CHAPTER 8 | THE SUN 143
All cannot live on the piazza, but everyone
may enjoy the sun.
— ITALIAN PROVERBA
wit once remarked that solar astronomers would
know a lot more about the sun if it were farther
away. Th e sun is so close that Earth’s astronomers can
see swirling currents of gas and arched bridges of magnetic
force. Th e details seem overwhelming. But the sun is just an
average star; and, in a sense, it is a simple object. It is made
up almost entirely of the gases hydrogen and helium confi ned
by its own gravity in a sphere 109 times Earth’s diameter
(Celestial Profi le 1). Th e gases of the sun’s surface are hot
and radiate the light and heat that make life possible on
Earth. Th at solar atmosphere is where you can begin your
exploration.
The Solar Atmosphere
The sun’s atmosphere is made up of three layers. Th e visible
surface is the photosphere, and above that are the chromosphere
and the corona. (You fi rst met these terms in Chapter 3 when
you learned about solar eclipses.)
When you look at the sun you see a hot, glowing surface
with a temperature of about 5800 K. At that temperature, every
square millimeter of the sun’s surface must be radiating more
energy than a 60-watt light bulb. With all that energy radiating
into space, the sun’s surface would cool rapidly if energy did not
fl ow up from the interior to keep the surface hot, so simple logic
tells you that energy in the form of heat is fl owing outward from
the sun’s interior. Not until the 1930s did astronomers under-
stand that the sun makes its energy by nuclear reactions at the
center. Th ose nuclear reactions are discussed in detail later in this
chapter.
For now, you can consider the sun’s atmosphere in its quies-
cent, average state. Later you can add the details of its continuous
activity as heat fl ows up from the sun’s interior and makes the
outer layers churn like a pot of boiling soup.
The Photosphere
Th e visible surface of the sun looks like a smooth layer of gas
marked only by a few dark sunspots that come and go over a few
weeks. Although the photosphere seems to be a distinct surface, it
is not solid. In fact, the sun is gaseous from its outer atmosphere
right down to its center. Th e photosphere is the thin layer of gas
from which Earth receives most of the sun’s light. It is less than
500 km deep, and if the sun magically shrank to the size of a bowl-
ing ball, the photosphere would be no thicker than a layer of tissue
paper wrapped around the ball (■ Figure 8-1).
8-1
Th e photosphere is the layer in the sun’s atmosphere that is
dense enough to emit plenty of light but not so dense that the
light can’t escape. Below the photosphere, the gas is denser and
hotter and therefore radiates plenty of light, but that light cannot
escape from the sun because it is blocked by the outer layers of
gas. So you cannot detect light from these deeper layers. Above
the photosphere, the gas is less dense and so is unable to radiate
much light.
Although the photosphere appears to be substantial, it is
really a very-low-density gas. Even in its deepest and densest lay-
ers, the photosphere is less than 1/3000 as dense as the air you
breathe. To fi nd gases as dense as the air at Earth’s surface, you
would have to descend about 70,000 km below the photosphere,aChromosphere
PhotosphereCoronab Visual-wavelength image■ Figure 8-1
(a) A cross section at the edge of the sun shows the relative thickness
of the photosphere and chromosphere. Earth is shown for scale. On this
scale, the disk of the sun would be more than 1.5 m (5 ft) in diameter. The
corona extends from the top of the chromosphere to great height above the
photosphere. (b) This photograph, made during a total solar eclipse, shows
only the inner part of the corona. (Daniel Good)