• 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