Clusters, Galaxies, Black Holes and Stars 285
every second. At this rate the Sun with 2 × 10^30 kilograms of material
would last 150 billion years. Only 10% of the hydrogen of the Sun can
be converted into helium, however, since the very high temperatures
needed for thermonuclear fusion only occur at the center of a star, a more
realistic estimate of the remaining lifetime of the Sun is 15 billion years.
The Sun has already existed in its present form 5 billion years. The
lifetime of other stars have been estimated to range from half a million
years for the brightest and heaviest stars to 200 billion years for the
lightest and faintest stars.
The Sun
Of all the stars in the heavens the Sun is the best known because of its
proximity. The Sun is quite a typical star. It shares many of its features
with other stars. The current mass of the Sun is 2 × 10^30 kg, which is in
the center of the range of stellar masses. The radius of the Sun, which is
spherical in shape, is 700,000 kilometers, approximately 100 times the
radius of the Earth. The density of the Sun is only 1.4 grams per cm^3 less
than the density of the Earth (5.5 grams per cm^3 ) and only slightly
greater than the density of water. The temperature of the Sun at its
surface is 6000 K and at its center 13 million K.
The surface of the Sun is observed to rotate from west to east. Unlike
the surface of the Earth all of which rotates at the same rate the various
parts of the Sun rotate at different rates. The equator completes a rotation
once every 25 days. The period of the higher latitudes, however, is
greater, increasing with latitude and reaching values as high as 33 days.
The rotation of the interior of the Sun is not known. Measurements of the
oblateness of the Sun, flattening at the Sun’s poles, is greater than that
one would expect from the centrifugal forces generated by its observed
rotation. This might indicate the interior is rotating at a greater rate than
the surface.
The structure of the Sun is quite complicated. The interior of the Sun
is divided into a number of zones. At the very center energy is being
generated by thermonuclear fusion. Above this zone lies the region
where energy is transported toward the surface by radiation. Above the
radiation zone lies the convection zone where energy is brought to the
surface by convection currents. Pockets of hot gas rise to the surface cool
off by radiation becoming heavier and then sink below the surface where
they are reheated and once again rise. In stars considerably more massive