Post on 02-Jan-2016
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!
Example: HR diagram of the star cluster M 55
High-mass stars evolved onto the
giant branch
Low-mass stars still on the main
sequence
Turn-off point
Evidence for Stellar Evolution: Variable Stars
Some stars show intrinsic brightness variations not caused by eclipsing in binary systems.
Most important example:
Cephei
Light curve of Cephei
Cepheid Variables: The Period-Luminosity Relation
The variability period of a Cepheid variable is correlated with its luminosity.
=> Measuring a Cepheid’s period, we can determine its absolute magnitude!
The more luminous it is, the more slowly it pulsates.
Cepheid Distance MeasurementsComparing 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.