Lecture 17 Star Clusters, HR Diagrams, and the ISM

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Lecture 17 Star Clusters, H-R Diagrams, and
the ISM
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Lecture 17 Star Clusters, H-R Diagrams the
Interstellar Medium

• Star clusters offer good test of stellar
  evolution theory because the stars in a given
  cluster all have the same distance, initial
  chemical composition, and age (each need to be
  determined). The only variable is the mass of
  individual stars.
• Interstellar dust causes extinction and reddening
  of optical light from background stars, as well
  as reflection nebulae around embedded or nearby
  stars.
• Interstellar gas is observed in optical,
  infrared, and radio radiation as emission
  nebulae, with the emission arising from thermal
  processes in ionized nebulae (H II regions) and
  planetary nebulae. In supernova remnants,
  emission in X-rays can arise from very hot
  shocked gas (T gt K), or from the nonthermal
  process of synchrotron radiation of relativistic
  electrons gyrating wildly in magnetic fields.

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Summary of Lives of Stars
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Measuring Distance by Parallax Works for
Relatively Close-by Stars
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Distances Based on Apparent and Absolute
Brightnesses Must Account for Interstellar
Extinction
Discovery of Interstellar Extinction by Robert
J. Trumpler (1886-1956)
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Dark Cloud Seen AgainstRich Background of Stars
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Dark Clouds Seen Against Bright Background of
Emission Nebula (H II Region)
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Why Interstellar Medium is Interesting

• ISM is not empty it contains
• small solid particles dust grains,
• gas, atomic and molecular, as well as plasma,
• relativistic particles cosmic rays,
• magnetic fields.
• ISM is kept active by interactions with stars
• Luminous stars provide continuous source of
  energy in form of starlight, especially UV light.
• Dying stars provide source of processed matter as
  well as input of kinetic energy.
• ISM provides the cradles of star birth, supplying
  the raw material for new generations of stars and
  planets.
• The last two processes provide the explanation
  for the differences between Population I and
  Population II stars.

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Panorama Showing Interaction of Gas, Dust, and
Stars
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Blue Sky Coupled with Reddening of Sun at Sunset
Sunrise
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Observational Evidence for Interstellar Dust and
Gas

• Reflection nebula ? Dust surrounding star.
• Stationary absorption lines in spectra of binary
  ? Cool gas between us and stars.

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Reflected Starlight Looks Bluer Than Illuminating
Stars
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Halo of Light from Pleiades Classical
Reflection Nebula
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Hot Young Stars Ionize the Material Around Them,
Producing H II Regions
Central star emits ultraviolet radiation, which H
II region degrades by fluorescence into visible
and infrared radiation.
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H II Regions Show Characteristic Red Light of
Fluorescing H
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A Star Cluster at Birth is Closely Associated
with Gas and Dust
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Open Star Clusters Are Found in the Disk of the
Galaxy
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Globular Star Clusters Are Foundin the Halo of
the Galaxy
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H-R Diagram of Open Clusters

• Investigation by Trumpler of H-R diagram of open
  star clusters.
• Recognition that main-sequence turnoff represents
  core-hydrogen exhaustion.
• Nature of stellar nuclear reactions explained by
  Bethe, Hoyle, Salpeter, Cameron, and their
  collaborators.
• Modern computation of stellar structure and
  evolution by computers.

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H-R Diagram of Pleiades Shows Both
Pre-Main-Sequence and Post-Main-Sequence Stars
Stars leaving the main sequence
Stars still contracting to the main sequence
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Evolution of Stars of Different Masses Off the
Main Sequence
Evolutionary tracks of stars of different masses
with similar abundances of heavy elements as Sun
(Population I).
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Turnoff Points of Different Open Clusters
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H-R Diagram of Globular Cluster Implies Its Stars
Are Very Old (and Deficient in Heavy Elements)
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A Globular Cluster Replete with White Dwarfs
Verifies Its Great Age
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Supernova Remnants Produce Both Thermal and
Nonthermal Emission Nebulae
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Nonthermal Continuum Emission Synchrotron
Emission
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Different States of Gas in the ISM
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Radio Observations of Interstellar Atomic
Hydrogen
Also possible with transitions between various
rotational states of molecules, but not H .
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Molecular Clouds Are Where Stars Are Being Born
Today
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Gas-Star Cycle in the Galaxy