1. Evidence of Stellar Evolution
Physics 113 Goderya
Chapter(s): 12
Learning Outcomes:

2. Evidence for Stellar Evolution: Star Clusters
Stars in a star cluster all have approximately the same age!
More massive stars evolve more quickly than less massive ones.
If you put all the stars of a star cluster on a HR diagram, the most massive stars (upper left) will be missing!

3. HR Diagram of a Star Cluster

4. Cluster Turnoff
(SLIDESHOW MODE ONLY)

5. Example: HR diagram of the star cluster M 55
High-mass stars evolved onto the giant branch
Turn-off point
Low-mass stars still on the main sequence

6. Estimating the Age of a Cluster
The lower on the MS the turn-off point, the older the cluster.

7. Evidence for Stellar Evolution: Variable Stars
Some stars show intrinsic brightness variations not caused by eclipsing in binary systems.
Most important example:
d Cephei
Light curve of d Cephei

8. RR Lyrae Variable
Brightness
12 hours
Cepheid Variable
Brightness
1 Week
Time

9. Cepheid Variables: The Period-Luminosity Relation
The variability period of a Cepheid variable is correlated with its luminosity.
The more luminous it is, the more slowly it pulsates.
=> Measuring a Cepheid’s period, we can determine its absolute magnitude!

10. Cepheid Distance Measurements
Comparing absolute and apparent magnitudes of Cepheids, we can measure their distances (using the 1/d2 law)!
The Cepheid distance measurements were the first distance determinations that worked out to distances beyond our Milky Way!
Cepheids are up to ~ 40,000 times more luminous than our sun
=> can be identified in other galaxies.

11. Pulsating Variables: The Instability Strip
For specific combinations of radius and temperature, stars can maintain periodic oscillations.
Those combinations correspond to locations in the Instability Strip
Cepheids pulsate with radius changes of ~ 5 – 10 %.

12. Pulsating Variables: The Valve Mechanism
Partial He ionization zone is opaque and absorbs more energy than necessary to balance the weight from higher layers. => Expansion
Upon expansion, partial He ionization zone becomes more transparent, absorbs less energy => weight from higher layers pushes it back inward. => Contraction.
Upon compression, partial He ionization zone becomes more opaque again, absorbs more energy than needed for equilibrium => Expansion

13. Period Changes in Variable Stars
Periods of some Variables are not constant over time
because of stellar evolution.
 Another piece of evidence for stellar evolution.

14. Evolution of Stars
(SLIDESHOW MODE ONLY)