CHAPTER 8 | THE SUN 145
granule. Spectra of granules show that the centers are a few hun-
dred degrees hotter than the edges, and Doppler shifts reveal that
the centers are rising and the edges are sinking at speeds of about
0.4 km/s.
From this evidence, astronomers recognize granulation as
the surface eff ects of convection just below the photosphere.
Convection occurs when hot fl uid rises and cool fl uid sinks, as
when, for example, a convection current of hot gas rises above a
candle fl ame. You can watch convection in a liquid by adding a
bit of cool non-dairy creamer to an unstirred cup of hot coff ee.
Th e cool creamer sinks, warms, expands, rises, cools, contracts,
sinks again, and so on, creating small regions on the surface of
the coff ee that mark the tops of convection currents. Viewed
from above, these regions look much like solar granules.
In the sun, rising currents of hot gas heat small regions of the
photosphere, which, being slightly hotter, emit more blackbody
radiation and look brighter. Th e cool sinking gas of the edges
emits less light and thus looks darker (Figure 8-2b). Th e presence
of granulation is clear evidence that energy is fl owing upward
through the photosphere.
Spectroscopic studies of the solar surface have revealed
another less obvious kind of granulation. Supergranules are
regions a little over twice Earth’s diameter that include about
300 granules each. Th ese supergranules are regions of very
slowly rising currents that last a day or two. Th ey appear to be
produced by larger gas currents that lie deeper under the
photosphere.
Th e edge, or limb, of the solar disk is dimmer than the cen-
ter (see the fi gure in Celestial Profi le 1). Th is limb darkening is
caused by the absorption of light in the photosphere. When you
look at the center of the solar disk, you are looking directly down
into the sun, and you see deep, hot, bright layers in the photo-
sphere. In contrast, when you look near the limb of the solar
disk, you are looking at a steep angle to the surface and cannot
see as deeply. Th e photons you see come from shallower, cooler,
dimmer layers in the photosphere. Limb darkening
proves that the temperature in the photosphere
increases with depth, yet another confi rmation that
energy is fl owing up from below.
The Chromosphere
Above the photosphere lies the chromosphere. Solar
astronomers defi ne the lower edge of the chromo-
sphere as lying just above the visible surface of the
sun, with its upper regions blending gradually with
the corona. You can think of the chromosphere as an
irregular layer with a depth on average less than
Earth’s diameter (see Figure 8-1). Because the chro-
mosphere is roughly 1000 times fainter than the
photosphere, you can see it with your unaided eyes
only during a total solar eclipse when the moon cov-
ers the brilliant photosphere. Th en, the chromosphere
fl ashes into view as a thin line of pink just above the photosphere.
Th e word chromosphere comes from the Greek word chroma,
meaning “color.” Th e pink color is produced by the combined
light of three bright emission lines—the red, blue, and violet
Balmer lines of hydrogen.
Astronomers know a great deal about the chromosphere
from its spectrum. Th e chromosphere produces an emission
spectrum, and Kirchhoff ’s second law tells you it must be an
excited, low-density gas. Th e chromosphere is about 10^8 times
less dense than the air you breathe.
Spectra reveal that atoms in the lower chromosphere are ion-
ized, and atoms in the higher layers of the chromosphere are even
more highly ionized. Th at is, they have lost more electrons.
Hydrogen atoms contain only one electron, but atoms like cal-
cium and iron contain many more, and at high enough tempera-
tures they can lose a number of electrons and become highly
ionized. From the ionization state of the gas, astronomers can
fi nd the temperature in diff erent parts of the chromosphere. Just
above the photosphere the temperature falls to a minimum of
about 4500 K and then rises rapidly (■ Figure 8-3) to the
extremely high temperatures of the corona. Th e upper chromo-
sphere is hot enough to emit X-rays and can be studied by X-ray
telescopes in space.
Solar astronomers can take advantage of some elegant phys-
ics to study the chromosphere. Th e gases of the chromosphere are
transparent to nearly all visible light, but atoms in the gas are
very good at absorbing photons of specifi c wavelengths. Th is
produces certain dark absorption lines in the spectrum of the
photosphere. A photon at one of those wavelengths is very■ Figure 8-3
The chromosphere. If you could place thermometers in the sun’s atmosphere,
you would discover that the temperature increases from 5800 K at the pho-
tosphere to 1,000,000 K at the top of the chromosphere.30001000040002000Height above photosphere (km)Temperature (K)1000 10,000 100,000 1,000,000ChromospherePhotosphereTo corona