1. 15.3 Variable Stars & Star Clusters

2. Variable Stars
Any star that varies significantly in brightness with time is called a variable star
Some stars vary in brightness because they cannot achieve proper balance between power welling up from the core and power radiated from the surface
Such a star alternately expands and contracts, varying in brightness as it tries to find a balance

3. Pulsating Variable Stars
The light curve of this pulsating variable star shows that its brightness alternately rises and falls over a 50-day period

4. Cepheid Variable Stars
Most pulsating variable stars inhabit an instability strip on the H-R diagram

5. Star Clusters

6. Open cluster: A few thousand loosely packed stars

7. Possess no regular shape
Contain hundreds of stars
Found in the Galactic plane (disk)
HR diagrams show long main sequences that indicate that the clusters are young and contain young hot stars (O and B type)

8. Useful movies to download:
Globular cluster visualization
Stellar collision simulation:
Globular cluster: Up to a million or more stars in a dense ball bound together by gravity

9. Possess a regular shape like a ball with stars more densely packed moving toward the center
Contain 10,000 to 1 million stars
Found in the Galactic halo
HR diagrams show short main sequences that indicate that the clusters are old (Spectral type O, B, A, F, and G stars are absent)

10. How do we measure the age of a star cluster?

11. Massive blue stars die first, followed by white, yellow,
orange,
and
red stars
Star_cluster_evolving.swf

12. Main-sequence turnoff point of a cluster tells us its age

13. Detailed modeling of the oldest globular clusters reveals that they are about 13 billion years old