• Homework Problems Chapter 13– Review Questions: 1-3, 9-11– Review Problems: 1, 2, 7– Web Inquiries: 1, 4

Stellar Clusters

An H-R diagram reveals three

significant features.

• The Main Sequence– core hydrogen fusion

• Red Giants– hydrogen shell source– (core helium fusion)

• White Dwarfs– carbon-oxygen core leftovers

This is a snapshot in the life of many,

unrelated stars.

H-R Diagram

O B A F G K M

L/Lsun

Other areas of the H-R diagram are

sparsely populated either because

stars just don’t exist with those

specific combinations of

luminosity and

temperature, or because the

stars evolve so quickly and

spend so little time in that area

that the probability of seeing

stars located there is very low.

(Example: “Hertzsprung Gap”)

H-R Diagram

O B A F G K M

L/Lsun

• Stars from the same formation epoch (same “birthday”)• Same Age • Similar Chemical Composition• Same Distance from the Earth

Clusters Formation

• OPEN Cluster– Bright– Hot, Young stars– Thousands of stars– Loose Associations

• GLOBULAR Cluster– Fainter– Cooler, Older stars– Hundreds of Thousands of stars– Tightly Bound Systems

Stellar Clusters

• OPEN Cluster

Very few evolved

stars.

Double Cluster

Many evolved stars.

Globular Cluster

H-R diagrams

revealing

a variety of

ages among

clusters.

Who is the youngest?

Who is the oldest?

Cluster HR Diagrams

Young Cluster

Young Cluster HR Diagram

Cluster Evolution

Combined Cluster H-R diagrams

Hyades

M44 Cluster

M41 Cluster

M2001 Cluster

Globular Cluster M5

Since stellar evolution

is highly dependent

upon the mass of a star,

the turn-off point

of a clusters

main-sequence

is a measure of the

clusters age.

Main Sequence Turnoff

Horizontal Giant Branch

Pulsating Variable Stars

Instability Strip

The Pulsating Stars

Cepheid Pulsation Sequence

Light curves for two

different Cepheid

variables showing

the periodic changes

in brightness.

Pulsating Variables

Cepheids

The interplay

of Radius

and Temperature.

L = 4R2T4

Cepheid Pulsation

Period - Luminosity Relation

Cepheid Period - LuminosityObserve apparent magnitude (m)

Observe Period of pulsation (P)

Utilize P-L relation to infer absolute magnitude (M)

Use distance modulus to

obtain distance:

m-M = 5 log D - 5

The Milky Ways two irregular satellite galaxies.

SMC and LMC

Distance Determinations

Distance Indicators