01:750:109 Chapter Notes - Chapter 9.3: Globular Cluster, Triple-Alpha Process, Main Sequence
Testing Stellar Models with Star Clusters
●Observations of star clusters have been particularly useful in testing stellar models for 2 key
reasons:
○All the stars in a cluster lie at about the same distance from Earth, meaning that the
apparent brightness of each star directly tells how its luminosity compares with that of
other stars in the cluster
○All the stars in a cluster formed at about the same time from the same large molecule
cloud, meaning that they are now all about the same age
●Here, will analyze how star clusters enable us to test our models of stellar lives
What do star clusters reveal about the lives of stars?
●2 basic types of star clusters (differs based on how densely packed they are and their locations
and ages):
○Modest-size open clusters
■Found in the galaxy's relatively flat disk
■Contain a few hundred to a few thousand stars in a region about 30 lys across
■All the stars are relatively young
H-R diagram for Pleiades
●Most of the stars fall along the main sequence, except for when the Pleiades'
stars trail away to the right of the main sequence at the upper end
(main-sequence turnoff point)
○Theoretical models indicate that the main-sequence lifetime of stars at
this point on the main sequence is about 100 million years aka the age
of Pleiades
○Stars w lifetimes longer than 100 million yrs are still fusing hydrogen in
their cores and hence remain main-sequence stars
●That is, the host, short-lived stars of the spectral O type are missing from the
main sequence
○Fact is consistent with what we expect from the assumption that hot,
massive stars do indeed have shorter lifetimes than cooler, less massive
stars
