148 PART 2^ |^ THE STARS
in the low-density, high-temperature gases of a chromosphere
and corona. Also, many stars are sources of X-rays, which appear
to have been produced by the high-temperature gas in their chro-
mospheres and coronae. Th is observational evidence gives astron-
omers good reason to believe that the sun, for all its complexity,
is a typical star.
Th e layers of the solar atmosphere are all that astronomers
can observe directly, but there are phenomena in those layers that
reveal what it’s like inside the sun—your next destination.
Below the Photosphere
Almost no light emerges from below the photosphere, so you
can’t see into the solar interior. However, solar astronomers can
study naturally occurring vibrations in the sun to explore its
depths in a process called helioseismology. Random convective
movements of gas in the sun constantly produce vibrations—
rumbles that would be much too low to hear with human ears
even if your ears could survive a visit to the sun’s atmosphere.
Some of these vibrations resonate in the sun like sound waves in
organ pipes. A vibration with a period of 5 minutes is strongest,
but the periods range from 3 to 20 minutes. Th ese are very, very
low-pitched sounds!
Astronomers can detect these vibrations by observing
Doppler shifts in the solar surface. As a vibrational wave travels
down into the sun, the increasing density and temperature curve
its path, and it returns to the surface, where it makes the photo-
sphere heave up and down by small amounts—roughly plus or
minus 15 km. Many of these vibrations occurring simultane-
ously cover the surface of the sun with a pattern of rising and
falling regions that can be mapped using the Doppler eff ect
(■ Figure 8-7). By observing these motions, astronomers can
determine which vibrations resonate and become stronger and
which become weaker. Short-wavelength waves penetrate less
deeply and travel shorter distances than longer-wavelength waves,
so the diff erent wavelength vibrations explore diff erent layers in
the sun. Just as geologists can study Earth’s interior by analyzing
vibrations from earthquakes, so solar astronomers can use helio-
seismology to explore the sun’s interior.
You can better understand how helioseismology works if you
think of a duck pond. If you stood at the shore of a duck pond
and looked down at the water, you would see ripples arriving
from all parts of the pond. Because every duck on the pond con-
tributes to the ripples, you could, in principle, study the ripples
near the shore and draw a map showing the position and velocity■ Figure 8-6
Flying through the magnetic car-
pet. This computer model shows an
extreme-ultraviolet image of a section
of the sun’s lower corona (green) with
black and white areas marking regions
of opposite magnetic polarity. A math-
ematical model has drawn lines to show
how the areas are linked by loops of
magnetic force. The largest loops here
could encircle Earth. (Stanford-Lockheed
Institute for Space Research, Palo Alto, CA,
and NASA GSFC)