160 PART 2^ |^ THE STARS
Chromospheric and Coronal Activity
Th e solar magnetic fi elds extend high into the chromosphere and
corona, where they produce beautiful and powerful phenomena.
Study Magnetic Solar Phenomena on pages 162–163
and notice three important points and seven new terms:All solar activity is magnetic. Th e arched shapes of promi-
nences are produced by magnetic fi elds, and fi laments are
prominences seen from above.
Tremendous energy can be stored in arches of magnetic fi eld,
and when two arches encounter each other a reconnection
can release powerful eruptions called fl ares. Although these
eruptions occur far from Earth, they can aff ect us in dra-
matic ways, and coronal mass ejections (CMEs) can trigger
communications blackouts and auroras.
In some regions of the solar surface, the magnetic fi eld does
not loop back. High-energy gas from these coronal holes
fl ows outward and produces much of the solar wind.You may have heard the Common Misconception that
an auroral display in the night sky is caused by sunlight refl ecting off
of the ice and snow at Earth’s North Pole. It is fun to think of polar
bears standing on sunlit slabs of ice, but that doesn’t cause auroras.
You know that auroras are produced by gases in Earth’s upper atmo-
sphere excited to glowing by energy from the solar wind.1
2
3
certain stars (■ Figure 8-16a). Such results confi rm that the
sunspots you see on our sun are not unusual.
Certain features found in stellar spectra are associated with
magnetic fi elds. Regions of strong magnetic fi elds on the solar
surface emit strongly at the central wavelengths of the two stron-
gest lines of ionized calcium. Th is calcium emission appears in
the spectra of other sun-like stars and suggests that these stars,
too, have strong magnetic fi elds on their surfaces. In some cases,
the strength of this calcium emission varies over periods of days
or weeks and suggests that the stars have active regions and are
rotating with periods similar to that of the sun. Th ese stars pre-
sumably have sunspots (“starspots”) as well.
In 1966, astronomers began a long-term project that moni-
tored the strengths of these calcium emission features in the
spectra of 91 stars with temperatures ranging from 1000 K hotter
than the sun to 3000 K cooler, considered most likely to have
sun-like magnetic activity on their surfaces. Th e observations
show that the strength of the calcium emission varies over peri-
ods of years. Th e calcium emission averaged over the sun’s disk
varies with the sunspot cycle, and similar periodic variations can
be seen in the spectra of some of the stars studied (Figure 8-16b).
Th e star 107 Piscium, for example, appears to have a starspot
cycle lasting nine years. At least one star, tau Bootis, has been
observed to reverse its magnetic fi eld. Th is kind of evidence sug-
gests that stars like the sun have similar magnetic cycles, and that
the sun is normal in this respect.
Confi rmation and Consolidation
What do scientists do all day? The scientifi c
method is sometimes portrayed as a kind
of assembly line where scientists crank out
new hypotheses and then test them through
observation. In reality, scientists don’t often
generate entirely new hypotheses. It is rare
that an astronomer makes an observation that
disproves a long-held theory and triggers a
revolution in science. Then what is the daily
grind of science really about?
Many observations and experiments confi rm
already-tested hypotheses. The biologist
knows that all worker bees in a hive are sisters
because they are all female, and they all had
the same mother, the queen bee. A biolo-
gist can study the DNA from many workers
and confi rm that hypothesis. By repeatedly
confi rming a hypothesis, scientists build con-
fi dence in the hypothesis and may be able to
extend it. Do all of the workers in a hive have
the same father, or did the queen mate with
more than one male drone?
Another aspect of routine science is
consolidation, the linking of a hypothesis to
other well-studied phenomena. A biologist
can study yellow jacket wasps from a single
nest and discover that the wasps, too, are
sisters. There must be a queen wasp who lays
all of the eggs in a nest. But in a few nests,
the scientist may fi nd two sets of unrelated
sister workers. Those nests must contain
two queens sharing the nest for convenience
and protection. From his study of wasps, the
biologist consolidates what he knows about
bees with what others have learned about
wasps and reveals something new: That bees
and wasps have evolved in similar ways for
similar reasons.
Confi rmation and consolidation allow
scientists to build confi dence in theirunderstanding and extend it to explain
more about nature.8-2
A yellow jacket is a wasp from a nest
containing a queen wasp. (Michael Dunham/
Getty Images)