Stellar%20Evolution

1
Stellar Evolution

• Chapters 12 and 13

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Topics

• Humble beginnings
• cloud
• core
• pre-main-sequence star
• Fusion
• main sequence star
• brown dwarf
• Life on the main sequence
• Retirement
• low mass stars (lt10 solar masses)
• high mass stars (gt10 solar masses)

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H-R Diagram
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Mass related to luminosity

• For binary stars, that we can reliably measure
  their masses and luminosities, graph luminosity
  vs. mass
• HUGE changes in luminosity correspond to small
  changes in mass -- power relationship!
• L M4 for main sequence stars

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So how do stars grow?
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How do we know?

• Develop computer models and theories based on
  physics
• Compare observations with predictions
• Although changes to stars generally occur over
  large time scales, there are enough stars that we
  occasionally see a change occur (like novae and
  supernovae)

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Oh, honey, lets have a baby...

• Cloud of dust and gas
• mostly gas
• lots of hydrogen
• diameter 10,000Ds.s.
• density lt 1000 atoms/cm3
• in equilibrium

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Milky Way
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Milky Way in infrared (COBE)
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Emission Nebulae
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M20 Trifid Nebula (900 pc)
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Barnard 68 Dark Nebula
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Horsehead Nebula in Orion
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Eagle Nebula in M16
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Pickles and Lamaze

• Internal temperature and pressure increases
• loss of gravitational energy results in a gain of
  kinetic energy and thermal energy
• temperature and pressure at the core increases
• rate of collapse slows down
• continues to contract although at a slower rate
• pre-main-sequence star

• Gravitational Collapse
• A shock wave likely produced by a nearby nova or
  supernova disturbs the cloud.
• The cloud is no longer in equilibrium.
• Local regions of higher density.
• Some of the dust and gas get close enough to each
  other that the gravitational force is significant
  enough that they collide and begin to clump.
• dense cores form
• these cores are protostars

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The water breaks!A star is born!

• fusion
• as the star contracts, the temperature and
  pressure at the core increase
• high temperature allows fusion to take place
• most common type of fusion at this stage is the
  proton-proton chain six hydrogen atoms yield one
  helium and two hydrogen atoms
• mass is transformed into energy (Emc2)

• equilibrium
• temperature and pressure increase in the core
• the outward pressure balances the inward
  gravitational force
• star is in hydrostatic equilibrium
• main-sequence star

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Fat stars die young

• A greater mass star requires a greater pressure
  to achieve equilibrium.
• Greater mass stars are thus hotter.
• M - L relationship!
• The more massive stars burn energy (i.e.
  convert hydrogen to helium) at a much higher
  rate.
• More massive stars die younger.

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When the birth goes wrong

• What if the temperature of the star is not high
  enough for fusion to begin?
• miscarriage brown dwarf
• brown dwarfs are different from planets in how
  they form
• they have approximately the same mass of large
  Jovian planets (gas giants)
• hard to detect looking for lithium is one way
• we define a brown dwarf as having mass 10-80
  Jupiter masses

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Adolescence to adulthood

• The star is on the main sequence.
• It continues to convert mass to energy by the
  process of fusion.
• The more massive stars will burn out sooner.
• So which stars on the H-R diagram are younger?

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H-R Diagram
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Two retirement plans

• what happens next depends on the stars mass
• low mass stars (lt10 solar masses when on the
  main sequence)
• red giant
• planetary nebula
• white dwarf (lt1.4 solar masses)
• high mass stars (gt10 solar masses when on the
  main sequence)
• red giant
• Type II supernova
• neutron star or black hole

23
Low mass stars

• evolve from main sequence stars to red giants as
  it exhausts its hydrogen supply in its core for
  fusion and subsequently cools
• as it cools, its outer layers expand to form a
  planetary nebula
• its core contracts until reaching equilibrium
• the core is so small and so dense that electrons
  cannot be packed closer together
• it is a white dwarf a corpse
• stable for Mlt1.4 solar massses (the Chandrasekhar
  limit)

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High mass stars

• If at the end of a stars life, the mass of its
  core is greater than 1.4 solar masses, the
  pressure due to the electron gas is not great
  enough to balance gravitation.
• It undergoes further collapse until it reaches a
  new equilibrium where the pressure of a neutron
  gas is great enough to counteract gravitation.
• It is a neutron star.
• For M gt 2 or 3 solar masses, even a neutron gas
  cannot withstand the gravitational forces.
• For these masses, it becomes a black hole.

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Novae

• A binary system of a white dwarf and red giant.
• The high gravitational force of the white dwarf
  attracts loosely held matter from the outer
  surface of the giant.
• As the matter accretes onto the white dwarf, its
  temperature increases.
• When fusion begins, the outer layer of the dwarf
  explodes.
• Process can be repeated over and over.
• Luminosity can be 10 or 100 times the luminosity
  of the Sun.

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Supernovae

• Type I
• a white dwarf increases enough mass to exceed 1.4
  solar masses
• the entire star and core explode
• nothing is left
• Type II
• death of a massive star (blue or red giant)
• core rapidly collapses, mass exceeds 1.4 solar
  masses
• explosion
• birth of a neutron star (or pulsar)

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Crab Nebula - supernova remnant from 1054 A.D.
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SN 1987A
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Summary

• Gravitation births stars in clouds
• Gravitation kills massive stars through in
  supernovae explosions.
• Fusion generates heavier elements.
• Supernovae expel dust and gas back into the
  interstellar medium, only to form stars again.