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

Star cluster website

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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