Lecture 11 Stellar Evolution

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Lecture 11Stellar Evolution

• How are stars born?
• How do they die?

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Exam 2

• Avg 34.9
• Median 34.9
• Curve
• 45- A 6
• 35-45 B 9
• 25-35 C 9
• 15-25 D 6
• 0-15 F 0
• 2 no-show, 1 excused absence

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Quiz today.One question1. What is your name?
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Temperature-Pressure relation
Balloon shrinks until inside and outside
pressures again balance
Pressure inside balances Pressure outside
Balloon cools, molecules inside slow down,
pressure inside decreases
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Hydrostatic Equilibrium
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Pressure-Temperature Thermostat

• In a star, inward pull of gravity balanced by the
  internal pressure
• As the star loses energy, the T and P would drop,
  except nuclear fusion is generating just enough
  energy to maintain the balance
• If reactions begin to produce too much energy,
  this extra energy raises T, which raises P, so
  star expands, which cools it slightly. This slows
  the nuclear reactions.
• If reactions slow, then inner T drops, lowering
  P. Gravity compresses the star slightly.
  Compression of gas raises T P increasing
  nuclear fusion rate.

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M-L Relation explained

• Remember that most massive MS stars are also the
  most luminous?
• Explained by GRAVITATIONAL EQUILIBRIUM

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Interstellar Medium
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Dark Nebulae
Dark Cloud
Dark Cloud / Cluster
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Interstellar Medium - Gas

• Narrow absorption lines in stellar spectra
• Line from the atmosphere of the star are broad
  due to doppler broadening. (Remember
  temperature is motion of atoms).
• Cool interstellar gas (not much motion) results
  in narrow lines.
• Emission nebulae
• Usually pink/red because of energies of electrons
  transitions

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Emission Nebulae
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Reflection Nebulae

• Look Blue!

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Collapse of a Protostar

• Stars form from the collapse of dense (1000
  atoms/cm3) molecular clouds
• Cloud has few 100,000 or a million solar masses
  of material
• Temperature 10K (COLD!)
• Why do they collapse? GRAVITY
• Sitting in gravitational equilibrium, compressed
  slightly, gravity takes over!
• Converts gravitational potential energy to
  THERMAL energy (infalling material heats up)
• Cloud fragments as it collapses each fragment
  becomes a PROTOSTAR, emitting radiation because
  it is hot

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Collapsing Interstellar Cloud
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From Protostar to Star

• What slows and eventually stops the collapse?
  PRESSURE
• Gas falls in, heats up As the temperature
  rises, so does the pressure!
• Three kinds of pressure
• Thermal pressure (Temp-Pressure related)
• Radiation Pressure (due to photons)
• Degeneracy Pressure (later)
• When the temperature rises high enough, FUSION
  (OH) begins, and A STAR IS BORN!
• Surrounding gas/dust get blown away

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Births of Stars

• Where on the HR diagram do new stars lie?
• THE MAIN SEQUENCE
• Tracks follow position of single star during
  its life (models)

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Formation Timescales

• Most massive stars form the most quickly
• Gravity collapses the cloud fragment more quickly
  in these cases

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Evidence of Star Formation Theory

• I should present EVIDENCE to support this theory
• Not much time see text for more details
• See objects
• that match our
• expectations

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Hot young stars evaporate surrounding material,
revealing the cores where other stars are forming
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Stellar Evolution

• Evolution means what happens to a star DURING
  its lifetime
• (not over generations of stars)
• How can we see this, since we dont see any
  single star evolve significantly during our
  lifetime?
• Observe many different stars of different ages
  and try to piece together the story
• Like taking a snapshot of the human population
  and figuring out how humans age

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Use theory to model the evolution
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Rules of Stellar Evolution

• Births of stars governed by balance between
  gravity and pressure
• Structure of Main-Sequence stars governed by the
  same gravitational equilibrium (OH 81)
• EVERYTHING that happens to a star, from birth to
  death, is governed by a competition between
  gravity and pressure!

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Modelling Stellar Evolution

• Apply the same rules that we did before to model
  MS stars
• Gravitational Equilibrium
• Energy Generation
• Energy Transport
• Energy conservation

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Main Sequence Lifetimes

• Once a star is born, how long does it live on the
  Main Sequence? (OH Table 9-2)
• Stays on the main sequence while fusing H to He
• Eventually, runs out of H in its center (core)
• Energy generation changes Leaves MS
• Stars spend 90 of lifetime on MS
• More massive stars use up fuel more quickly, so
  run out of H FIRST! They spend LESS time on the
  MS! Thats one reason why there are more dim red
  MS stars than luminous blue MS stars

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ConcepTest discussion

• Would you expect to find intelligent life on
  planets orbiting hot, blue, luminous
  main-sequence stars?

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Post-Main-Sequence Stars

• What about the other stars on the HR diagram?
• These stars have run out H in their cores
• Core out of H He ash in core no energy
  generated there T,P drop
• H is still fusing (burning) in a shell
• Gravity collapses core T,P rise at center
  He begins to fuse to Carbon

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Post-MS He fusion

• Core collapses, heats and Helium fusion begins
  due to higher Temperature
• Luminosity goes up (larger volume of material
  involved in fusion)
• Surface temperature increase results in pressure
  increase
• Since force of gravity (mass) hasn't changed,
  increased pressure causes outer layers to expand.
• Expansion causes outer layers to cool
• Star gets larger and cooler!

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Red Giants

• We now have a bigger, cooler star, so where does
  it fall on the HR diagram?
• Sun-like stars will be red giants, very massive
  ones will be red supergiants

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Red Giants Now What?

• Eventually, will run out of He in core!
• He burning wont last as long as H burning
  because He generates less Energy per unit mass
  (OH 60)
• MASS controls what happens next!

HST image of Betelgeuse
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Fate of Low-Mass Stars

• Stars like the Sun are considered low-mass
• When they run out of He in core
• Left with Carbon ash
• Core contracts
• Outer layers blown away by stellar
  winds/radiation pressure
• Collapsed core is called a White Dwarf (hot,
  low-luminosity more later)
• Outer layers called a Planetary Nebula
• NOTHING to do with planets!

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Low-Mass Star Evolutionary Track

• Stars run out of H at center, leave MS and become
  Red Giants
• Run out of He at center, eject outer layers,
  leaving a hot, small, Carbon core

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Planetary NebulaeTesting Theory

• EVIDENCE Do we see systems like we expect?
• YES we see planetary nebulae with hot central
  stars! This SUPPORTS our theory.

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White Dwarves

• Continuing the story of low-mass stars like the
  sun what is this white dwarf that remains?
• T,P never rise high enough for Carbon to fuse,
  because theres ANOTHER source of PRESSURE
• Degeneracy Pressure - electrons refuse to pack
  themselves into a higher volume

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Degeneracy Pressure

• Quantum Mechanical Effects important for high
  density
• Pauli Exclusion Principle no two electrons can
  occupy the same state
• Properties
• Degenerate material RESISTS compression
• Pressure NOT related to temperature (unlike
  normal gas) only depends on E levels
• Add mass increase gravity material squeezed
  tighter SHRINKS!
• Chandrasekhar Limit Mass 1.4 solar masses
  would imply a radius of 0, so is impossible!

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White Dwarves (2)

• Supported against gravity by pressure of
  degenerate electrons
• Can never be more massive than 1.4 solar masses
• Shining because hot, but SMALL, so not very
  luminous
• Generating NO MORE ENERGY, just cooling
• DENSE mass of Sun in object size of Earth!
• 1 cubic cm would weigh 1 ton on Earth

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Fate of High-Mass Stars

• Remember, evolution of stars depends on MASS
  weve been discussing the fate of stars like the
  sun
• High-mass stars go through similar initial
  stages, but faster (remember M-L relation!)
• Runs out of H in core, core collapses until T,P
  increase enough for He to fuse, meanwhile outer
  layers expand and cool red SUPERgiant

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High-Mass vs Low-Mass
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Fate of High-Mass Stars (2)

• Core becomes hot enough to fuse C and O into
  higher elements
• As the core runs out of each, it contracts due to
  gravity, heats up, and begins another round of
  fusion, resulting in an onion-like structure

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High-Mass fates (3)

• Finally, Si is fused to iron (Fe)
• Fe is most tightly bound nucleus (OH 60)
• No reaction (either fission or fusion) results in
  energy generation
• Can NO LONGER support itself against GRAVITY!

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Death of a High-Mass star

• Core still supported by degenerate electrons
• BUT matter is raining down from above
• Eventually, core cant hold itself up any more,
  and COLLAPSES (forcing protons electrons
  together pe-n?, Energy is carried away by the
  neutrinos)
• Collapsing core becomes a NEUTRON STAR (held up
  by neutron degeneracy pressure) or a BLACK HOLE
  (held up by nothing)
• Envelope blasted apart in a SUPERNOVA

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Death of a High-Mass star

• Core still supported by degenerate electrons
• BUT matter is raining down from above
• Eventually, core cant hold itself up any more,
  and COLLAPSES (forcing protons electrons
  together pe-n?, Energy is carried away by the
  neutrinos)
• Collapsing core becomes a NEUTRON STAR (held up
  by neutron degeneracy pressure) or a BLACK HOLE
  (held up by nothing)
• Envelope blasted apart in a SUPERNOVA

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Neutron Star

• Core (1.4-3 solar masses) supported by degenerate
  neutrons
• Young ones have a rapid spin and strong magnetic
  fields
• A beam of light comes out of the magnetic poles
  if we're in the beam, we see pulsing. (Pulsar)

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Black Hole

• So massive, nothing can hold it up.
• Star collapses to a single point? (Singularity)
• Event horizon, point at which not even light can
  escape.
• The only things we can find out about a BH are
  mass, charge and spin.

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Life-Cycle of High-Mass Star
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Comparison of Life CyclesLow-Mass vs High-Mass
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Star Stuff!

• Well see that universe began with mostly H, He
• Therefore all heavy elements were made in stars
  prior to (lighter than iron) and during (heavier
  than iron) explosions
• These were sent out to enrich the ISM by ejection
  of outer layers (planetary nebulae formation) or
  supernovae (exploding stars)
• New stars formed from these new IS clouds!
• We are made of dead stars!

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Evidence on Origin of Elements

• Stars in older clusters (formed earlier) have
  fewer heavy elements
• Elements generated only in SN (like gold) are
  rare (as predicted)
• We are made of elements formed in stars!

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Gas to Gas Dust to Dust
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Massive-Star Supernovae Theory

• When core mass reaches 1.4 M_sun, core collapses
  RAPIDLY
• No pressure to hold them up, so outer layers fall
  in
• They bounce off the dense core, and absorb some
  of the neutrinos
• BANG!

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Massive-Star Supernovae Theory
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Why and How Bang?

• Energy is transferred to the surface layers,
  accelerating them away from the star
• VERY HOT during explosion, so fusion creates
  elements that are heavier than Fe
• Gas expands with velocities 10,000 km/s
• When Betelgeuse explodes, it will be 10x
  brighter than full moon (tonight to 30,000 years
  from now)

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SN Predictions of Theory

• Previously normal star suddenly (few days)
  becomes MUCH more luminous (1010 L_sun), fades
  over months/years
• Surrounding ISM should be disturbed by exploding
  matter, disturbance should grow
• Neutron stars or black holes should be found in
  or near supernova remnants (SNR)
• Neutrinos should be emitted during a SN as
  electrons and protons combine to form neutrons
• Elements heavier than Fe (iron) should be present
  in the spectra of supernovae

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SN Observational Support

• 1054 AD Crab SN seen by Chinese, Native
  American, African observers
• Bright as full moon for several weeks, slowly
  faded
• Tycho saw one in 1572, Kepler in 1604
• We observe some in other galaxies!
• Can rival entire galaxy in brightness for a few
  weeks
• SN 1993J in M31 11 million ly away
• Not there one night, there the next (OH S.12)

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Supernova Remnants
Disturbed ISM Left Cygnus Loop SNR Right
Vela SNR
Cygnus Loop SNR in X-rays Filled with hot gas!
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Crab Supernova 1054 AD

• Top
• green synchrotron (radio)
• Redhydrogen emission lines (optical)
• Bottom
• Radiored
• Opticalgreen
• X-rayblue

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Crab Supernova Remnant

• Theory predicts this remnant should be EXPANDING
  into the surrounding ISM
• Next week well also see theres a PULSAR in this
  SNR (a pulsar is a neutron star)

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SN1006
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SN 1987A In a nearby galaxy
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Originally a 20 M_sun supergiant
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SN 1987A Expanding materialAND neutrino
detection!
First confirmation of FIFTY year-old theory!
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SN 1987A Light curve
Matches predictions!
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SN 1987A More support for theory

• Saw gamma-rays with particular energies that
  could only come from short-lived radioactive
  Cobalt fusion occurring during SN!
• At IR wavelengths, saw emission lines of freshly
  made cobalt, nickel, etc.

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TESTING Stellar Evolution Models

• Evolution Models PREDICT
• Least massive stars take longest to form
• Most massive stars leave MS first
• Stars become red giants or supergiants after they
  leave the MS
• Some low and medium-mass stars become WDs
• How to test?
• Watch sun for 10 billion yrs? IMPRACTICAL!
• Look at sets of stars that are ALL THE SAME AGE!

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Star Clusters Testing our Model

• Giant Molecular Cloud fragmented and collapsed
  into MANY stars!
• All these formed at same time in same place
  same AGE and DISTANCE
• What do our models predict for a population of
  stars of varying masses, but all the same age?

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Star Clusters Predictions

• YOUNG CLUSTER PREDICTIONS
• Lowest mass stars not yet on MS (not fusing H
  yet)
• MOST stars still on MS (not run out of H yet)
• Luminous stars are BLUE (not yet time to evolve
  red giants, WDs)
• OLD CLUSTER PREDICTIONS
• High-mass stars have left MS (run out of H)
• Only low-mass stars still on MS
• Lots of red giants, supergiants, WDs

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Estimating Ages

• Theories predicted that some clusters should have
  HR diagrams like our prediction for young
  clusters, while others should match the old
  cluster predictions.
• Since this is the case, we can use the HR
  diagrams to estimate the AGES of stars in
  clusters!

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Star Clusters - Observations

• Observations Match Predictions
• HR diagram of observed old cluster

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Open Clusters

• Young, mostly MS stars (no RG/WD)

Pleiades
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Globular Clusters

• Old, no massive MS stars