Evidence of Stellar EvolutionPhysics 113 Goderya

Chapter(s): 12Learning 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

Low-mass stars still on the main

sequence

Turn-off point

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:

Cephei

Light curve of Cephei

0 12 hours

0 1 Week

Bri

ghtn

ess

Bri

ghtn

ess

RR Lyrae Variable

Cepheid Variable

Time

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.

Evolution of Stars

(SLIDESHOW MODE ONLY)