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Lecture 23

Stellar Evolution &

Death (High Mass)

November 21, 2018

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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3

• 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

• 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

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

Supernova 1987a11

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.

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

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.

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

• 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

• 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

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

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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