CHAPTER 8 | THE SUN 165
▶ (^) Alternate sunspot cycles have reversed magnetic polarity, which
has been explained by the Babcock model (p. 158), in which the
differential rotation (p. 158) of the sun winds up the magnetic fi eld.
Tangles in the fi eld arch above the surface and cause active regions
visible to your eyes as sunspot pairs. When the fi eld becomes strongly
tangled, it reorders itself into a simpler but reversed fi eld, and the
cycle starts over.
▶ (^) Arches of magnetic fi eld are visible as prominences (p. 162) in the
chromosphere and corona. Seen from above in fi ltergrams, promi-
nences are visible as dark fi laments (p. 162) silhouetted against the
bright chromosphere.
▶ (^) Reconnections (p. 163) of magnetic fi elds can produce powerful
fl ares (p. 163), sudden eruptions of X-ray, ultraviolet, and
visible radiation plus high-energy atomic particles. Flares are
important because they can have dramatic effects on Earth, such as
communications blackouts.
▶ (^) The solar wind originates in regions on the solar surface called coronal
holes (p. 163), where the sun’s magnetic fi eld leads out into space
and does not loop back to the sun.
▶ (^) Coronal mass ejections, or CMEs (p. 163), occur when magnetic
fi elds on the surface of the sun eject bursts of ionized gas that fl ow
outward in the solar wind. Such bursts can produce auroras (p. 163)
and other phenomena if they strike Earth.
▶ (^) Other stars are too far away for starspots to be visible, but some stars
vary in brightness in ways that show they have spots on their surfaces.
Also, spectroscopic observations reveal that many other stars have
spots and magnetic fi elds that follow long-term cycles like the sun’s.
▶ (^) Small changes in the solar constant (p. 161) over decades can affect
Earth’s climate and may be responsible or the Little Ice Age and other
climate fl uctuations in Earth’s history.
Review Questions
- Why can’t you see deeper into the sun than the photosphere?
- What evidence can you give that granulation is caused by convection?
- How are granules and supergranules related? How do they differ?
- How can astronomers detect structure in the chromosphere?
- What evidence can you give that the corona has a very high
temperature? - What heats the chromosphere and corona to a high temperature?
- How are astronomers able to explore the layers of the sun below the
photosphere? - Why does nuclear fusion require high temperatures?
- Why does nuclear fusion in the sun occur only near the center?
- How can astronomers detect neutrinos from the sun?
- How did neutrino oscillation affect the detection of solar neutrinos by
the Davis experiment? - What evidence can you give that sunspots are magnetic?
- How does the Babcock model explain the sunspot cycle?
- What does the spectrum of a prominence reveal? What does its shape
reveal? - How can solar fl ares affect Earth?
- How Do We Know? What does it mean when scientists say they are
certain? What does scientifi c certainty really mean? - How Do We Know? How does consolidation extend scientifi c
understanding?
Discussion Questions
- What energy sources on Earth cannot be thought of as stored
sunlight? - What would the spectrum of an auroral display look like? Why?
▶ (^) The corona is the sun’s outermost atmospheric layer and can be
imaged using a coronagraph (p. 146). It is composed of a very-low-
density, very hot gas extending many solar radii from the visible sun.
Its high temperature—over 2,000,000 K—is believed to be maintained
by the magnetic fi eld extending up through the photosphere—the
magnetic carpet (p. 147)—and by magnetic waves coming from
below the photosphere.
▶ (^) Parts of the corona give rise to the solar wind (p. 147), a breeze of
low-density ionized gas streaming away from the sun.
▶ (^) Solar astronomers can study the motion, density, and temperature of
gases inside the sun by analyzing the way the solar surface oscillates.
Known as helioseismology (p. 148), this fi eld of study requires large
amounts of data and extensive computer analysis.
▶ (^) There are only four forces in nature: the electromagnetic force, the
gravitational force, the weak force (p. 150), and the strong force
(p. 150). In nuclear fi ssion or nuclear fusion, the energy comes from
the strong force. Physicists are working on unifying the four forces
under one mathematical description.
▶ (^) Nuclear reactors on Earth generate energy through nuclear fi ssion
(p. 150), during which large nuclei such as uranium break into smaller
fragments. The sun generates its energy through nuclear fusion
(p. 150), during which hydrogen nuclei fuse to produce helium nuclei.
▶ (^) Hydrogen fusion in the sun proceeds in three steps known as the
proton–proton chain (p. 151). The fi rst step in the chain combines
two hydrogen nuclei to produce a heavy hydrogen nucleus called deu-
terium (p. 151). The second step forms light helium, and the third
step combines the light helium nuclei to form normal helium. Energy
is released as positrons (p. 151), neutrinos (p. 151), gamma rays,
and the rapid motion of particles fl ying away.
▶ (^) Fusion can occur only at the center of the sun because charged particles
repel each other, and high temperatures are needed to give particles
high enough velocities to penetrate this Coulomb barrier (p. 152).
High densities are needed to provide large numbers of reactions.
▶ (^) Neutrinos escape from the sun’s core at nearly the speed of light,
carrying away about 2 percent of the energy produced by fusion.
Observations of fewer neutrinos than expected coming from the sun’s
core are now explained by the oscillation of neutrinos among three
different types (fl avors). The detection of solar neutrinos confi rms the
theory that the sun’s energy comes from hydrogen fusion.
▶ (^) Energy fl ows out of the sun’s core as photons traveling through the
radiative zone (p. 152) and closer to the surface as rising currents
of hot gas and sinking currents of cooler gas in the convective zone
(p. 152).
▶ (^) Sunspots seem dark because they are slightly cooler than the rest of
the photosphere. The average sunspot is about twice the size of Earth.
They appear for a month or so and then fade away, and the number of
spots on the sun varies with an 11-year cycle.
▶ (^) Early in a sunspot cycle, spots appear farther from the sun’s equator,
and later in the cycle they appear closer to the equator. This is shown
in the Maunder butterfl y diagram (p. 155).
▶ (^) Astronomers can use the Zeeman effect (p. 155) to measure mag-
netic fi elds on the sun. The average sunspot contains magnetic fi elds a
few thousand times stronger than Earth’s. This is part of the evidence
that the sunspot cycle is produced by a solar magnetic cycle.
▶ (^) The sunspot cycle does not repeat exactly each cycle, and the decades
from 1645 to 1715, known as the Maunder minimum (p. 155), seem
to have been a time when solar activity was very low and Earth’s
climate was slightly colder.
▶ (^) Sunspots are the visible consequences of active regions (p. 155)
where the sun’s magnetic fi eld is strong. Arches of magnetic fi eld can
produce sunspots where the fi eld passes through the photosphere.
▶ (^) The sun’s magnetic fi eld is produced by the dynamo effect (p. 158)
operating at the base of the convection zone.