Chapter 13: Star Clusters

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Chapter 13 Star Clusters
Stars (almost) always form in clusters
globular cluster
Open cluster

• The stars all formed at the same time
• They are all at the same distance
• They formed from the same material
• Only difference is their masses!

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h Chi Persei Double Cluster in Perseus
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Messier 35 and NGC2158 open clusters of
different ages
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Pleiades

• Open clusters
• Contain lt 1000 stars
• Loosely gravitationally bound together
• Younger than globular (some still contain O and B
  stars)
• More enriched in heavy elements (Population I
  stars)

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Messier 50 An open cluster of stars
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Open clusters that have become gravitationally
unbound are stellar associations. If they
continue to have similar proper motion, this is a
moving group.
Ursa Major moving group
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• globular clusters
• Contains 1000-106 stars
• Extremely old billions of years
• Population II (low in heavy elements)

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Globular cluster M2
He who cannot see gravity at work here has no
soul. - Richard Feynman
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HD Diagram for star clusters only difference is
mass
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Isochrones Lines of constant age for stars of
different masses
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M.S. LIFETIME for STAR at TURNOFF AGE of
CLUSTER
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Star cluster website
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• Star clusters are useful to
• Verify stellar evolution models
• Determine distances to nearby galaxies
• Determine chemical composition of universe in the
  very distant past

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