• During its stay on the main sequence, any fluctuations in a star’s condition are quickly restored; the star is in equilibrium.© 2017 Pearson Education, Inc... • Stages of a star leavin
Trang 1© 2017 Pearson Education, Inc.
Trang 2© 2017 Pearson Education, Inc.
Trang 3Units of Chapter 12
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Trang 4• During its stay on the main sequence, any fluctuations in a star’s condition are quickly restored; the star is in equilibrium.
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Trang 512.1 Leaving the Main Sequence
sequence
– Low-mass stars go quietly.
– High-mass stars go out with a bang!
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Trang 6• Even while on the main sequence, the composition
of a star’s core is changing
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Trang 712.2 Evolution of a Sun-like Star
collapse
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Trang 8• Stages of a star leaving the main sequence.
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Trang 912.2 Evolution of a Sun-like Star
– As the core continues to shrink, the outer layers of the star expand and cool
– It is now a red giant, extending out as far as the orbit of Mercury.
– Despite its cooler temperature, its luminosity increases enormously due to its large size.
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Trang 10• The red giant stage on the H–R diagram
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Trang 1112.2 Evolution of a Sun-like Star
100,000,000 K, the helium in the core starts to fuse
– Helium begins to fuse extremely rapidly; within hours the enormous energy output is over, and the star once again reaches equilibrium.
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Trang 12• Stage 10 on the H–R diagram
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Trang 1312.2 Evolution of a Sun-like Star
– As the helium in the core fuses to
carbon and oxygen, the core becomes hotter and hotter, and the helium burns faster and faster
– The star is now similar to its condition just as it left the main sequence, except now there are two shells.
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Trang 14• The star has become a red giant for the second time.
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Trang 1512.3 The Death of a Low-Mass Star
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Trang 16• There is no more outward fusion pressure being generated
in the core, which continues to contract
planetary nebula
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Trang 1712.3 The Death of a Low-Mass Star
– A small, extremely dense carbon core
– An envelope about the size of our solar system.
early astronomers viewing the fuzzy envelope thought it resembled a planetary system
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Trang 18• Stages 13 and 14: White and black
dwarfs:
– Once the nebula has gone, the remaining core is extremely dense and extremely hot, but quite small.
– It is luminous only due to its high
temperature.
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Trang 1912.3 The Death of a Low-Mass Star
companion of the much larger and brighter Sirius A
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Trang 20• The Hubble Space Telescope has detected white dwarf stars in globular clusters.
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Trang 2112.3 The Death of a Low-Mass Star
dimmer, and finally ceases to glow
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Trang 22• A nova is a star that flares up very suddenly and then returns slowly to its former luminosity.
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Trang 2312.3 The Death of a Low-Mass Star
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Trang 24• Material falls onto the white dwarf from its main-sequence companion
new material
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Trang 2512.4 Evolution of Stars More Massive than the Sun
that stars more massive than the Sun
follow very different paths when leaving
the main sequence
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Trang 26• High-mass stars, like all stars, leave the main sequence when there is no more hydrogen fuel in their cores.
core burning helium to carbon, surrounded by helium- and hydrogen-burning shells
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Trang 2712.4 Evolution of Stars More Massive than the Sun
masses do not experience a helium flash—
helium burning starts gradually
Trang 28• A star of more than 8 solar masses can fuse elements far beyond carbon in its core, leading to a very different fate.
same luminosity as it cools off
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Trang 2912.4 Evolution of Stars More Massive than the Sun
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Trang 30• A supernova is incredibly luminous, as can be seen from these curves—more than a million times as bright as a nova
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Trang 3112.5 Supernova Explosions
progenitor star
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Trang 32• Carbon-detonation supernova: This white dwarf has accumulated too much mass from
its binary companion
keep the core from collapsing
explosion
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Trang 3312.5 Supernova Explosions
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Trang 34• Supernovae leave remnants—the expanding clouds of material from the explosion
Trang 3512.6 Observing Stellar Evolution in Star Clusters
H–R diagrams show how stars of the same
age, but different masses, appear as the
cluster as a whole ages
have already left the main sequence,
whereas many of the least massive have not
even reached it yet
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Trang 36• After 100 million years, a distinct
main-sequence turnoff begins to develop This
shows the highest-mass stars that are still
on the main sequence
turnoff is much clearer
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Trang 3712.6 Observing Stellar Evolution in Star Clusters
– The red giant, subgiant, asymptotic giant, and horizontal branches are all clearly populated.
– White dwarfs, indicating that solar-mass stars are in their last phases, also appear.
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Trang 38• This double cluster, h and χ Persei, must be
quite young—its H–R diagram is that of a
newborn cluster Its age cannot be more
than about 10 million years
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Trang 3912.6 Observing Stellar Evolution in Star Clusters
main-sequence turnoff indicates an age of about 600 million years
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Trang 40• This globular cluster, M80, is about 10–12 billion years old, much older than the previous examples.
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Trang 4112.7 The Cycle of Stellar Evolution
Stars form, evolve, and
die
heavy elements into the
interstellar medium
become parts of new
stars
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Trang 42• Once hydrogen is gone in the core, a star burns hydrogen in the surrounding shell The core contracts and heats; the outer atmosphere expands and cools.
the core continues to collapse The envelope blows off, leaving a white dwarf to gradually cool
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Trang 43Summary of Chapter 12, cont.
iron At the end, the core collapses and rebounds as a Type II supernova
white dwarf
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