Structure Of The Sun
Different layers of the sun are described by those which we cannot directly
view and those which comprise the atmosphere. On a cursory examination of the
Sun, one would find it to be a pretty, yellow ball hanging in the sky. Since
we cannot (and I really mean CANNOT) look directly at the Sun without professional
protection lest we permanently damage our eyes in a fraction of a second, we
rely on photographs to discern some of the Sun's secrets. On the surface, it
really looks like a yellow ball. What is surprising is the discovery that the
Sun is made entirely of gas, all the way from the most exterior solar corona
to the exotic and super-dense core. We know that the Sun is made of gas based
on how we study it. All gases increase in temperature with an increase in the
crowding of the gas particles. The more spread out the gas particles are from
each other, the more cool the temperature will be. Any photograph of the Sun
will reveal that the edges (termed "limbs" by astronomers) are darker
than the middle of the picture.
photograph to your right demonstrates this limb-darkening. As we look at the
edge of the solar disk in the picture, it is obvious that the edges are darker
in color than the rest of the picture. We are seeing the gas layer of the Sun
that exists outside the visible photosphere layer. These gases are more spread
out, cooler in temperature, and therefore darker in color. From this knowledge
alone, it can be inferred that the Sun will get more hot as one proceeds on
an imaginary journey from the outer regions in toward the center. Although the
material is always gaseous, the gravitational pressure upon the gases farther
and farther into the interior brings about a steady increase in temperature
until you reach a region inside where that temperature exceeds 10,000,000 K.
Additionally, you can see that this picture shows the presence of dark spots
known as sunspots. You will learn more about these interesting features later
in this unit. For now, the sunspot is a region of unusual magnetic activity
that causes the area to be cooler in temperature by as much as 2000 K, and yet
far more energetic that the surrounding regions.
Finally, the Sun is not quite the smooth yellow ball it appears to be to the
untrained eye. Close examination, even of this simple picture, reveals a pattern
of circle called granulation. This granulation is the result of convection of
the solar material, much like the convection of a pot of boiling water. The
Sun literally is boiling upward from its interior and spewing out at the "surface."
Sometimes, material may be ejected from the Sun, as solar flares, while other
gases remain trapped in the beautiful corona. Unfortunately, the corona is only
visible during a total solar eclipse, and very people have ever seen it.
image to the left depicts the layers of the Sun, and a description of each is
Core = The site of thermonuclear fusion, which is the engine of the sun. Temperatures
here approach 15,000,000 K and pressures exceed 250,000,000,000 atmospheres.
The core makes up 50% of the sun's mass but only 1/64th of the Sun's volume.
All of the energy that emits from the Sun is produced in the core.
Radiative Zone = The region where energy from the core begins its journey
outward, but the material is too dense and hot for heat transfer, and therefore
the energy radiates out by creating alternating parallel magnetic and electrical
fields, thus moving outward as electromagnetic radiation.
Convective Zone = A region of less dense material, the energy is primarily
carried toward the surface by heat convection currrents which carry hot gases
toward the surface before they cool and fall back inward. A really cool picture
of convection at the Sun's surface is seen below. Hot gases from the interior
have welled up and bulged to the surface where the heated gas encounters the
cool region of space above. The cooled gas sinks back into the Sun, and the
appearance of the cooler gaas changes in color from the hot white gas in the
center of each little (size of the USA on this scale) convection cell to a brown
color that outlines each convection cell.
Photosphere = A shallow region of 200 km thickness where the sun's energy
escapes as visible light. This is the area we directly see, and it is always
the coolest region of any star. Sunspots are visible on this layer, and these
objects and their cyclical appearance are areas of much intense study.
Chromosphere = A region where spicules of hot gas erupt above the photosphere
and where temperatures climb back above 10,000 K. This area demonstrates the
strongest hydrogen emission spectra.
Corona = The site of solar wind blowing out from the sun, where the out-flowing
energy energizes the thin gases to 2,000,000 K, by unknown mechanisms.
have included this drawing of the Sun's layers simply because I thought it looked
cool when I was developing this course.
This concludes your examination of the structure of the Sun. I am not even
attempting to teach some of the myriad details surrounding our nearest star,
but only want to help you become acquainted a little better
with the Sun.
From here, please move forward to Thermonuclear
Fusion to learn how the Sun generates its life-giving light and heat, or
you may return to the Sun
Introduction, or to the Syllabus.
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