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Chapter 13

AY 101 Chapter Notes - Chapter 13: Planetary Nebula, Brown Dwarf, White Dwarf

Course Code
AY 101
Raymond White

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The Big Picture
In this chapter, we have seen how the origin of the elements, first discussed in Chapter 1, is linked to the
lives and deaths of stars. As you look back, keep in mind these “big picture” ideas:
Virtually all elements in the universe besides hydrogen and helium were forged in stars. We and our
planet are therefore made of stuff produced by stars that lived and died long ago.
Low-mass stars like our Sun live long lives and die with the ejection of planetary nebulae, leaving
behind white dwarfs.
High-mass stars live fast and die young, exploding dramatically as supernovae and leaving behind
neutron stars or black holes.
Close binary stars can exchange mass, altering the usual course of stellar evolution.
Summary of Key Concepts
How do stars form?
Stars are born in cold, relatively dense molecular clouds. As a cloud fragment collapses under gravity,
it becomes a rapidly rotating protostar surrounded by a spinning disk of gas in which planets may
form. The protostar may also fire jets of matter outward along its poles.
How massive are newborn stars?
On the upper end, the most massive newborn stars are about 150MSun. On the lower end, stars cannot be
less massive than 0.08MSun; below this mass, degeneracy pressure prevents gravity from making the
core hot enough for efficient hydrogen fusion, and the object becomes a brown dwarf. Low-mass stars
far outnumber high-mass stars.
What are the life stages of a low-mass star?
A low-mass star spends most of its life generating energy by fusing hydrogen in its core. When the
core’s hydrogen is exhausted, the core begins to shrink while the star as a whole expands to become a
red giant, with hydrogen shel fusion occurring around an inert helium core. When the core becomes
hot enough, a helium flash initiates helium fusion in the core, which fuses helium into carbon; the star
shrinks somewhat in size and luminosity during this time. The core shrinks again when helium core
fusion ceases, while both helium and hydrogen fusion occur in shells around the inert carbon core and
cause the outer layers to expand once more.
How does a low-mass star die?
A low-mass star like the Sun never gets hot enough to fuse carbon in its core, because degeneracy
pressure stops the gravitational collapse of the core. The star expels its outer layers into space as a
planetary nebula, leaving its exposed core behind as a white dwarf.
What are the life stages of a high-mass star?
A high-mass star lives a much shorter life than a low-mass star, fusing hydrogen into helium via the
CNO cycle. After exhausting its core hydrogen, a high-mass star begins hydrogen shell fusion and then
goes through a series of stages fusing successively heavier elements. The furious rate of this fusion
makes the star swell in size to become a supergiant.
How do high-mass stars make the elements necessary for life?
In its final stages of life, a high-mass stars core becomes hot enough to fuse carbon and other heavy
elements. The variety of different fusion reactions produces a wide range of elements-including all the
elements necessary for life-that are then released into space when the star dies.
How does a high-mass star die?
A high-mass star dies in a cataclysmic explosion called a supernova, scattering newly produced
elements into space and leaving behind a neutron star or black hole. The supernova occurs after fusion
begins to pile up iron in the high-mass stars core. Because iron fusion cannot release energy, the core
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