1

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

2

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.

3

Evidence for Stellar Evolution: Variable Stars

Some stars show intrinsic

brightness variations not caused by eclipsing in binary systems.

Most important example:

δδδδ Cephei

Light curve of δ 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.

4

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

5

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)