Chapter 13: Star Clusters
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Open cluster globular cluster
Stars (almost) always form in clusters
• 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!
h & Chi Persei – Double Cluster in Perseus
Messier 35 and NGC2158 – “open clusters” of different ages
Open clusters:
1. Contain < ~ 1000 stars
2. Loosely gravitationally bound together
3. Younger than globular (some still contain O and B stars)
4. More enriched in heavy elements (“Population I” stars)
Pleiades
Messier 50 – An “open cluster” of stars
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
globular clusters:1. Contains ~1000-106 stars2. Extremely old: billions of years3. Population II (low in heavy elements)
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“He who cannot see gravity at work here has no soul.”
- Richard Feynman
Globular cluster M2
HD Diagram for star clusters: only difference is mass
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 clusters are useful to:
1. Verify stellar evolution models2. Determine distances to nearby galaxies3. Determine chemical composition of
universe in the very distant past
