Solar Structure
The main regions of the Sun, not drawn to
scale, with some physical dimensions labeled.
Solar
Oscillations
(a)
The Sun has been found to vibrate in
a very complex way. By observing
the motion of the solar surface,
scientists can determine the
wavelength and the frequencies of
the individual waves and deduce
information about the solar interior
not obtainable by other means. The
alternating patches represent gas
moving down (red) and up (blue).
(b)
(b) Depending on their initial
directions, the waves contributing to
the observed oscillations may travel
deep inside the Sun, providing vital
information about the solar
interior. (National Solar Observatory)
Solar Interior
Theoretically modeled profiles of density (b) and temperature (c) for the solar
interior, presented for perspective in (a). All three parts describe a cross-sectional
cut through the center of the Sun.
Solar Convection
Physical transport of energy in the Sun’s
convection zone. We can visualize the upper
interior as a boiling, seething sea of gas. Each
convective loop is about 1000 km across. The
convective cell sizes become progressively
smaller closer to the surface. (This is a highly
simplified diagram; there are many different cell
sizes, and they are not so neatly arranged.)
Solar
Granulation P
hotograph of the granulated solar
photosphere. Typical solar
granules are comparable in size
to Earth’s continents. The bright
portions of the image are regions
where hot material is upwelling
from below. The dark regions
correspond to cooler gas that is
sinking back down into the
interior. (Big Bear Solar
Observatory)
Figure
16.8 Solar
Spectrum A detailed
spectrum of our Sun shows thousands
of Fraunhofer spectral lines which
indicate the presence of some 67
different elements in various stages of
excitation and ionization in the lower
solar atmosphere. The numbers give
wavelengths, in nanometers.(Palomar
Observatory/Caltech)
Figure
16.9 Spectral Line
Formation Photons with
energies well away from any atomic transition
can escape from relatively deep in the
photosphere, but those with energies close to a
transition are more likely to be reabsorbed
before escaping, so the ones we see on Earth
tend to come from higher, cooler levels in the
solar atmosphere. The inset shows a close-up
tracing of two of the thousands of solar
absorption lines, the “H” and “K” lines of calcium
at about 395 nm.
Solar
Chromosphere
This photograph of a total solar eclipse
shows the solar chromosphere, a few
thousand kilometers above the Sun’s
surface. (G. Schneider)
Figure 16.11 Solar
Spicules Short-lived narrow jets of gas
that typically last mere minutes, spicules can be seen
sprouting up from the solar chromosphere in this Ha
image of the Sun. The spicules are the thin, dark,
spikelike regions. They appear dark against the face of
the Sun because they are cooler than the solar
photosphere. (NOAO)
Solar
Corona
When both the
photosphere and the
chromosphere are
obscured by the Moon
during a solar eclipse, the
faint corona becomes
visible. This photograph
shows clearly the
emission of radiation from
the solar corona. (G.
Schneider)
Solar
Atmospheric
Temperature The change
of gas temperature in the lower solar
atmosphere is dramatic. The minimum
temperature occurs in the chromosphere.
Beyond that, the temperature rises sharply in
the transition zone, finally leveling off at around
3 million K in the corona.
Sunspots This
photograph of the entire Sun, taken
during a period of maximum solar
activity, shows several groups of
sunspots. The largest spots in this
image are over 20,000 km across—
twice the diameter of Earth. Typical
sunspots are only about half this size.
(Palomar Observatory/Caltech)
Sunspots, Up Close (a) An
enlarged photograph of the largest pair of sunspots in Figure
16.15. Each spot consists of a cool, dark inner region called
the umbra, surrounded by a warmer, brighter region called the
penumbra. The spots appear dark because they are slightly
cooler than the surrounding photosphere. (b) A highresolution, true-color image of a single sunspot shows details
of its structure as well as much surface granularity
surrounding it. The spot is about the size of Earth. (Palomar
Observatory/Caltech; National Solar Observatory
Sunspot Rotation
The evolution of some sunspots and lower
chromospheric activity over a period of 12 days.
The sequence runs from left to right. An
H
filter was used to make these
photographs, taken from the Skylab space
station in 1975. An arrow follows one set of
sunspots over the course of a week as they are
carried around the Sun by its rotation. (NASA)
Solar Magnetism
(a) Sunspot pairs are linked by magnetic field
lines. The Sun’s magnetic field lines emerge
from the surface through one member of a pair
and reenter the Sun through the other member.
The leading members of all sunspot pairs in the
solar northern hemisphere have the same
polarity (labeled N or S, as described in the
text). If the magnetic field lines are directed into
the Sun in one leading spot, they are inwardly
directed in all other leading spots in that
hemisphere. The same is true in the southern
hemisphere, except that the polarities are
always opposite those in the north. The overall
direction of the magnetic field reverses itself
roughly every 11 years. (b) A far-ultraviolet
image taken by NASA’s Transition Region and
Coronal Explorer (TRACE) satellite in 1999,
showing magnetic field lines arching between
two sunspot groups. Note the complex structure
of the field lines, which are seen here via the
radiation emitted by superheated gas flowing
along them. Resolution here is about 700 km. In
this negative image (which shows the lines more
clearly), the darkest regions have temperatures
of about 2 million K. (NASA)
Solar Rotation The
Sun’s differential rotation wraps and distorts the
solar magnetic field. Occasionally, the field lines
burst out of the surface and loop through the
lower atmosphere, thereby creating a sunspot
pair. The underlying pattern of the solar field
lines explains the observed pattern of sunspot
polarities. If the loop happens to occur on the
limb of the Sun and is seen against the
blackness of space, we see a phenomenon
called a prominence, described in Section 16.4.
Sunspot Cycle (a)
Annual number of sunspots throughout the
twentieth century, showing the five-year
average of the annual data to make long-term
trends more evident. The (roughly) 11-year
solar cycle is clearly visible. At the time of solar
minimum, hardly any sunspots are seen. About
four years later, at solar maximum, as many as
100–200 spots are observed per year. The
most recent solar maximum occurred in 2000.
(b) Sunspots cluster at high latitudes when solar
activity is at a minimum. They appear at lower
latitudes as the number of sunspots peaks.
Finally, they are prominent near the Sun’s
equator as solar minimum is again approached.
Maunder
Minimum
Number of sunspots occurring
each year over the past four
centuries. Note the absence of
spots during the late
seventeenth century.
Is Solar
Luminosity
Related to Sun
Spot Numbers?
Solar
Prominences
(a) This particularly large solar
prominence was observed by
ultraviolet detectors aboard the
SOHO spacecraft in September,
1999. (b) Like a phoenix rising from
the solar surface, this filament of
hot gas measures more than
100,000 km in length. Earth could
easily fit between its outstretched
“arms.” Dark regions in this TRACE
image have temperatures less than
20,000 K; the brightest regions are
about 1 million K. The ionized gas
follows the solar magnetic field
lines away from the Sun. Most of it
will subsequently cool and fall back
to the photosphere. (NASA)
Solar Flares (a) Much
more violent than a prominence, a solar flare is
an explosion on the Sun’s surface that sweeps
across an active region in a matter of minutes,
accelerating solar material to high speeds and
blasting it into space. (b) A flare occurs when
hot gas breaks free of the magnetic field
confining it and bursts into space. This
composite image shows a dark sunspot group
(visible light), the surrounding solar
photosphere (ultraviolet, shown in red here) and
a collection of magnetic loops (extreme
ultraviolet, colored green here) confining milliondegree gas a few minutes before a major flare
in June 2000. (c) This remarkable image,
obtained by the TRACE satellite, shows an
active region shortly after a flare. The green
“slinky spring” is actually an arcade of many
magnetic field lines arching through the Sun’s
lower atmosphere roughly from top to bottom,
confining hot gas as the Sun’s magnetism
reestablishes control. (NASA)
sun
A little about the solar interior
Time scale for contraction under gravity: ~30 Myrs
Time scale for hydrostatic adjustment: ~1 hr
Convective layer:
When the adiabatic lapse rate is too large in the
Radiative Equilibrium Model, convection takes over.
In the atmosphere this is the troposphere, but in the star
it is the outer 200,000 km.