Chapter 17 Star Formation and Evolution to the Main Sequence

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Chapter 17Star Formation and Evolution to the
Main Sequence
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Eagle Nebula

• Hydrogen (red)
• Dust clouds
• Bok Globules (smallest darkest clouds)
• Young hot stars that ionize the hydrogen

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Molecular Hydrogen

• Extensive and non-uniform distribution of
  molecular hydrogen in the plane of the Milky Way

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Interstellar Dust Particles

• Dust is 1 of the interstellar medium
• Dust is small, like soot

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Horsehead Nebula

• Dust against bright background of ionized hydrogen

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Molecular Clouds

• Object Size Density
• Giant Molecule Cloud Complex 150 ly, 100
• Giant Moleculaar cloud 12 ly, 1000
• Core of molecular cloud 3, 4000
• Dark Cloud complex 30, 400
• Dark cloud 1.5 10,000
• Dark cloud core 0.6, 40,000

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Molecular Hydrogen

• Ultraviolet spectrum of molecular hydrogen dust
  grains needed for formation

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Two Mechanisms of Cloud Collapse

• (a) Radiation pressure from nearby hot star
• (b) Collisions of colliding clouds
• Other mechanisms
• Supernova explosion blast wave
• Galaxy density waves

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Dynamical PhaseNot to scale, subject to change
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Mechanisms of Cloud CollapseDynamical Phase,
short time scales

• (a)Giant molecular cloud fragments into small
  pieces about 100,000 AU in size
• (b) Interior collapses faster making a core
• (c) Collapsing core has so much pressure, core
  collapse halted
• (d)Core collapse reverses direction the core
  bounce
• (ef) expanding core meets collapsing outer
  regions then entire object collapses again
• (g) Core becomes opaque, light particles not free
  to escape as density is so high
• (hi) collapse continues with increasing speed
  until a 2nd core bounce happens increase in temp
  and brightness put object on the HR diagram

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Protostar Evolution during Dynamical Collapse
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Approach to Main Sequence
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From cloud to star

• (1-3) dynamical phase energy still transported
  by convection, turbulent!! No visible radiation,
  absorbed by dust, infrared is main observational
  tool
• (4) Shrinking continues, energy transport
  converts from convection to radiation
• (5) Core temp 10 Million K, hydrogen burning
• (6) Increase in outward pressure due to H burning
  slows and eventually stops collapse, H burning
  becomes dominant energy source
• A STAR is born! (ZAMS -gt Zero age main sequence)

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Events after HydrogenBurning Starts
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Timelines

• 30 Solar masses O star, 30,000 years to reach
  ZAMS
• 10 Solar masses B star, 300,000 years
• 4 , B, 1,000,000 years
• 2, A, 8,000,000 years
• 1, G2, 30,000,000 years
• 0.5 K 100,000,000 years
• 0.2, M 1,000,000,000 years

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Bipolar CO Molecular Cloud

• Radio observations
• Blue shifted and red shifted lobes

• Model to explain observations
• Note accretion disk

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BokGlobules

• Small dark clouds associated with star formation

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IR View of Star Formation Region
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T Tauri Stars

• T Tauri stars in Taurus-Auriga dark cloud
• Evolutionary paths
• Birthline is the theoretical line below which
  stars should appear on H-R diagram
• Consistent with accretion disks around protostar
• Lots of Lithium -gt young stars Li is destroyed
  at temps in MS stars

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Stages of Star Formation for Low-Mass Star

• The predicted infrared spectrum at each stage is
  shown
• Varies from blackbody because of IR emission from
  dust

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Herbig-Haro Object
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Herbig-Haro Object inside Bok Globule
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Formation of Expanding OB Association
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Eagle Nebula
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Eagle Nebula molecular gas
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Star Cluster

• Cannot observe changes in stars over time
  available time scales
• Clusters help us
• All are at same distance
• All formed out of same material
• All formed about the same time

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Pleiades star Cluster

• Young stars illuminating and reflecting light
  from dust in cloud through which stars are passing

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Pleiades star Cluster
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M92
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M80
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M13
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Evolution of Imaginary Cluster
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Star Formation in NGC 2264

• Gas, dust, molecular cloud
• T Tauri stars
• Hot, young OB stars
• Physical association of all these are consistent
  with star formation ideas

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H-R Diagram of NGC 2264
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Brown Dwarf Gliese 229B
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Orion Nebula

• Nebula shines because of UV radiation from hot
  stars at center

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Orion Nebula

• Optical Infrared

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H-R Diagram for Orion

• Low-mass stars
• Theoretical lines for stars of constant age

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Protoplanetary Disks in Orion
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Chapter 17 HW Problems

• Summary Questions 3,4,6
• Applying Your Knowledge 5