146 PART 2^ |^ THE STARS
■ Figure 8-4
Hα fi ltergrams reveal complex structure
in the chromosphere that cannot be
seen at visual wavelengths, including
spicules springing from the edges of
supergranules over twice the diameter
of Earth. Seen at the edge of the solar
disk, spicules look like a burning prai-
rie, but they are not at all related to
burning. Compare with Figure 8-1.
(BBSO; © 1971 NOAO/NSO; Hinode)unlikely to escape from deeper layers. A fi ltergram is an image
of the sun made using light in one of those dark absorption lines.
Th ose photons can only have escaped from higher in the sun’s
atmosphere. In this way, fi ltergrams reveal detail in the upper
layers of the chromosphere. Another way to study these high, hot
layers of gas is to record solar images in the far-ultraviolet or in
the X-ray part of the spectrum.
■ Figure 8-4 shows a fi ltergram made at the wavelength of
the Hα Balmer line. Th is image shows complex structure in the
chromosphere. Spicules are fl amelike jets of gas extending
upward into the chromosphere and lasting 5 to 15 minutes. Seen
at the limb of the sun’s disk, these spicules blend together and
look like fl ames covering a burning prairie (Figure 8-1), but they
are not fl ames at all. Spectra show that spicules are cooler gas
from the lower chromosphere extending upward into hotter
regions. Images at the center of the solar disk show that spicules
spring up around the edge of supergranules like weeds around
fl agstones (Figure 8-4).
The Solar Corona
Th e outermost part of the sun’s atmosphere is called the corona,
after the Greek word for crown. Th e corona is so dim that, like
the chromosphere, it is not visible in Earth’s daytime sky because
of the glare of scattered light from the sun’s brilliant photosphere.
During a total solar eclipse, the innermost parts of the corona are
visible to the unaided eye, as shown in Figure 8-1b. Observations
made with specialized telescopes called coronagraphs can block
the light of the photosphere and record the corona out beyond
20 solar radii, almost 10 percent of the way to Earth. Such
images reveal streamers in the corona that follow magnetic lines
of force in the sun’s magnetic fi eld (■ Figure 8-5).
Th e spectrum of the corona can tell you a great deal about
the coronal gases and simultaneously illustrate how astronomers
analyze a spectrum. Some of the light from the outer corona
produces a spectrum with absorption lines that are the same as
the photosphere’s spectrum. Th is light is just sunlight refl ected
from dust particles in the corona. In contrast, some of the light
from the corona produces a continuous spectrum that lacks
absorption lines, and that happens when sunlight from the pho-
tosphere is scattered off free electrons in the ionized coronal gas.
Because the coronal gas has a temperature over 1 million K and
the electrons travel very fast, the refl ected photons suff er large,
random Doppler shifts that smear out absorption lines to pro-
duce a continuous spectrum.
Superimposed on the corona’s continuous spectrum are emis-
sion lines of highly ionized gases. In the lower corona, the atoms
are not as highly ionized as they are at higher altitudes, and this
tells you that the temperature of the corona rises with altitude. Just
above the chromosphere, the temperature is about 500,000 K, but
in the outer corona the temperature can be 2 million K or more.Hα imageVisual-wavelength imageHα imageSpiculesDiameter of the Earth (8000 miles)