CHAPTER 8 | THE SUN 149
Sun’s
centerSurfaceofsunComputer model of one of 10 million
possible modes of vibration for the sun.A short-wavelength wave does
not penetrate far into the sun.Rising regions have a
blueshift, and sinking
regions have a redshift.Long-wavelength waves
move deeper through the sun.of every duck on the pond. Of course, it would be diffi cult to
untangle the diff erent ripples, so you would need lots of data and
a big computer. Nevertheless, all of the information would be
there, lapping at the shore.
Helioseismology demands huge amounts of data, so astron-
omers have used a network of telescopes around the world oper-
ated by the Global Oscillation Network Group (GONG). Th e
network can observe the sun continuously for weeks at a time as
Earth rotates. Th e sun never sets on GONG. Th e SOHO satel-
lite in space can observe solar oscillations continuously and can
detect motions as slow as 1 mm/s (0.002 mph). Solar astrono-
mers can then use high-speed computers to separate the diff erent
patterns on the solar surface and measure the strength of the
waves at many diff erent wavelengths.
Helioseismology has allowed astronomers to map the tem-
perature, density, and rate of rotation inside the sun. Th ey have
been able to map great currents of gas fl owing below the photo-
sphere and the development of sunspots before they become
■ Figure 8-7
Helioseismology: The sun can vibrate
in millions of different patterns or
modes, and each mode corresponds
to a different wavelength vibration
penetrating to a different level.
By measuring Doppler shifts as the
surface moves gently up and down,
astronomers can map the inside of
the sun. (AURA/NOAO/NSF)
SCIENTIFIC ARGUMENT
What evidence leads astronomers to conclude that temperature
increases with height in the chromosphere and corona?
Scientifi c arguments usually involve evidence, and in astronomy
that means observations. Solar astronomers can observe the spec-
trum of the chromosphere, and they find that atoms there are
more highly ionized (have lost more electrons) than atoms in the
photosphere. Atoms in the corona are even more highly ionized.
That must mean the chromosphere and corona are hotter than the
photosphere.
Evidence is the key to understanding how science works. Now
it is time to build a new argument. What evidence leads astrono-
mers to conclude that other stars have chromospheres and cor-
onae like those of the sun?visible in the photosphere. Helioseismology can even locate sun-
spots on the back side of the sun, sunspots that are not yet visible
from Earth.