The Surface of the Sun
ATLAS OF THE UNIVERSE
U
se a telescope to project the Sun’s image, and you will
see that the yellow disk is brightest at its centre and
less brilliant at the edges; this is because towards the cen-
tre we are seeing into deeper and therefore hotter layers.
There may be one or more darker patches which are
known as sunspots. The spots are not genuinely black, but
appear so because they are cooler than the surrounding
regions of the photosphere.
A major spot is made up of a dark central portion or
umbra, surrounded by a lighter penumbra. Sometimes the
shapes are regular; sometimes they are very complex, with
many umbrae contained in a single mass of penumbra. The
temperature of the umbra is about 4500 degrees C, and
of the penumbra 5000 degrees C (as opposed to 6000
degrees C for the surrounding unaffected photosphere), so
that if a spot could be seen shining on its own, the surface
brilliance would be greater than that of an arc-lamp.
Spots generally appear in groups. An ‘average’ two-
spot group begins as a pair of tiny pores at the limit of
visibility. The pores develop into proper spots, growing and
separating in longitude; within two weeks the group has
reached its maximum length, with a fairly regular leading
spot and a less regular follower, together with many smaller
spots spread around in the area. A slow decline then sets
in, usually leaving the leader as the last survivor. Around
75 per cent of groups fit into this pattern, but there are
many variations, and single spots are also common.
Sunspots may be huge; the largest on record, that of
April 1947, covered an area of over 18,000 million square
kilometres (7000 million square miles) when at its largest.
Obviously they are not permanent. A major group may
persist for anything up to six months, though very small
spots often have lifetimes of less than a couple of hours.
Spots are essentially magnetic phenomena, and there is
a fairly predictable cycle of events. Maxima, with many
groups on view simultaneously, occur every 11 years or1660 1680 1700 1720 1740 1760 1780 1800 1820 1840 1860 1880 1900 1920 1940 1960 1980 2000
200
160
120
80
40
0
▼ Solar rotation.This
sequence shows the giant
sunspot group of 1947.
After passing round the far
side of the Sun, it reappeared
to make a second crossing.▲ Differential rotation. The
rotation period of the
photosphere increases with
increasing latitude. In the
idealized situation shown
here, if a row of sunspots
lay along the Sun’s central
meridian, then, after one
rotation, the spots would
be spread out in a curve. The Great Sunspot of
1947 – the largest known.
On April 8, it covered
18,000 million square km
(7000 million square miles). Sunspotsphotographed
by H. J. P. Arnold.▼ The solar cycle, 1650 to
present. Not all maxima are
equally energetic, and during
the ‘Maunder Minimum’,
1645–1715, it seems that the
cycle was suspended, though
therecords are incomplete.
The vertical scale is the
Zürich number, calculated
from the number of groups
and the number of spots.E152-191 UNIVERSE UK 2003mb 7/4/03 5:41 pm Page 156