AST201H1 Lecture 6: Lecture 6-Chapter 14-Our Star
4 Aug 2010
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Chapter 14 ± Our Star
Why Does the Sun Shine?
x %\(LQVWHLQ¶VWKHRU\RIUHODWLYLW\WKH6XQFRQYHUWVPDVVLQWRHQHUJ\WKURXJKnuclear
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.
x (YHUVLQFHHQHUJ\OLEHUDWHGE\IXVLRQKDVPDLQWDLQHGWKH6XQ¶Vgravitational
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 ±
gravitational equilibrium.
x 7KH6XQ¶VLQWHUQDOSUHVVXUHSUHFLVHO\EDODQFHVJUDYLW\DWHYHU\SRLQWZLWKLQLW
keeping the Sun stable in size.
x The deeper into the Sun, with the greater weight of overlying layers, the greater
the pressure.
x 7KXVGHHSLQWKH6XQ¶VFRUHWKHSUHVVXUHPDNHVWKHJDVKRWDQGGHQVHHQRXJKWR
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
QXFOHDUIXVLRQHQHUJ\JHQHUDWLRQLQWKH6XQ¶VLQWHULRUFDPHLQWREDODQFHZLWKWKH
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.
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DIAGRAM 14.3
x 7KH6XQ¶VUDGLXVLVDERXWNLWVPDVVLV;ANJWLPHVWKHPDVV
of Earth).
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
poles.
x 7KH6XQ¶VWRWDOSRZHURXWSXW±luminosity ± is an incredible 3.8 X 10^26 watts.
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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
PLOOLRQ.H[SODLQVZK\LWHPLWVPRVWRIWKH6XQ¶V;UD\V+RZHYHUWKHFRURQD¶V
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.
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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
convection.
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
helium.
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
small distances.
x Nuclear fusion is to push the positively charged nuclei close enough together for
the strong force to outmuscle electromagnetic repulsion.
DIAGRAM 14.6
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