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

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!

HR Diagram of a Star Cluster

Cluster Turnoff (SLIDESHOW MODE ONLY)

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

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

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

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

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!

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.

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 %.

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

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.

Evolution of Stars (SLIDESHOW MODE ONLY)