Chapter 15--Our Sun - Geological Sciences

I say Live, Live, because of the Sun,
The dream, the excitable gift.
Anne Sexton (1928–1974)
Astronomy today involves the study of the
entire universe, but the root of the word
astronomy comes from the Greek word
for “star.” Although we have learned a lot about the
universe up to this point in the book, only now do
we turn our attention to the study of the stars, the
namesakes of astronomy.
When we think of stars, we usually think of the
beautiful points of light visible on a clear night. The
nearest and most easily studied star is visible only
in the daytime—our Sun. Of course, the Sun is important
to us in many more ways than as an object
for astronomical study. The Sun is the source of virtually
all light, heat, and energy reaching Earth, and life
on Earth’s surface could not survive without it.
In this chapter, we will study the Sun in some
depth. We will learn how the Sun makes life possible
on Earth. Equally important, we will study our Sun as
a star so that in subsequent chapters we can more
easily understand stars throughout the universe.
15.1 Why Does the Sun Shine?
Ancient peoples recognized the vital role of the Sun in their
lives. Some worshiped the Sun as a god, and others created
elaborate mythologies to explain its daily rise and set. Only
recently, however, have we learned how the Sun provides us
with light and heat.
Most ancient thinkers viewed the Sun as some type of
fire, perhaps a lump of burning coal or wood. The Greek
philosopher Anaxagoras (c. 500–428 B.C.) imagined the Sun
to be a very hot, glowing rock about the size of the Greek
peninsula of Peloponnesus (comparable in size to Massachusetts).
Thus, he was one of the first people in history to
believe that the heavens and Earth are made from the same
types of materials.
By the mid-1800s, the size and distance of the Sun
were reasonably well known, and scientists seriously began
to address the question of how the Sun shines. Two early
ideas held either that the Sun was a cooling ember that had
once been much hotter or that the Sun generated energy
from some type of chemical burning similar to the burning
of coal or wood. Simple calculations showed that a cooling
or chemically burning Sun could shine for a few thousand
years—an age that squared well with biblically based estimates
of Earth’s age that were popular at the time. However,
these ideas suffered from fatal flaws. If the Sun were a cooling
ember, it would have been much hotter just a few hundred
years earlier, making it too hot for civilization to have
existed. Chemical burning was ruled out because it cannot
generate enough energy to account for the rate of radiation
observed from the Sun’s surface.
A more plausible hypothesis of the late 1800s suggested
that the Sun generates energy by contracting in size,
a process called gravitational contraction.Ifthe Sun were
shrinking, it would constantly be converting gravitational
potential energy into thermal energy, thereby keeping the
Sun hot. Because of its large mass, the Sun would need to
contract only very slightly each year to maintain its temperature—so
slightly that the contraction would be unnoticeable.
Calculations showed that the Sun could shine
for up to about 25 million years generating energy by gravitational
contraction. However, geologists of the late 1800s
had already established the age of Earth to be far older than
25 million years, leaving astronomers in an embarrassing
position.
Only after Einstein published his special theory of
relativity, which included his discovery of E mc 2 ,did
the true energy-generation mechanism of the Sun become
clear. We now know that the Sun generates energy by nuclear
fusion, a source so efficient that the Sun can shine for
about 10 billion years. Because the Sun is only 4.6 billion
years old today [Section 9.5],we expect it to keep shining
for some 5 billion more years.
According to our current model of solar-energy generation
by nuclear fusion, the Sun maintains its size through
a balance between two competing forces: gravity pulling
inward and pressure pushing outward. This balance is called
gravitational equilibrium (or hydrostatic equilibrium).
It means that, at any point within the Sun, the weight of
overlying material is supported by the underlying pressure.
A stack of acrobats provides a simple example of this balance
(Figure 15.1). The bottom person supports the weight
of everybody above him, so the pressure on his body is
very great. At each higher level, the overlying weight is less,
so the pressure decreases. Gravitational equilibrium in the
Sun means that the pressure increases with depth, making
the Sun extremely hot and dense in its central core (Figure
15.2).
THINK ABOUT IT
Earth’s atmosphere is also in gravitational equilibrium, with the
weight of upper layers supported by the pressure in lower
layers. Use this idea to explain why the air gets thinner at higher
altitudes.
chapter 15 • Our Star 497