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Star evolution
Chapters 17 & 18
(Yes, we skip chap. 16, star birth)
Goals & Learning Objectives
• Learn some simple astronomical terminology
• Develop a sense of what scientists know about
the overall universe, its constituents, and our
location
• Describe stellar evolution
• Contrast the life history of a low-mass star
with the life history of a high-mass star.
• Explain how black holes are formed and their
effect on their surrounding environment.
3 star groups (p. 565)
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3 categories of stars:
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Low mass (<2 Msun)
Intermediate mass (2  8 Msun)
High mass (>8 Msun)
Intermediate similar to both high and low mass. Book
focuses more on similarities with high mass (in section
17.1).
One major difference: high mass stars die very
differently!
Which star group has the highest core
pressure?
1. Low mass
2. Intermediate mass
3. High mass
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Which star group has the hottest core
temperature?
1. Low mass
2. Intermediate mass
3. High mass
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So what can you conclude about the fusion rate? Luminosity?
Which stars live longer? Why?
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The end of the Sun
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Eventually core runs out of hydrogen.
What did the core need fusion for?
What will happen to it as a result of losing fusion?
What happens to gas balls when they shrink?
What happens to the temperature of the material
surrounding the core?
CLICKER QUESTION (next slide).
What are the surrounding layers made of?
What can happen if they get hot enough?
For Sun, this takes hundreds of millions of years.
Is there Hydrogen outside the Sun’s core?
1. Yes
2. No
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Shell “burning”
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In fact, the outer layers get hotter than 15 million K.
What does that tell us about hydrogen fusion rate?
What should we observe as a result? CLICKER
The light “gets stuck” and pushes the outer layers
out.
What happens to gas when you expand it?
Color of outside? What kind of star do we have?
What is the core made of?
What is the structure?
See fig. 17.4 page 568
Star becomes ______ luminous
1. More
2. Less
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What’s happening to the mass of the
HELIUM core as the shell “burns”?
1. Increasing
2. Decreasing
3. Staying the same
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Inside the core…
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Core shrinks
Core gets hotter
More hot helium dumped onto core
Something must stop the core from shrinking.
– Low mass stars: degeneracy pressure
• Read section 16.3, page 557 and S4.4 pp. 481-483
• Mosh pit
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Intermediate &
High mass: fusion causing thermal & gas pressure.
• Helium Fusion turns on at 100 million K
– Low mass: whole core starts fusing simultaneously: helium “flash”
– Intermediate & high mass: “regular” fusion
Next phase
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Structure of the star now?
Figure 17.5
This lasts until …
What happens to the core?
– Low & intermediate mass: core shrinks until degeneracy
pressure stops it. Focus on that now.
– [for High mass: next fusion turns on]
• Back to low mass: What’s the core made of?
• Shrinks to size of Earth.
• What happens outside the core?
– Temp, composition
Double shell burning
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Not stable
Outer layers pulsate
Outer layers come off
See pictures around the planetarium
– Cat’s eye, Butterfly, Ring: all “planetary nebula”
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See also figure 17.7 – more examples
NOT related to planets
What’s in the center of a planetary nebula?
End of low & intermediate mass stars…
Show interactive figure 17.4
Do low mass stars like the Sun fuse Carbon into
anything?
1. Yes
2. No
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If the universe contained only low mass stars, would
there be elements heavier than carbon?
1. Yes
2. No
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High mass star differences
• Degeneracy pressure never turns on
– Gas & thermal pressure always stronger
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Can fuse carbon with helium into Oxygen
Can fuse Oxygen with helium into neon
Etc. (magnesium, silicon, sulfur)
When core hot enough, can fuse carbon with carbon,
carbon with oxygen …
• Etc.
• Big picture: carbon and stuff fuses until you get to a
core made of …
• Iron (Fe on the periodic table, #26, middle section,
top row, see page A-13, Appendix C)
Iron
• Most stable nucleus
• Can’t release energy by fusing it
– Fusion USES energy (uses instead of ___________)
• True for everything heavier than iron, too.
– Fission USES energy
• True for most things lighter than iron, too.
• Iron is the last element made in stable reactions in
stars
• Look at the periodic table on page A-13
– Find iron
– Gold = Au. Mercury = Hg. Xenon = Xe. Are these made in
stable stars?
What we see
• See figure 17.12, page 575 for onion skin
model
• See HR diagram on p. 575 (fig. 17.13)
– Runs out of core fuel, goes right
– Next fuel turns on, goes back left
– Repeat until core is made of Iron
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After the Iron core forms
Iron core shrinks
Gravity is stronger than Electron degeneracy pressure
Electrons squeezed more than they can tolerate
Electrons merge with protons
Result: neutrons
– And neutrinos!
– (Fly straight out! We observe them first!)
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No more electron degeneracy pressure support.
Rapidly shrinks: Earth-size shrinks to town-size in 1 second!
Lots of energy released. Turn on neutron degeneracy pressure.
Core bounces. Demo
Supernova explosion. Leaves behind core
Core is made of … Called …
Interactive figure 17.12 & 17.17 (crab nebula in 1054)
(If the core is too heavy for neutron degeneracy pressure…)
Production of Elements
• High mass stars make up to Iron
• Everything heavier made DURING the
supernova
– Lots of neutrons around
– They merge with nuclei quickly (r-process)
– Eventually nucleus decays to something stable
– Like Gold, Silver, Platinum, Lead, Mercury, etc.
Stellar remnants
• End states for stars
– Low mass stars become …
– Intermediate mass also become … (Oxygen)
– & high mass stars become …
– The highest mass stars (O & B) become …
Which stars should begin with the most
heavy elements inside them?
1. The stars that formed earliest
2. The most recently formed stars
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Summary of star death
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When fusion runs out, core ____ & _____
Shell fusing occurs. Many shells possible.
Core fusion can turn on.
What’s different for low mass & high mass?
Which elements get made in low & high?
What’s special about iron?
Degeneracy pressure (electron & neutron)
– What, where, why
• Possible end states; which stars make them
– RG  PN  WD, RG  SN  NS or BH
Chapter 18: Stellar remnants
• The next few slides are material from chap 18.
White dwarfs
• Radius
– Earth sized (4000 miles)
• What kind of pressure resists gravity?
– Electron degeneracy pressure
• Temperature
– Start hot. [Clicker question]
– Cool down (black dwarf eventually)
• Composition:
– Usually carbon
– sometimes oxygen (intermediate mass) or helium (very
low mass)
• Gravity: teaspoon weighs 5 tons!
What kind of light would a white dwarf emit
most when it is first detectable?
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X-rays
Visible light
Infrared
Radio waves
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White dwarf limit
• Observed around 1 Msun
• Can be up to 1.4 Msun
• If heavier, electrons can’t push out strongly enough to resist
gravity. [they’d have to move faster than c]
• What happens if you add mass to a 1.4 Msun white
dwarf?
– Where could extra mass come from?
– Supernova explosion!
– “White dwarf supernova” (“Type 1a”)
• Are a “standard candle”. What’s that?
– Leaves NOTHING behind, unlike massive star supernovae
– LESS VIOLENT: Nova if add small amount of stuff to lower
mass WD.
Sirius binary system
What you’d see through a telescope
Ignore the spikes
X-ray image & visible image
superimposed
Neutron stars
• Composition?
– Gigantic nuclei.
– No empty space like in atoms (99.999% empty)
• Paper clip of neutrons weighs as much as a mountain!
• Dropping brick: energy = an atom bomb!
– As stuff falls onto a neutron star, releases X-rays!
• Mass
– Observed: 1-1.4 Msun
– Can be up to 2-3 Msun (we don’t know exact upper limit)
– Any heavier & neutrons can’t push out strongly enough to resist
gravity.
• Radius: City sized (6 miles). WD = 4000 miles!
• What kind of pressure resists gravity?
– Neutron degeneracy pressure
• Neat trivia: Escape speed = ½ c. (Gravity very strong!)
Pulsars
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See figures 18.7 & 18.8
Jocelyn Bell
Should’ve won the Nobel Prize
Rapidly spinning neutron stars
1800 known pulsars, pulsing radio, but some also emit
other types: visible + X-rays and sometimes gamma.
– 1 pulsar, discovered in October 2008 emits only gamma
• See figure 18.9
• Is it possible to be a neutron star that’s not a pulsar? How
about vice versa? [2 clicker Q’s]
• Spin up to 600 times per SECOND! (Show movie!)
– Larger objects would break apart
Is it possible to be a neutron star but not a
pulsar, as seen on Earth?
1. Yes
2. No
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Is it possible to be a pulsar but not a
neutron star, as seen on Earth?
1. Yes
2. No
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Black holes
• Black holes don’t “suck”
– Strong gravity. Things FALL in; don’t get SUCKED
• Event horizon / escape speed
– What happens if further away than event horizon?
• Schwarzschild radius: 3km per solar mass.
• Falling in
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Redshift
Time dilation; time “travel”
Tidal stretching
Friends won’t see you die if fall into high mass
• How do we know they exist?
– Cygnus X-1, XRB, accretion disks
– Looking for BH collisions emitting gravitational waves, LIGO.
– Gravitational lenses (MACHOs)
• Hawking radiation – black hole evaporation
Chap. 18, #18: If a black hole 10 times as massive as our Sun
were lurking just beyond Pluto’s orbit, we’d have no way of
knowing it was there.
1. True
2. False
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Summary of stellar “graveyard”
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White dwarf properties: mass, radius, pressure
White dwarf limit, results of exceeding it
Neutron star properties
Pulsars
Black holes
– Falling in
– Gravity far away
– How we can find them