The Solar System

156 PART 2^ |^ THE STARS

90N

30N

0 °

30S

90S

1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Year

Penumbra

A typical sunspot is about twice the

size of Earth, but there is a wide

range of sizes. They appear, last a

few weeks to as long as 2 months,

and then shrink

away. Usually,

sunspots

occur in

pairs or

complex

groups.

1

2

2a

The dark spots that appear on the sun are only the visible

traces of complex regions of activity. Observations over

many years and at a range of wavelengths tell you that

sunspots are clearly linked to the sun’s magnetic field.

Spectra show that sunspots are cooler than the

photosphere with a temperature of about

4200 K. The photosphere has a

temperature of about 5800 K. Because

the total amount of energy radiated

by a surface depends on its

temperature raised to the fourth

power, sunspots look dark in

comparison. Actually, a

sunspot emits quite a bit of

radiation. If the sun were

removed and only an

average-size sunspot

were left behind, it

would be brighter than

the full moon.

Umbra

A typical sunspot is about twice the

size of Earth, but there is a wide

range of sizes. They appear, last a

few weeks to as long as 2 months,

and then shrink

away. Usually,

sunspots

occur in

pairs or

complex

groups.

Earth

to scale

Penumbra

Sunspots are not

shadows, but

astronomers refer to the

dark core of a sunspot as its

umbra and the outer, lighter

region as the penumbra.

Sunspots are not

shadows, but

astronomers refer to the

dark core of a sunspot as its

umbra and the outer, lighter

region as the penumbra.

The number of spots visible on the

sun varies in a cycle with a period of 11

years. At maximum, there are often over 100

spots visible. At minimum, there are very few.

Early in the cycle, spots appear at high

latitudes north and south of the sun’s

equator. Later in the cycle, the spots appear

closer to the sun’s equator. If you plot the

latitude of sunspots versus time, the graph

looks like butterfly wings, as shown in this

Maunder butterfly diagram, named after

E. Walter Maunder of Greenwich Observatory.

Hinode JAXA/NASA

NASA

Hinode JAXA/NASA

Hinode JAXA/NASA

NASA

The dark spots that appear on the sun are only the visible
traces of complex regions of activity. Observations over
many years and at a range of wavelengths tell you that
sunspots are clearly linked to the sun’s magnetic field.

Spectra show that sunspots are cooler than the
photosphere with a temperature of about
4200 K. The photosphere has a
temperature of about 5800 K. Because
the total amount of energy radiated
by a surface depends on its
temperature raised to the fourth
power, sunspots look dark in
comparison. Actually, a
sunspot emits quite a bit of
radiation. If the sun were
removed and only an
average-size sunspot
were left behind, it
would be brighter than
the full moon.

Earth

to scale

Equator

Visual wavelength imageVisual wavelength image

Hinode JAXA/NASA

Streamers above

a sunspot suggest

a magnetic field.

Year

Number of sunspots

1950 1960 1970

50

1980 1990 2000 2010

100

150

200

250

Sunspot

minimum

Sunspot

maximum