Chapter 15--Our Sun - Geological Sciences

pressure
gravity
Figure 15.2 Gravitational equilibrium in the Sun: At each point
inside, the pressure pushing outward balances the weight of the
overlying layers.
Figure 15.1 An acrobat stack is in gravitational equilibrium: The
lowest person supports the most weight and feels the greatest
pressure, and the overlying weight and underlying pressure decrease
for those higher up.
Although the Sun today maintains its gravitational
equilibrium with energy generated by nuclear fusion, the
energy-generation mechanism of gravitational contraction
was important in the distant past and will be important
again in the distant future. Our Sun was born from a collapsing
cloud of interstellar gas. The contraction of the
cloud released gravitational potential energy, raising the
interior temperature higher and higher—but not high
enough to halt the contraction. The cloud continued to
shrink because thermal radiation from the cloud’s surface
carried away much of the energy released by contraction,
even while the interior temperature was rising. When the
central temperature and density eventually reached the
values necessary to sustain nuclear fusion, energy generation
in the Sun’s interior matched the energy lost from the
surface in the form of radiation. With the onset of fusion,
the Sun entered a long-lasting state of gravitational equilibrium
that has persisted for the last 4.6 billion years.
About 5 billion years from now, when the Sun finally
exhausts its nuclear fuel, the internal pressure will drop, and
gravitational contraction will begin once again. As we will see
later, some of the most important and spectacular processes
in astronomy hinge on this ongoing “battle” between the
crush of gravity and a star’s internal sources of pressure.
In summary, the answer to the question “Why does the
Sun shine?” is that about 4.6 billion years ago gravitational
contraction made the Sun hot enough to sustain nuclear
fusion in its core. Ever since, energy liberated by fusion has
maintained the Sun’s gravitational equilibrium and kept the
Sun shining steadily, supplying the light and heat that sustain
life on Earth.
15.2 Plunging to the
Center of the Sun:
An Imaginary Journey
In the rest of this chapter, we will discuss in detail how
the Sun produces energy and how that energy travels to
Earth. First, to get a “big picture” view of the Sun, let’s
imagine you have a spaceship that can somehow withstand
the immense heat and pressure of the solar interior
and take an imaginary journey from Earth to the center
of the Sun.
Approaching the Surface
As you begin your voyage from Earth, the Sun appears as a
whitish ball of glowing gas. With spectroscopy [Section 7.3],
you verify that the Sun’s mass is 70% hydrogen and 28%
helium. Heavier elements make up the remaining 2%.
The total power output of the Sun, called its luminosity,
is an incredible 3.8 10 26 watts. That is, every second,
the Sun radiates a total of 3.8 10 26 joules of energy into
space (recall that 1 watt 1 joule/s). If we could somehow
capture and store just 1 second’s worth of the Sun’s lumi-
498 part V • Stellar Alchemy