CHAPTER 8 | THE SUN 155
It is not safe to look directly at the sun, and it is even more
dangerous to look at the sun through any optical instrument such
as a telescope, binoculars, or even the viewfi nder of a camera. Th e
light-gathering power of such an optical system concentrates the
sunlight and can cause severe injury. Never look at the sun with any
optical instrument unless you are certain it is safe. ■ Figure 8-12a
shows a safe way to observe the sun with a small telescope.
In the early 17th century, Galileo observed the sun and saw
spots on its surface; day by day he saw the spots moving across
the sun’s disk. He rightly concluded that the sun was a sphere
and was rotating. If you repeated his observations, you would
probably also detect sunspots, a view that would look something
like Figure 8-12b.
Sunspots
Th e dark sunspots that you see at visible wavelengths only hint
at the complex processes that go on in the sun’s atmosphere. To
explore those processes, you must analyze images and spectra at
a wide range of wavelengths.
Study Sunspots and the Sunspot Cycle on pages
156–157 and notice fi ve important points and four new terms:
Sunspots are cool spots on the sun’s surface caused by strong
magnetic fi elds.
Sunspots follow an 11-year cycle, becoming more numer-
ous, reaching a maximum, and then becoming much less
numerous. Th e Maunder butterfl y diagram shows how the
location of sunspots changes during a cycle.
Th e Zeeman eff ect gives astronomers a way to measure the
strength of magnetic fi elds on the sun and provide evidence
that sunspots contain strong magnetic fi elds.
Th e intensity of the sunspot cycle can vary from cycle to
cycle and appears to have almost faded away during the
Maunder minimum in the late 17th century. Th at seems to
have aff ected Earth’s climate.
Th e evidence is clear that sunspots are part of active regions
dominated by magnetic fi elds that involve all layers of the
sun’s atmosphere.
Th e sunspot groups are merely the visible traces of magneti-
cally active regions. But what causes this magnetic activity? Th e
answer is linked to the growth and decay cycle of the sun’s overall
magnetic fi eld.1
2
3
4
5
a b■ Figure 8-12
(a) Looking through a telescope at the sun is
dangerous, but you can always view the sun
safely with a small telescope by projecting its
image on a white screen. (b) If you sketch the
location and structure of sunspots on succes-
sive days, you will see the rotation of the sun
and gradual changes in the size and structure
of sunspots, just as Galileo did in 1610.