lecture 23 stellar evolution & death (high mass)bennett, the essential cosmic perspective, 7th...
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Lecture 23
Stellar Evolution &
Death (High Mass)
November 21, 2018
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High Mass Stars (M > 5 M
)Section 13.3 Bennett, The Essential Cosmic Perspective, 7th ed.
• High mass stars have:
– More mass
– Greater gravity
– Higher temperatures and pressures in the core.
• Fusion reactions do not stop with Helium
burning in the core as they do in smaller
stars.
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• Star becomes giant similar to small mass star.
– Helium burning ends in core.
– Core contracts.
– Temp and pressure in core increase.
– He shell burning begins.
– Core continues collapse.
• Then carbon fusion begins in the core. Carbon
fuses into higher-mass elements.
• Process continues as core runs out of fuel.
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• Fusion of different elements continues through
neon, oxygen, silicon and finally iron.
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• Star expands to
become a
Supergiant.
• Star moves
back and forth
on the HR
diagram with
each type of
fusion.
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• Each stage of fusion lasts for a shorter
period of time
Fusion Temp
(million K)
Duration
H 40 7 mill. yrs
He 200 500000 yrs
C 600 600 yrs
Ne 1200 1 yr
O 1500 6 mo.
Si 2700 1 day
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Death of High Mass Star
• Iron builds up in the core.
• Iron cannot be fused and produce
more energy.
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• What keeps iron
core from
collapsing?
First: electron
degeneracy
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• After core has a mass greater than 1.4 M
(Chandrasekhar limit) the electron
degeneracy is not strong enough.
• Electrons are forced to combine with the
protons to create neutrons.
• Core collapses until pressure from physical
force of neutrons bouncing against each
other stops it.
• Core rebounds and runs into outer material
that is still falling inward.
Death of a High Mass Star9
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Supernova• Collision produces huge
shock wave pushing all
material outward in an
immense explosion called a
supernova.
• Explosion can be as bright as
an entire galaxy (billions of
stars) for a few days
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• Some of the energy creates elements heavier than
iron. These elements are distributed to the rest of the
galaxy.
• Interactive Fig 13.17 core detail
• Interactive Fig 13.15 track on HR diagram
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Supernova 1987a11
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Eta Carinae (100-150 Solar Masses)Last outburst in 1841
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• Supernova leave a large shell of slowly
expanding material around a central core
(supernova remnant).
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Stars like the Sun probably do not form
iron cores during their evolution because
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A. all the iron is ejected when they become
planetary nebulas.
B. their cores never get hot enough for them to
make iron by nucleosynthesis.
C. the iron they make by nucleosynthesis is all
fused into uranium.
D. their strong magnetic fields keep their iron in
their atmospheres.
E. they live such a short time that it is impossible
for iron to form in their cores.
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Neutron StarsSection 14.2 Bennett, The Essential Cosmic Perspective, 7th ed.
• Supernova remnant
• Tightly packed neutron core.
• Size ~ 20 km (small asteroid or city)
• Mass ~ 1.4 to 3 M
• Density very high
– 1 tsp. > 100,000,000 tons on Earth.
• Some stars rotate many times per second
– Conservation of angular momentum
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• Strong magnetic field
– When star collapses, magnetic field is
concentrated.
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Neutron Star -- HST17
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A. Region A
B. Region B
C. Region C
D. Region D
E. Region E
F. Region F
G. Region G
H. Region H
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Where would a neutron star be found
on an H-R diagram?
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A. Region A
B. Region B
C. Region C
D. Region D
E. Region E
F. Region F
G. Region G
H. Region H
Where would a neutron star be found
on an H-R diagram?
Neutron stars are hot and very tiny so they’d be found
near region F on an H-R diagram.
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Pulsars
• 1967 Jocelyn Bell
– Observed object emitting pulses of radio waves.
– Pulses repeated every 1.34 seconds
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• Hundreds more have been found.
• Some pulse in optical, X-rays, or gamma rays.
• Periods typically range from 0.03 to 0.30 sec.
Periods gradually increase with pulsar’s age
– Angular momentum is not fully conserved
– Earth slows due to tidal friction
– Pulsars slow due to radiated energy
• Some pulsars are associated with supernova
remnants.
Pulsars21
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• Hewitt proposed it is
a rapidly rotating
neutron star beaming
radiation.
– Magnetic pole and
rotational axis not
quite lined up.
– Strong magnetic
field.
– Charged particles at
poles of magnetic
fields and emit large
amounts of energy.
“Lighthouse Model”
Pulsars22
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• Not all neutron stars are seen to pulse
– Beam may not be pointed at the Earth
– Animation (Arny & Schneider, Explorations, 5th ed., Figure 14-9)
– Unknown if all neutron stars are pulsars
EarthEarth never sees
beam of energy
EarthEarth sees beam of
energy
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Crab Nebula
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Crab Pulsar
The pulsar must be young
because it is seen at visible and
X-ray wavelengths. Old
pulsars emit at lower energy
radio wavelengths.
Comins & Kaufmann, Discovering the
Universe 7th ed., Fig. 13-18.
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As time progresses, the pulse rate for most solitary
pulsars is
A. decreasing, because rotational energy is
being used to generate the pulses.
B. remaining constant due to conservation of
angular momentum.
C. varying periodically as the neutron star
expands and contracts
D. increasing, because the neutron star slowly
contracts.
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The Age of Star ClustersSection 12.3 Bennett, The Essential Cosmic Perspective, 7th ed.
• Open Clusters --loose clusters of 10-100 stars
• Globular Clusters -- Old, tightly bound group of
100’s or 1000’s of stars
• All stars in a cluster are formed at the same time.
• Age of a cluster can be determined by looking at
what point the stars turn off of the main sequence
“turn-off point”.
• Age of Cluster = Lifetime of star at turn-off point.
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28 Figure 20.17,
Chaisson and McMillan,
5th ed. Astronomy Today,
© 2005 Pearson Prentice Hall
Interactive Figure 12.17
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29 Illustrative movie
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• Young Cluster -- Hyades cluster
• Around 600 million years old
Figure 20.19,
Chaisson and McMillan,
5th ed. Astronomy Today,
© 2005 Pearson Prentice Hall
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• Old Cluster -- 47 Tucanae
• One of the oldest clusters, about 12 to 14 billion years old
Figure 20.20,
Chaisson and McMillan,
5th ed. Astronomy Today,
© 2005 Pearson Prentice Hall
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The Pleiades is a very young cluster. What would you
expect its overall color to be when observed from the Earth?
A. Blue
B. Yellow/White
C. Red
D. None of the above
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End of material on Exam 3
Exam 3 Information
• Bring a #2 pencil!
• Bring a calculator. No cell phones or tablets
allowed!
• Contents:
– Worked-out problems (2 questions, 10 points)
– True/False (10 questions, 20 points)
– Multiple Choice (35 questions, 70 points.
None of these require a calculation.)
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