Star evolution

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Star evolution

• Chapters 17 18
• (Yes, we skip chap. 16, star birth)

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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.

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3 star groups (p. 565)

• 3 categories of stars
• Low mass (lt2 Msun)
• Intermediate mass (2 ? 8 Msun)
• High mass (gt8 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!

4
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

So what can you conclude about the fusion rate?
Luminosity? Which stars live longer? Why?
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The end of the Sun

• 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.

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Is there Hydrogen outside the Suns core?

1. Yes
2. No

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Shell burning

• 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

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Star becomes ______ luminous

1. More
2. Less

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Whats 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

• 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
• 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

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Next phase

• 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 Whats the core made of?
• Shrinks to size of Earth.
• What happens outside the core?
• Temp, composition

13
Double shell burning

• Not stable
• Outer layers pulsate
• Outer layers come off
• See pictures around the planetarium
• Cats eye, Butterfly, Ring all planetary
  nebula
• See also figure 17.7 more examples
• NOT related to planets
• Whats in the center of a planetary nebula?
• End of low intermediate mass stars
• Show interactive figure 17.4

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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
• 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)

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Iron

• Most stable nucleus
• Cant 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?

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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!)
• 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)

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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.

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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

22
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

• When fusion runs out, core ____ _____
• Shell fusing occurs. Many shells possible.
• Core fusion can turn on.
• Whats different for low mass high mass?
• Which elements get made in low high?
• Whats 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

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Chapter 18 Stellar remnants

• The next few slides are material from chap 18.

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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!

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What kind of light would a white dwarf emit most
when it is first detectable?

1. X-rays
2. Visible light
3. Infrared
4. Radio waves

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White dwarf limit

• Observed around 1 Msun
• Can be up to 1.4 Msun
• If heavier, electrons cant push out strongly
  enough to resist gravity. theyd 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. Whats that?
• Leaves NOTHING behind, unlike massive star
  supernovae
• LESS VIOLENT Nova if add small amount of stuff
  to lower mass WD.

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Sirius binary system
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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 dont know exact upper
  limit)
• Any heavier neutrons cant 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!)

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Pulsars

• See figures 18.7 18.8
• Jocelyn Bell
• Shouldve 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 thats not a
  pulsar? How about vice versa? 2 clicker Qs
• Spin up to 600 times per SECOND! (Show movie!)
• Larger objects would break apart

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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 dont suck
• Strong gravity. Things FALL in dont get SUCKED
• Event horizon / escape speed
• What happens if further away than event horizon?
• Schwarzschild radius 3km per solar mass.
• Falling in
• Redshift
• Time dilation time travel
• Tidal stretching
• Friends wont 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

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Chap. 18, 18 If a black hole 10 times as
massive as our Sun were lurking just beyond
Plutos orbit, wed have no way of knowing it was
there.

1. True
2. False

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Summary of stellar graveyard

• 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