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12/05/2001
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Name
Date
What Happens as a Star Runs Out of Hydrogen?
ES2810
Use another sheet of paper if you need extra space to write complete answers.
1 What determines the size (volume) of a main
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sequence star?
5 Describe how a medium-sized star can grow
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as it runs out of hydrogen fuel.
The size of a main sequence star depends on the initial
The extra energy given off when a stellar core begins
mass of matter in the star and the balance between the
fusing helium speeds up the hydrogen-to-helium
inward force of gravity and outward force generated by
nuclear reactions occurring in the shell. This causes
fusion.
higher outward pressure in the layers of gas above the
core that isn’t balanced by an increase in gravitational
force, so the star’s volume increases.
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2 Which size star uses up its available
hydrogen most quickly? Explain your
answer.
6 How is the formation of a planetary nebula
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different from a supernova?
Increased outward force from the core of a red giant
creates a series of pulsations that gradually blow the
The more massive the star, the hotter the interior
outer shell of material away from the surface of the star.
becomes due to gravitational contraction. The hotter
This outer shell of material sometimes appears as a
the core is, the more quickly its hydrogen fuel is
“cloud” around the central star.
converted to helium. The gravitational force of larger
Supernova explosions eject the outer layers of a star in
stars squeezes the hydrogen into a smaller volume than
one incredibly violent explosion following the collapse
in smaller stars, so fusion reactions take place at a
of the iron core. The star is thought to implode, crush
greater rate.
the core, then rebound. The supernova is the rebound.
The amount of mass crashing down on the core
determines the density of the star’s final state.
3 After a massive star’s hydrogen is depleted in
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Chapter 28
Internet Investigation
the core, what does it use for nuclear fuel?
7 Describe the final form of matter for
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medium- and small-sized stars.
Helium and carbon atoms fuse into progressively
Medium- and small-sized stars eventually become
heavier elements, up to iron.
white dwarfs, stars that are extremely small (Earth-size)
and very dense."
4 Describe the final form of matter for large
●
stars.
96
8 How does the initial mass of a star compare
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to its final size (volume)?
The largest stars end as neutron stars—matter squeezed
The larger (more massive) the initial star size, the
so tightly that electrons are forced into atomic nuclei
smaller the final size. Large stars form black holes (with
and neutralize their protons, or as black holes—matter
zero diameter) or neutron stars which are about as large
so dense that its gravitational field will not allow light to
as a city; medium and small stars form white dwarves
escape.
that are Earth-sized objects.
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Earth Science