Download Universe 8e Lecture Chapter 17 Nature of Stars

Star Formation - 6
(Chapter 5 – Universe)
Death of Stars
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Key Ideas
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Stellar Evolution: Because stars shine by
thermonuclear reactions, they have a finite life span. The
theory of stellar evolution describes how stars form and
change during that life span.
Mass Loss by Protostars: In the final stages of pre–
main-sequence contraction, when thermonuclear
reactions are about to begin in its core, a protostar may
eject large amounts of gas into space.
Low-mass stars that vigorously eject gas are called T
Tauri stars.
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The Main-Sequence Lifetime: The duration of a star’s
main-sequence lifetime depends on the amount of
hydrogen available to be consumed in the star’s core
and the rate at which this hydrogen is consumed.
The more massive a star, the shorter its main-sequence
lifetime. The Sun has been a main-sequence star for
about 4.56 billion years and should remain one for about
another 7 billion years.
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Becoming a Red Giant: Core hydrogen fusion ceases
when the hydrogen has been exhausted in the core of a
main-sequence star with mass greater than about 0.4
M. This leaves a core of nearly pure helium surrounded
by a shell through which hydrogen fusion works its way
outward in the star. The core shrinks and becomes
hotter, while the star’s outer layers expand and cool. The
result is a red giant star.
As a star becomes a red giant, its evolutionary track
moves rapidly from the main sequence to the red-giant
region of the H-R diagram. The more massive the star,
the more rapidly this evolution takes place.
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Helium Fusion: When the central temperature of a red
giant reaches about 100 million K, helium fusion begins
in the core. This process, also called the triple alpha
process, converts helium to carbon and oxygen.
In a more massive red giant, helium fusion begins
gradually; in a less massive red giant, it begins suddenly,
in a process called the helium flash.
After the helium flash, a low-mass star moves quickly
from the red-giant region of the H-R diagram to the
horizontal branch.
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Late Evolution of Low-Mass Stars: A star of moderately low mass
(about 0.4 M to about 4 M) becomes a red giant when shell
hydrogen fusion begins, a horizontal-branch star when core helium
fusion begins, and an asymptotic giant branch (AGB) star when the
helium in the core is exhausted and shell helium fusion begins.
Planetary Nebulae and White Dwarfs: Helium shell flashes in an
old, moderately low-mass star produce thermal pulses during which
more than half the star’s mass may be ejected into space. This
exposes the hot carbon-oxygen core of the star.
No further nuclear reactions take place within the exposed core.
Instead, it becomes a degenerate, dense sphere about the size of
the Earth and is called a white dwarf. It glows from thermal radiation;
as a white dwarf cools, it becomes dimmer.
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Late Evolution of High-Mass Stars: Unlike a
moderately low-mass star, a high-mass star (initial mass
more than about 4 M) undergoes an extended
sequence of thermonuclear reactions in its core and
shells. These include carbon fusion, neon fusion, oxygen
fusion, and silicon fusion.
The Deaths of the Most Massive Stars: A star with an
initial mass greater than 8 M dies in a violent cataclysm
in which its core collapses and most of its matter is
ejected into space at high speeds. The luminosity of the
star increases suddenly by a factor of around 108 during
this explosion, producing a supernova.
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Other Types of Supernovae: An accreting white dwarf
in a close binary system can also become a supernova
when carbon fusion ignites explosively throughout such
a degenerate star. This is called a thermonuclear
supernova.
A Type Ia supernova is produced by accreting white
dwarfs in close binaries. A Type II supernova is the
result of the collapse of the core of a massive star, as
are supernovae of Type Ib and Type Ic; these latter
types occur when the star has lost a substantial part of
its outer layers before exploding.
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Neutron stars form from supernova of original M-S stars
greater than 8 M
Neutron Stars: A neutron star is a dense stellar corpse
consisting primarily of closely packed degenerate
neutrons.
A neutron star typically has a diameter of about 20 km, a
mass less than 3 M, a magnetic field 1012 times
stronger than that of the Sun, and a rotation period of
roughly 1 second.
Pulsars: A pulsar is a source of periodic pulses of radio
radiation. These pulses are produced as beams of radio
waves from a neutron star’s magnetic poles sweep past
the Earth.
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Black Holes: form from supernova of original M-S stars
greater than 8 M
If a stellar core has a mass greater than about 2 to 3 M,
gravitational compression will overwhelm any and all
forms of internal pressure
The stellar corpse will collapse to such a high density
that its escape speed exceeds the speed of light.