Chapter 15--Our Sun - Geological Sciences

electron neutrinos. However, recent experiments have shown
that some of the electron neutrinos might change into muon
and tau neutrinos as they fly out through the solar plasma.
In that case, our detectors would count fewer than the expected
number of electron neutrinos. Early results from
the Sudbury Neutrino Observatory in Canada, a new detector
designed to search for all types of neutrinos, suggest
that neutrinos changing type is indeed the solution to the
solar neutrino problem. The observations are ongoing, and
it will probably be several more years before this solution
can be definitively confirmed.
Because of their roles in detecting solar neutrinos and
identifying the solar neutrino problem, Raymond Davis,
leader of the Homestake experiment, and Masatoshi Koshiba,
leader of Super-Kamiokande, shared in the 2002 Nobel
Prize for physics.
15.4 From Core to Corona
Energy liberated by nuclear fusion in the Sun’s core must
eventually reach the solar surface, where it can be radiated
into space. The path that the energy takes to the surface
is long and complex. In this section, we follow that
long path.
The Path Through the Solar Interior
In Chapter 6, we discussed how atoms can absorb or emit
photons. In fact, photons can also interact with any charged
particle, and a photon that “collides” with an electron can
be deflected into a completely new direction.
Deep in the solar interior, the plasma is so dense that
the gamma-ray photons resulting from fusion travel only
a fraction of a millimeter before colliding with an electron.
Because each collision sends the photon in a random new
direction, the photon bounces around the core in a haphazard
way, sometimes called a random walk.With each
random bounce, the photon drifts farther and farther,
on average, from its point of origin. As a result, photons
from the solar core gradually work their way outward (Figure
15.13). The technical term for this slow, outward migration
of photons is radiative diffusion (to diffuse means
to “spread out” and radiative refers to the photons of light
or radiation).
Along the way, the photons exchange energy with their
surroundings. Because the surrounding temperature declines
as the photons move outward through the Sun, they
are gradually transformed from gamma rays to photons
of lower energy. (Because energy must be conserved, each
gamma-ray photon becomes many lower-energy photons.)
By the time the energy of fusion reaches the surface, the
photons are primarily visible light. On average, the energy
released in a fusion reaction takes about a million years
to reach the solar surface.
Figure 15.13 A photon in the solar interior bounces randomly
among electrons, slowly working its way outward in a process
called radiative diffusion.
Radiative diffusion is just one type of diffusion. Another is the
diffusion of dye through a glass of water. If you place a concentrated
spot of dye at one point in the water, each individual dye
molecule begins a random walk as it bounces among the water
molecules. The result is that the dye gradually spreads through
the entire glass. Can you think of any other examples of diffusion
in the world around you?
Radiative diffusion is the primary way by which energy
moves outward through the radiation zone, which stretches
from the core to about 70% of the Sun’s radius (see Figure
15.4). Above this point, where the temperature has
dropped to about 2 million K, the solar plasma absorbs photons
more readily (rather than just bouncing them around).
This point is the beginning of the solar convection zone,
where the buildup of heat resulting from photon absorption
causes bubbles of hot plasma to rise upward in the process
known as convection [Section 10.2].Convection occurs
because hot gas is less dense than cool gas. Like a hot-air
balloon, a hot bubble of solar plasma rises upward through
the cooler plasma above it. Meanwhile, cooler plasma from
above slides around the rising bubble and sinks to lower
layers, where it is heated. The rising of hot plasma and sinking
of cool plasma form a cycle that transports energy outward
from the top of the radiation zone to the solar surface
(Figure 15.14a).
The Solar Surface
THINK ABOUT IT
Earth has a solid crust, so its surface is well defined. In
contrast, the Sun is made entirely of gaseous plasma. Defining
where the surface of the Sun begins is therefore something
like defining the surface of a cloud: From a distance
it looks quite distinct, but up close the surface is fuzzy, not
sharp. We generally define the solar surface as the layer that
appears distinct from a distance. This is the layer we identified
as the photosphere when we took our imaginary journey
into the Sun. More technically, the photosphere is the
layer of the Sun from which photons finally escape into
space after the million-year journey of solar energy outward
from the core.
508 part V • Stellar Alchemy