Chapter 15--Our Sun - Geological Sciences

COMMON MISCONCEPTIONS
The Sun Is Not on Fire
fission
fusion
We are accustomed to saying that the Sun is “burning,”
a way of speaking that conjures up images of a giant bonfire
in the sky. However, the Sun does not burn in the
same sense as a fire burns on Earth. Fires on Earth generate
light through chemical changes that consume oxygen
and produce a flame. The glow of the Sun has more
in common with the glowing embers left over after the
flames have burned out. Much like hot embers, the Sun’s
surface shines with the visible thermal radiation produced
by any object that is sufficiently hot [Section 6.4].
However, hot embers quickly stop glowing as they
cool, while the Sun keeps shining because its surface is
kept hot by the energy rising from the Sun’s core. Because
this energy is generated by nuclear fusion, we
sometimes say that it is the result of “nuclear burning”—
a term that suggests nuclear changes in much the same
way that “chemical burning” suggests chemical changes.
Nevertheless, while it is reasonable to say that the Sun
undergoes nuclear burning in its core, it is not accurate
to speak of any kind of burning on the Sun’s surface,
where light is produced primarily by thermal radiation.
No real spacecraft could survive, but your imaginary
one keeps plunging straight down to the solar core.There
you finally find the source of the Sun’s energy: nuclear fusion
transforming hydrogen into helium. At the Sun’s center,
the temperature is about 15 million K, the density is more
than 100 times that of water, and the pressure is 200 billion
times that on the surface of Earth. The energy produced
in the core today will take about a million years to
reach the surface.
With your journey complete, it’s time to turn around
and head back home. We’ll continue this chapter by studying
fusion in the solar core and then tracing the flow of the
energy generated by fusion as it moves outward through
the Sun.
15.3 The Cosmic Crucible
The prospect of turning common metals like lead into
gold enthralled those who pursued the medieval practice
of alchemy. Sometimes they tried primitive scientific approaches,
such as melting various ores together in a vessel
called a crucible. Other times they tried magic. Their getrich-quick
schemes never managed to work. Today we
know that there is no easy way to turn other elements into
gold, but it is possible to transmute one element or isotope
into another.
If a nucleus gains or loses protons, its atomic number
changes and it becomes a different element. If it gains or
Figure 15.5 Nuclear fission splits a nucleus into smaller nuclei
(not usually of equal size), while nuclear fusion combines smaller
nuclei into a larger nucleus.
loses neutrons, its atomic mass changes and it becomes a
different isotope [Section 4.3].The process of splitting a nucleus
into two smaller nuclei is called nuclear fission.The
process of combining nuclei to make a nucleus with a greater
number of protons or neutrons is called nuclear fusion
(Figure 15.5). Human-built nuclear power plants rely
on nuclear fission of uranium or plutonium. The nuclear
power plant at the center of the Sun relies on nuclear fusion,
turning hydrogen into helium.
Nuclear Fusion
The 15 million K plasma in the solar core is like a “soup”
of hot gas, with bare, positively charged atomic nuclei (and
negatively charged electrons) whizzing about at extremely
high speeds. At any one time, some of these nuclei are on
high-speed collision courses with each other. In most cases,
electromagnetic forces deflect the nuclei, preventing actual
collisions, because positive charges repel one another. If
nuclei collide with sufficient energy, however, they can stick
together to form a heavier nucleus (Figure 15.6).
Sticking positively charged nuclei together is not easy.
The strong force,which binds protons and neutrons together
in atomic nuclei, is the only force in nature that can
At low speeds, electromagnetic
repulsion prevents the collision
of nuclei.
At high speeds, nuclei come close
enough for the strong force to bind
them together.
Figure 15.6 Positively charged nuclei can fuse only if a highspeed
collision brings them close enough for the strong force
to come into play.
chapter 15 • Our Star 501