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

H-R Diagram An H-R diagram reveals three significant features. L/Lsun 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. L/Lsun O B A F G K M

H-R Diagram 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”) L/Lsun O B A F G K M

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

Stellar Clusters OPEN Cluster GLOBULAR Cluster Bright Hot, Young stars Thousands of stars Loose Associations GLOBULAR Cluster Fainter Cooler, Older stars Hundreds of Thousands of stars Tightly Bound Systems

Double Cluster OPEN Cluster Very few evolved stars.

Globular Cluster Many evolved stars.

Cluster HR Diagrams H-R diagrams revealing a variety of ages among clusters. Who is the youngest? Who is the oldest?

Young Cluster

Young Cluster HR Diagram

Cluster Evolution

Combined Cluster H-R diagrams

Hyades

M44 Cluster

M41 Cluster

M2001 Cluster

Globular Cluster M5

Main Sequence Turnoff 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.

Horizontal Giant Branch

Pulsating Variable Stars

Instability Strip

The Pulsating Stars

Cepheid Pulsation Sequence

Pulsating Variables Light curves for two different Cepheid variables showing the periodic changes in brightness.

Cepheids

Cepheid Pulsation The interplay of Radius and Temperature. L = 4pR2sT4

Period - Luminosity Relation

Cepheid Period - Luminosity Observe 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

SMC and LMC The Milky Ways two irregular satellite galaxies.

Distance Determinations

Distance Indicators