
called the chromosphere, which lies just above the photosphere and is where
the temperature begins rising.
1600: Shows a mixture between the upper photosphere and what's called the
transition region, a region between the chromosphere and the upper most
layer of the sun's atmosphere called the corona. The transition region is
where the temperature rapidly rises.
304: This light is emitted from the chromosphere and transition region.
171: This wavelength shows the sun's atmosphere, or corona, when it's quiet.
It also shows giant magnetic arcs known as coronal loops.
193: Shows a slightly hotter region of the corona, and also the much hotter
material of a solar flare.
211: This wavelength shows hotter, magnetically active regions in the sun's
335: This wavelength also shows hotter, magnetically active regions in the
94: This highlights regions of the corona during a solar flare.
131: The hottest material in a flare.
Dopplergrams provide maps of velocity on the sun's surface. Solar Region:
Magnetograms show maps of the magnetic field on the sun’s surface, with
black showing magnetic field lines pointing away ...from Earth, and white
showing magnetic field lines coming toward Earth. Solar Region:
Continuums provide photographs of the solar surface, incorporating a broad
range of visible light. Solar Region: Photosphere
Ultraviolet light continuum, shows the surface of the sun. As well as a
layer of the sun's atmosphere called the chromosphere, which lies just
above the photosphere and is where the temperature begins rising.
Temperatures: 4500 Kelvin, Solar Region: Photosphere/Chromosphere
White light continuum showing the sun's surface or photosphere.
Temperatures: 6000 Kelvin, Solar Region: Photosphere
Emitted by carbon-4 (C IV) at around 10,000 Kelvin. C IV at these
temperatures is present in the upper photosphere and what's called the
transition region, a region between the chromosphere and the upper most
layer of the sun's atmosphere called the corona. The transition region is
where the temperature rapidly rises. SDO images of this wavelength are
typically colorized in dark yellow. Solar Region: Upper
Emitted by helium-2 (He II) at around 50,000 Kelvin. This light is emitted
from the chromosphere and transition region. SDO images of this wavelength
are typically colorized in red. Solar Region: Transition Region/Chromasphere
Emitted by iron-9 (Fe IX) at around 600,000 Kelvin. This wavelength shows
the quiet corona and coronal loops, and is typically colorized in gold.
Solar Region: Upper Transition Region/Quiet Corona
Emitted by iron-12 (Fe XII) at 1,000,000 K and iron 24 (Fe XXIV) at
20,000,000 Kelvin. The former represents a slightly hotter region of the
corona and the later represents the much hotter material of a solar flare.
This wavelength is typically colorized in light brown. Solar Region:
Emitted by iron-14 (Fe XIV) at temperatures of 2,000,000 Kelvin. These
images show hotter, magnetically active regions in the sun's corona and
are typically colorized in purple. Solar Region: Active Regions
Emitted by iron-16 (Fe XVI) at temperatures of 2,500,000 Kelvin. These
images also show hotter, magnetically active regions in the corona, and
are typically colorized in blue. Solar Region: Active Regions
Emitted by iron-18 (Fe XVIII) at temperatures of 6,000,000 Kelvin.
Temperatures like this represent regions of the corona during a solar
flare. The images are typically colorized in green. Solar Region: Flaring
Emitted by iron-20 (Fe XX) and iron-23 (Fe XXIII) at temperatures greater
than 10,000,000 Kelvin, representing the material in flares. The images
are typically colorized in teal. Solar Region: Flaring Regions
Karen C. Fox
NASA Goddard Space Flight Center, Greenbelt, MD
compiled from The British Antarctic Study, NASA, Environment Canada,
UNEP, EPA and other sources as stated and credited Researched by Charles
Welch-Updated daily This Website is a project of the The Ozone Hole Inc.
a 501(c)(3) Nonprofit Organization http://www.theozonehole.com