286 The Poetry of Physics and The Physics of Poetry
than the Sun the convection and radiation layers are reversed. A photon
takes approximately 10^9 seconds to journey from the center of the Sun to
the surface. In the course of its journey it is absorbed and re-emitted
several times its wavelength increasing each time.
The surface of the Sun is not well defined like that of the Earth. There
is a smooth transition from the interior of the Sun to its atmosphere. The
interior of the Sun is opaque and the atmosphere is, therefore, defined to
be that point where the gases become transparent. The surface of the Sun
or photosphere is by definition, that layer of the Sun we detect visually.
The atmosphere or the chromosphere extends several thousand
kilometers above the surface of the Sun. The temperature of the
chromosphere falls with altitude. Then suddenly the temperature of the
outer atmosphere increases very rapidly reaching temperatures as high as
a million degrees. This rise in temperature occurs when the density of the
atmosphere becomes very thin. This very thin region is known as the
corona. It serves as the launching point for the solar wind, the steady
stream of protons, alpha particles and electrons ejected by the Sun.
The surface of the Sun is mottled and granulated. The centers of the
granules are hotter than the edges. They form in sizes to about 1000
kilometers in diameter and are due to the hot gases rising to the surface
of the Sun. From time to time there appears on the photosphere dark
spots known as sunspots. The sunspots are dark because they are cooler
than the surrounding surface. Sunspots are caused by disturbances of the
Sun’s magnetic field and may be regarded as magnetic storms of the Sun.
Sunspot activity passes through very active and very tranquil cycles,
which reoccur every 11 years. During the periods of intensive sunspot
activities solar prominences and solar flares frequently take place. Solar
prominences are tremendous jets of solar material which shoot out of the
Sun several thousand kilometers above the surface and then fall back
again into the Sun. Solar flares are sudden outbursts of radioactive
energy from the surface of the Sun, which are several minutes or hours in
length. These flares are also associated with an increase in the activity of
the solar winds.
The magnetic properties of the Sun are not peculiar to it alone. Other
stars are known to have magnetic fields, considerably stronger than that
of the Sun, for those stars with smaller radii. The magnetic properties of
the Sun and other stars are detected by studying the spectra of their
emitted radiation. Magnetic fields split the energy levels of the atoms,
which show up in the radiation they emit.