PART 2 | THE STARS
In contrast, an earlier period called the Grand Maximum,
lasting from about ad 1100 to about 1250, saw a warming of
Earth’s climate. Th e Vikings were able to explore and colonize
Greenland, and native communities in parts of North America
were forced to abandon their settlements because of long droughts.
Th e Grand Maximum may have been caused by a small change
in solar activity, but the evidence is not conclusive.
Other minima and maxima have been found in climate data
taken from studies of the growth rings of trees. In good years,
trees add a thicker growth ring than in poor years, so measuring
tree rings can reveal the climate in the past. Evidently, solar activ-
ity can increase or decrease the solar constant very slightly and
aff ect Earth’s climate in dramatic ways. Th e future of our civiliza-
tion on Earth may depend on our learning to understand the
solar constant.
SCIENTIFIC ARGUMENT
What kind of activity would the sun have if it didn’t rotate
differentially?
Once again, it can help you understand a concept by constructing
an argument with one factor changed. Begin by thinking about the
Babcock model. If the sun didn’t rotate differentially, its equator
traveling faster than its higher latitudes, then the magnetic fi eld
might not get wound up, and there might not be a solar cycle.
Twisted tubes of magnetic fi eld might not form and rise through
the photosphere to produce prominences and fl ares, although con-
vection might tangle the magnetic fi eld slightly and produce some
activity. Is the magnetic activity that heats the chromosphere and
corona driven by differential rotation or by convection? It is hard
to guess; but, without differential rotation, the sun might not
have a strong magnetic fi eld and high-temperature gas above its
photosphere.
This is very speculative, but speculating within a scientific
argument can be revealing. For example, redo the argument above.
What do you think the sun would be like if it had no convec-
tion inside?What Are We? SunlightWe live very close to a star and depend on
it for survival. All of our food comes from
sunlight that was captured by plants on
land or by plankton in the oceans. We either
eat those plants directly or eat the animals
that feed on those plants. Whether you had
salad, seafood, or a cheeseburger for supper
last night, you dined on sunlight, thanks to
photosynthesis.
Almost all of the energy that powers
human civilization came from the sunthrough photosynthesis in ancient plants
that were buried and converted to coal, oil,
and natural gas. New technology is making
energy from plant products like corn, soy
beans, and sugar. It is all stored sunlight.
Windmills generate electrical power, and
the wind blows because of heat from the
sun. Photocells make electricity directly
from sunlight. Even our bodies have
adapted to use sunlight to manufacture
vitamin D.Our planet is warmed by the sun, and
without that warmth the oceans would be
ice and much of the atmosphere would be a
coating of frost. Books often refer to the
sun as “our sun” or “our star.” It is ours in
the sense that we live beside it and by its
light and warmth, but we can hardly say it
belongs to us. It is better to say that we
belong to the sun.Summary
▶ (^) The sun is very bright, and its light and infrared radiation can burn
your eyes, so you must take great care in observing it. At sunset or
sunrise when it is safe to look at the sun, you see the sun’s photo-
sphere, the level in the sun from which visible photons most easily
escape. Dark sunspots (p. 143) come and go on the sun, but only
rarely are they large enough to be visible to the unaided eye.
▶ (^) The solar atmosphere consists of three layers of hot, low-density gas:
the photosphere, chromosphere, and corona.
▶ (^) The granulation (p. 144) of the photosphere is produced by
convection (p. 145) currents of hot gas rising from below. Larger
supergranules (p. 145) appear to be caused by larger convection cur-
rents deeper in the sun.
▶ (^) The edge or limb (p. 145) of the solar disk is dimmer than the center.
This limb darkening (p. 145) is evidence that the temperature in the
solar atmosphere increases with depth.
▶ (^) The chromosphere is most easily visible during total solar eclipses,
when it fl ashes into view for a few seconds. It is a thin, hot layer of
gas just above the photosphere, and its pink color is caused by the
Balmer emission lines in its spectrum.
▶ (^) Filtergrams (p. 146) of the chromosphere reveal spicules (p. 146),
fl amelike structures extending upward into the lower corona.