Much remains unknown about the structure of the sun, particularly
in its interior. It is traditional for purposes of discussion
to organize the anatomy of the sun around the core and shells
surrounding the core, these shells being referred to as the
radiative layer (or zone), the convection layer, and the photosphere,
which is the surface "layer" that we are able to observe.
Actually the gaseous photosphere layer is somewhat vaguely
defined, as it probably is several hundred kilometers thick.
The density in the radiative zones 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.
At the extremely dense core, which holds about 50% of the
sun's total mass, but only about 1.5% of the total volume,
the conditions are extreme, to say the least. The temperature
is thought to be around 15 million Kelvins and these conditions
are so extreme that all atomic materials present are stripped
of their electrons to leave a hot brew of protons, neutrons,
nuclei, and free electrons. The pressure at the core is perhaps
250 billion times larger than the pressure of the Earth's
atmosphere. The sun does not suffer gravitational collapse
only because of this enormous outward pressure, which is generated
by the heat produced in the core. Nor does it explode like
a hydrogen bomb, because the stupendous mass of the gases
above the core contains its explosiveness. As noted above
the core density is extremely high. A bucket full of core
material would be so heavy that you would be unable to lift
it. At the core we find the nuclear inferno that produces
the energy that ultimately is spewed forth into space.
From the core to Solar Max
The sun's energy is manifested in the form of short wavelength
gamma rays, which can be regarded as tiny packets of energy
called photons, the particle component of electromagnetic
radiation. If we could see into the core it would appear black,
since none of the energy produced there lies in the visible
part of the spectrum. Through collisional losses the gamma
ray photons are soon reduced to longer wavelength and less
energetic x-ray photons, which continue to remain outside
of the visible part of the electromagnetic spectrum.
The Radiative Zone
The x-ray photons produced in the core follow a long and torturous
route as they work their way to the surface of the sun, following
a path of reduced temperature, pressure and density. Once
the photons escape from the core, they travel outward in the
radiative zone where some of the electrons in the radiative
zone are captured by helium nuclei (He2+) to form
ionized helium atoms (He+). The radiative zone
is packed with ionized hydrogen and helium atoms, and extends
about 70% of the distance from the center of the sun to the
surface. This mixture of ionized hot gases and electrons is
called plasma and is sometimes regarded as a fourth state
of matter. While moving through the radiative zone the photons
encounter less and less dense materials. Two-thirds of the
way through, the density is about the same as that of air
at the Earth's surface, and at the edge of the zone, the density
is thought to be around 0.1 g/cm3.
|Your suntan is one million years
Deep in the radiative zone the photons collide with particles
and change direction in random ways. Each photon may travel
only a few millimeters before it suffers another collision
and is set off in a different direction. Nevertheless, the
photons continue to work their way painfully toward the surface
by meandering in zig-zag fashion toward regions of lower temperature
and pressure. The time that it takes for them to complete
their journey to the surface is measured in millions of years,
which is an incredible fact given that photons travel at the
speed of light! To put it in more personal terms, the sunlight
that gave you your summer tan resulted from a nuclear reaction
that took place perhaps 1,000,000 years ago deep within the
core of the sun.
The Convection Layer
The collisions suffered by the photons rob them of part of
their energy, and consequently their wavelengths gradually
become longer and longer as they move toward the convection
zone. Ultimately wavelengths corresponding to visible light
are reached. Finally the photons arrive at the convection
layer 150,000 kilometers below the surface, where temperatures
have moderated to about 1 million degrees or less-a pleasant
day, by solar standards. Here nuclei are able to hold on to
electrons and neutral atoms are found. And by this time the
photon energies have been degraded to the point that the gaseous
atoms in the convection zone absorb the energy of the photons
and hold on to it rather than having it bounced off (or absorbed
and re-radiated). These atoms effectively block the outward
flow of radiative energy, and the energy absorbed by the atoms
makes them enormously hot. At that point the convection currents,
like those we have observed in warming liquids and air, take
over and carry the sun's energy to the photosphere on seething
rivers of hot gases.
As the temperature of the gas that absorbs the radiative energy
at the bottom of the convection zone increases, it expands,
becoming less than the other gas in its surroundings. These
bubbles of hot gas, being less density, move up to the surface,
where they radiate away their excess energy to space. In the
process, they become cooler and more dense and sink down again.
So you have a huge number of "conveyor belts" with hot gas
moving up and cooler gas moving down.
Energy "conveyor belts"
are key to solar wind.
At the surface (photosphere) the gas is very turbulent,
rising up in the center of structures called convection cells
(supergranules), flowing to the cell boundaries and then sinking.
The processes going on at these cell boundaries, where plasmas
with oppositely oriented magnetic fields collide and magnetic
energy is converted into kinetic energy, are probably responsible
for the heating of the corona and the acceleration of the
solar wind. So the convection zone is the key to the solar
Interestingly, although it may have taken the energy a million
years to reach the convection zone, the energy it delivers
rises through the entire convection zone in about three months.
It is important to realize that all of the energy emitted
at the surface of the sun is transported there by convection.
At the very top of the convection zone is the photosphere,
the visible bright surface of the sun. Here where the temperatures
are even more moderate and the gas densities are quite small
(estimated to be one-millionth the density of water or less),
the gaseous atoms no longer block radiative flow. As the hot
atoms cool they release their excess energy once again as
photons that stream unimpeded into space and ultimately provide
support for life on Earth.
The pebbly, granular surface of the sun, the photosphere,
is where early astronomers focused most of their attention,
because it is here that we find the easily observed blotches
that are now called sunspots. Sunspots come and go in a regular
rhythm about 11 years long but there is uncertainty about
the driving force behind their appearance and disappearance.
They vary in size and often occur in groups that sprawl over
hundreds of millions of square kilometers on the sun's surface.
They look dark because they are cooler than the surrounding
surface of the sun. Sunspots are thought to arise from the
temporary inhibition of convection currents by strong localized
magnetic fields. In other words, if a convection current is
prevented from carrying its load of thermal energy to the
surface, the surface served by that current will cool and
a sunspot will appear. Intervals of high sunspot activity
usually coincide with a wide range of other dramatic solar
events such as coronal mass ejections (CMEs) and flares, which
often manifest themselves by disrupting communications, and
arguably, even weather patterns on Earth.
The lower atmosphere of the sun-the chromosphere-escaped scrutiny
by early astronomers because it is invisible when compared
to the bright photosphere below it. The relatively miniscule
amount of light emitted by the chromosphere is only momentarily
visible to the unaided eye during a total solar eclipse when
the moon stands in front of the photosphere. The chromosphere
appears transiently under these conditions as a thin, bright
red ribbon that encircles the silhouette of the moon. Modern
astronomers have been able to study the chromosphere at their
convenience owing to the wide variety of instruments that
are available to them.
The chromosphere has indeed proved to be an exciting and unique
feature of the solar landscape. It is here that solar astronomers
have found a host of transient exotic structures, including
spicules, prominences, and plages. The spicules are abundant,
but short-lived, evanescent streams of hot gases that vault
high into the chromosphere. More impressive and photogenic
are the prominences, some of which are spectacular bright
loops of hot gas that arch high above the top of the chromosphere
and often extend into the corona. Some of them have widths
the size Earth while others may approach half the diameter
of the sun itself.
Prominences often are associated with sunspots and some of
them-the quiescent prominences-may hold their shape for months
before collapsing. Others-called eruptive prominences-erupt
from the chromosphere as gaseous streamers. Finally there
are the plages, which are bright, cloud-like structures that
are found in the vicinity of sunspots.
CMEs release plasma and flares
add electromagnetic radiation for a 70-hour trip to
Closely related to prominences are monstrously energetic
coronal mass ejections (CMEs) and flares, which coincide with
the explosive release of magnetic energy in the solar atmosphere.
CMEs spew all sorts of solar debris into space, and flares
add a component strong blast of electromagnetic radiation,
including x-rays and ultraviolet rays. The power of these
blasts is unimaginable, corresponding to the energy released
by the explosion of millions of hydrogen bombs. This electromagnetic
radiation arrives at earth in about eight minutes and can
cause severe disruptions in the Earth's upper atmosphere,
where the radiation is absorbed. Fortunately for the residents
of earth, the atmosphere and the earth's magnetic field provide
protection from the harmful effects of this powerful radiation.
After a period of around 70 hours, the particles of the CME
and its associated magnetic shock wave begin to arrive at
earth. The charged particles of the CME then begin to flow
around earth's magnetic field, inducing electrical currents
that can have calamitous effects. For example, in 1989 one
of the strongest flares/CMEs ever observed erupted, causing
a power failure all across the province of Quebec and creating
an aurora borealis that was seen as far south as Key West,
The power of coronal mass ejections (CMEs) and flares cannot
be over-estimated. Flare temperatures may reach 50 million
degrees, which is several times hotter than the core of the
sun. If the power of one CME or flare could be harnessed it
would be sufficient to provide the energy needs of the inhabitants
of the Earth for millions of years.
The chromosphere seems to derive its spectacular behavior
from the dominant force in the solar atmosphere, which is
magnetism. In contrast to the dense plasmas in the sun's lower
regions, the plasmas of the atmosphere are dilute and are
unable to contain the immense magnetic field of the sun. Rather,
the magnetic fields dictate the behavior of the plasma in
the sun's atmosphere, giving rise to the bizarre features
that characterize this region. Loop prominences, for example,
are observed when plasma is captured by magnetic fields and
bent back into the chromosphere.
The outermost layer of the solar atmosphere is the corona,
which in some ways is the most mysterious layer of all. Mysterious
because, contrary to expectations and seemingly to the laws
of thermodynamics, the temperature rises steadily from a minimum
around 4000 Kelvins in the chromosphere to more than a million
degrees Kelvin in the corona, making the corona the hottest
part of the solar region outside of the sun's core! How is
it possible for heat to be transported from a cooler body
(the chromosphere) to a hotter body (the corona)? Even today
there is uncertainty about the mechanism of energy transfer
to the corona, but it is thought by many astrophysicists to
be the result of magnetic waves transported along magnetic
field lines emerging from the sun.
The incredibly dilute, superheated gases of the corona reach
millions of miles into space. Those who have witnessed a total
eclipse have seen the corona as a luminous white halo surrounding
the solar disk, which is an effect that results from photospheric
light bouncing off free electrons in the coronal plasma. The
corona is synchronized with the solar activity cycle, changing
shape from a jagged ring around the sun during the peak of
the cycle to wispy plumes and streamers that reach millions
of miles into space at the end of the cycle.
The plasma-like or streamer features in the corona are pictures
of the solar wind leaving the sun. The streamers are the origin
of the dense, lower-speed component of the solar wind, the
subject of "What is Solar Wind?"