Chapter 14 ± Our Star
Why Does the Sun Shine?
fusion of hydrogen into helium.
x So the Sun shines because it generates energy through nuclear fusion.
x When the Sun was born, gravitational contraction made the Sun hot enough to sustain
nuclear fusion in its core.
equilibrium and kept the Sun shining steadily.
What Keeps the Sun Stable in Size?
x The sun keeps its core hot and dense through a natural balance between two
competing forces: gravity pulling inward and pressure pushing outward ±
keeping the Sun stable in size.
x The deeper into the Sun, with the greater weight of overlying layers, the greater
sustain nuclear fusion.
x The released energy by fusion heats the gas and keeps the balance against the
inward pull of gravity.
DIAGRAM 14.2, pg 479.
How Fusion Started?
x When the sun was born 4.6 billion years ago from a collapsing cloud of interstellar
gas, the contraction of the cloud released gravitational potential energy, causing
the interior temperature and pressure to rise.
x When the central temperature and density finally grew high enough to sustain
energy lost from the surface in the form of radiation.
x This energy balance stabilized the size of the Sun ± gravitational equilibrium
x Calculations show that the Sun was born with enough hydrogen in its core to shine
steadily and maintain gravitational equilibrium for about 10 billions years.
x Rather than spinning and rotating at the same rate, the solar equator completes one
rotation in about 25 days, increasing with latitude to about 30 days near the solar
x 7KH6XQ¶VWRWDOSRZHURXWSXW±luminosity ± is an incredible 3.8 X 10^26 watts.
x Solar wind is a stream of charged particles continually blown outward in all
directions from the Sun.
x It helps shape the magnetosphere of planets and blows back the material that forms
the plasma tails of comets. It also cleared away the gas of the solar nebula.
x The corona LVWKHRXWHUPRVWOD\HURIWKHVXQ¶VDWPRVSKHUH,WVWHPSHUDWXUHEHing 1
density is extremely low.
x The chromospehere is the middle layer of the solar atmosphere and the region that
UDGLDWHVPRVWRIWKH6XQ¶VXOWUDYLROHWZLWKDWHPSHUDWXUHRI about 10,000 K.
x The photosphere is the lowest layer of the atmosphere, which is the visible surface
of the Sun. Its temperature is about 6,000 K and it is where sunspots are found.
x The convection zone is where energy generated in the solar core travels upward,
transported by the rising of hot gas and falling of cool gas, a process called
x The radiation zone is where energy moves outward primarily in the form of
photons of light. The temperature is about 10 million K with predominant X rays.
x The solar core is where nuclear fusion transforms hydrogen into helium. The
temperature is about 15 million K with 100X the density of water and pressure that
is 200-billionX that on the surface of Earth.
The Cosmic Crucible
x Nuclear Fission ± the process of splitting a nucleus into two smaller nuclei.
x Nuclear Fusion ± the process of combining nuclei to make a nucleus with a greater
number of protons or neutrons.
x Human-built nuclear power plants rely on nuclear fission of uranium or plutonium.
x The nuclear power plant of the Sun relies on nuclear fusion, turning hydrogen to
Nuclear Fusion in the Sun
x Fusion occurs within the Sun, because of positively charged atomic nuclei in the
solar core moving at high speeds, colliding with one another.
x In most cases, electromagnetic forces deflect the nuclei, preventing collisions,
because positive charges repel one another.
x A collision with sufficient energy will stick together to form a heavier nucleus.
x Sticking positively charged nuclei require strong force (can overcome positive
repulsion), which binds protons and neutrons together in atomic nuclei.
x The strong is like glue/Velcro ± overpowers the electromagnetic force over very
x Nuclear fusion is to push the positively charged nuclei close enough together for
the strong force to outmuscle electromagnetic repulsion.