Chapter 14: Star Stuff

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Chapter 14 Star Stuff

• Star Formation
• Evolution of Low-Mass Stars
• Evolution of High-Mass Stars
• Evolution of Close Binary Stars

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14.1 Lives in the Balance

• Story of a stars life Battle between gravity
  and pressure.
• A star needs thermal pressure to balance gravity.
  
• Sources of pressure nuclear fusion or
  contraction.
• The evolution of a star depends almost entirely
  on its birth mass.
• We start looking at the process of stellar birth.
  

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14.2 Star Birth

• Star-forming clouds are cold (10-30 K) and dense.
  They are called molecular clouds because their
  low temperatures allow the formation of H2 and
  other molecules such as CO, H2O and dust.
• The cold temperatures and high density allow
  gravity to overcome thermal pressure, initiating
  the collapse toward new stars.

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Protostars (earliest form of stars)

• Contraction increases the clouds thermal energy,
  which is initially radiated away quickly.
• When thermal energy cannot longer escape easily,
  the internal temperature and pressure rise
  dramatically.
• Dense cloud fragments become protostars.

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Protostellar Disk

• The law of conservation of angular momentum (m x
  v x r) implies that cloud fragments spin faster
  as they collapse.
• As it shrinks, a rotating cloud must flatten to
  form a protostellar disk.
• Are all protostellar disks also protoplanetary
  disks?

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Protostellar Wind

• Rapid rotation generates a strong magnetic field.
  
• Angular momentum can be trasferred outward along
  magnetic field lines.
• An outward flow of particles similar to the solar
  wind, constitutes the protostellar wind.
• Many young stars fire high-speed collimated jets.
  
• H-H objects are shock fronts of jets with the ISM

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A Star Is Born

• In HRD we can plot evolutionary tracks for
  newborn stars.
• Young stars contract until their cores become hot
  enough for H fusion (10 million K).
• The pre-main-sequence lifetime depends on the
  stars mass. For 1 MSun it lasts about 50 million
  years.

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PMS Evolution

• T Tauri stars, named after variable star T in
  Taurus are PMS stars that have just emerged from
  their cocoons. They have accretion disks and
  strong magnetic fields.
• They occur in young star clusters and loose T
  associations.
• Post T Tauri stars are intermediate between T
  Tauris and MS stars.

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The Initial Mass Function

• Molecular clouds collapse and fragment leading to
  stars with a wide array of masses.
• Stars with low masses greatly outnumber stars
  with high masses.
• For every star with mass 10-100 MSun, there are
  10 stars with 2-10 MSun, 50 with 0.5-2 MSun and
  few 100s with mass

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Limits to Stellar Masses

• Above 100 MSun gravity cannot contain the
  radiation pressure.
• Below 0.08 MSun the central temperature never
  climbs sufficiently for stable H-burning.
  Degeneracy pressure halts gravitational
  contraction.

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Substellar Objects

• Paulis exclusion principle prevents two
  identical e- from occupying the same space at the
  same time.
• The resistance to squeezing exerts a deneracy
  pressure that halts contraction. This pressure
  does not depend on temperature, only on density.
• Brown dwarfs and giant planets.

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14.3 Evolution of Low-Mass Stars.

• Lower-mass stars have cooler interiors and deeper
  convection zones.
• In very low-mass stars and brown dwarfs, the
  convection zone extends all the way down to the
  core.

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Moving Away from the Main Sequence

• When the central supply of H is depleted, the
  core shrinks and outer layers expand.
• For 1 MSun, the evolution away from the MS,
  through the subgiant phase and into the red giant
  branch takes about 1 billion years.

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Hydrogen Shell Burning

• The He core is inert.
• Gravity shrinks a shell of H around the core and
  allows it to burn.
• The temperature is higher, the H burns faster
  than in the MS, the star expands and becomes more
  luminous. Slow winds carry away matter.
• Newly produced He amplifies core shrinking.

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Helium Burning

• He nuclei have two p and hence 2x positive charge
  than H.
• He fusion occurs at higher temperature than H
  fusion. Tburn(He)108K
• A 12C nucleus has slightly less mass than 3 4He
  nuclei.

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Structure of He Burning Stars

• He ignites very suddently (He flash), pushing the
  H-burning shell away, and lowering the
  temperature.
• The He-burning star becomes hotter and smaller
  than a red giant.

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Horizontal Branch

• In a stellar cluster, the He-burning stars are
  arranged along a horizontal branch.
• The He cores of all low-mass stars fuse He at
  about the same rate, so they have about the same
  luminosity.

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Final Stages of Evolution

• Core runs out of He in about 108 years.
• Core shrinks and He burns in a shell around it.
  Star expands again.
• Carbon fusion requires Tcore6x108 K. Low-mass
  stars develop degenerate cores before reaching
  such temperature.

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Carbon Stars

• Strong convection can dredge up C from the core
  to the surface.
• Carbon-rich red giants are called carbon stars.
  They have cool, slow winds where insterstellar
  dust grains condensate.
• Carbon stars are the largest dust polluters in
  the universe.

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Planetary Nebulae

• When the inert core becomes degenerate, a
  low-mass star ejects its outer layers into space.
  
• A planetary nebula is an expanding shell that
  glows brightly because it is heated by the UV
  radiation emitted by the exposed C-He core. A PN
  lasts only 106 years.

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Evolution of High-Mass Stars

• The hot core temperatures of high-mass stars
  enable a higher rate of H fusion.
• C,N,O atoms act as catalysts of a reaction chain
  called CNO cycle, which is a more efficient way
  of fusing H into He than the pp chain.
• Radiation pressure can drive strong, fast-moving
  winds.

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Onion Structure

• Chain of events H-core exhaustionH-shell
  burningGradual He fusionHe-shell
  fusionC-fusion C-shell burningO-fusionO-shell
  burning Ne-fusion Ne-shell burning Mg-fusion
  Mg-shell burning Si-fusion Si-shell burning.
• Layer upon layer of shells burning different
  elements with inert Fe core.

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Life Tracks of High-Mass Stars

• High-mass stars zigzag across the HRD.
• Each time a fuel is exhausted in the core, shell
  burning makes the star expand.
• At each stage of core fusion of a heavier
  element, the outer layers contract and the stars
  surface temperature increases.

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The Iron Wall

• Iron is the only one element from which it is not
  possible to generate any kind of nuclear energy.
• Iron has the lowest mass per nuclear particle of
  all nuclei. It cannot release energy by either
  fusion or fission. It is the most stable element
  in the universe.

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Massive Stars Final Collapse

• Electron degeneracy cannot support the structure
  of stars with M8MSun.
• Electrons combine with protons producing neutrons
  and neutrinos.
• About 1MSun of Fe in the core with a size
  1REarth collapses into a neutron corpse with
  size Honolulu.
• Neutron degeneracy halts further collapse.

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Supernova Explosion!!

• The gravitational collapse of the Fe core
  releases in a few days more than 100xLSun over
  its entire lifetime.
• It is not clear how this titanic energy is
  transported outwards. Neutrinos are a
  possibility.
• The stars outer layers are projected into space
  with speeds of up to 104 km/s.
• For about 1 week LSN1010LSun. As bright as a
  whole galaxy!

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The Origin of the Elements

• Stars in GCs have Z0.1, while stars in OCs have
  Z2.
• The total amount of elements heavier than He has
  increased over time in the Milky Way.
• Elements heavier than Fe are rare because they
  are produce only during supernova explosions.

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Crabs Supernova Remnant

• The Crab Nebula is the remnant of a SN observed
  in A.D.1054 by Chinese astronomers.
• A pulsar lies at the center.
• The nebula is expanding at a rate of several
  thousand km/s.
• Nebulas birth occurred some 2300 years ago.

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Supernova 1987A

• In modern times, the only supernova visible to
  the naked eye burst in the Large Magellanic Cloud
  (d50 Kpc).
• Precursor was a blue supergiant, not a red
  supergiant.
• Neutrinos were collected in Japan and Ohio,
  confiming the formation of a neutron core.

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Summary of Stellar Lives
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14.5 Evolution of Close Binaries

• Algol paradox a 3.7 MSun MS star and a 0.8MSun
  subgiant.
• How can the lower mass component of Algol be more
  evolved than the higher-mass component?
• When the more massive star expands out of the MS,
  mass exchange to the MS companion occurs.

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The Big Picture

• Most elements heavier than He are forged in
  stellar furnaces. Elements heavier than Fe are
  released in SN explosions.
• The initial stellar mass is the primary factor
  controlling stellar evolution.
• Low-mass stars live long and end up ejecting a
  planetary nebula and leaving behind a WD.
• High-mass stars live fast and explode as SN.
• Close binaries can exchange mass, altering the
  usual course of stellar evolution.