AY 101 Chapter Notes - Chapter 11: Coronal Mass Ejection, Convection Zone, Radiation Zone
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The Big Picture
In this chapter, we have examined our Sun, the nearest star. When you look back at this chapter, make
sure you understand the following “big picture” ideas:
•The ancient mystery of why the Sun shines is now solved. The Sun shines with energy generated by
fusion of hydrogen into helium in the Sun’s core. After a journey through the solar interior lasting
several hundred thousand years and an 8-minute journey through space, a small fraction of this energy
reaches Earth and supplies sunlight and heat.
•The Sun shines steadily thanks to the balance between pressure and gravity (gravitational equilibrium)
and the balance between energy production in the core and energy release at the surface (energy
balance). These two kinds of balance create a natural thermostat that regulates the Sun’s fusion rate,
keeping the Sun shining steadily and allowing life to ﬂourish on Earth.
•The Sun’s atmosphere displays its own version of weather and climate, governed by solar magnetic
ﬁelds. Some solar weather, such as coronal mass ejections, clearly affects Earth’s magnetosphere. Other
claimed connections between solar activity and Earth’s climate may or may not be real.
•The Sun is important not only as our source of light and heat, but also because it is the only star near
enough for us to study in great detail. In the coming chapters, we will use what we’ve learned about the
Sun to help us understand other stars.
Summary of Key Concepts
Why does the Sun shine?
•The Sun began to shine about 4.5 billion years ago when gravitational contraction made its core hot
enough to sustain nuclear fusion. It has shined steadily ever since because of two types of balance:
•Gravitational equilibrium, a balance between the outward push of pressure and the inward pull of
•Energy balance between the energy released by fusion in the core and the energy radiated into space
from the Sun’s surface
What is the Sun’s structure?
•The Sun’s interior layers, from the inside out, are the core, the radiation zone, and the convection
zone. Atop the convection zone lies the photosphere, the surface layer from which photons can freely
escape into space. Above the photosphere are the warmer chromosphere and the very hot corona.
How does nuclear fusion occur in the Sun?
•The core’s extreme temperature and density are just right for fusion of hydrogen into helium, which
occurs via the proton-proton chain. Because the fusion rate is so sensitive to temperature, gravitational
equilibrium and energy balance act together as a thermostat to keep that rate steady.
How does the energy from fusion get out of the Sun?
•Energy moves through the deepest layers of the Sun-the core and the radiation zone-in the form of
randomly bouncing photons. After energy emerges from the radiation zone, convection carries it the rest
of the way to the photosphere, where it is radiated into space as sunlight. Energy produced in the core
takes hundreds of thousands of years to reach the photosphere.
How do we know what is happening inside the Sun?
•We can construct theoretical models of the solar interior using known laws of physics and then check
the models against observations of the Sun’s size, surface temperature, and energy output. We also use
studies of solar vibrations and solar neutrinos.
What causes solar activity?
•Convection combined with the rotation pattern of the Sun-faster at the equator than at the poles-causes
solar activity because the gas motions stretch and twist the Sun’s magnetic ﬁeld. These contortions of
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