Binary stars and clusters. Chapter 11. Review. Properties of stars apparent brightness (apparent magnitude) measure energy/area/second luminosity (absolute magnitude) calculate distance, use inverse-square law distance to the star parallax temperature
Notice that the more luminous stars in the Pleiades are off the main sequence.
These are red giants.
Evidently, the higher mass stars have evolved into red giants.
This is a typical characteristic of open clusters.
The more stars that are on the main sequence, the younger the cluster.
The more stars that are off the main sequence, the older the cluster.
They are similar in age, probably formed early in the universe (11-14 billion y).
The horizontal branch is “signature” of globular clusters.
The horizontal branch may indicate fewer heavy metals in the star, as would be the case for very old stars.