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Lecture 18: Clusters of stars
Astronomy 111
Testing stellar evolutionTesting stellar evolution
• The Problem:– Stellar Evolution happens on billion-yearStellar Evolution happens on billion year
time scales.– Astronomers only live for a few 10’s of y
years.• The Solution:
– Make H-R Diagrams of star clusters with a wide range of ages.
ASTR111 Lecture 19
g g
Star clustersStar clusters
• Groups of 100’s to 1000’s of stars.• All stars in a clusterAll stars in a cluster...
– are at the same distanceeasy to measure relative Luminosityeasy to measure relative Luminositydon’t need distances to individual stars
have the same age– have the same age.– have the same chemical composition.
ASTR111 Lecture 19
The main sequence revisitedThe main sequence revisited
• The Main Sequence is a Mass Sequence:– High-mass stars are hotter and brighter.g g– Low-mass stars are cooler and fainter.
• Main Sequence Lifetime depends on q pMass:– High-mass stars have short M-S lifetimesg– Low-mass stars have long M-S lifetimes.
• Low-Mass stars take longer to form.
ASTR111 Lecture 19
g
Progressive evolutionProgressive evolution
• As a cluster ages:– High-mass stars reach the M-S first, with the g
low-mass stars still approaching.– High-mass stars run out of hydrogen in their
fi l i i icores first, evolving into supergiants.– As successively lower mass stars run out of
hydrogen in their cores they too evolve offhydrogen in their cores, they too evolve off.• Peel off the Main-Sequence from the top.
ASTR111 Lecture 19
Age: ~1 Myr
106
Age: 1 Myr
106
104
sun)
102
osity
(Ls
1
102Lum
ino
104
Zero Age
Main Sequence
ASTR111 Lecture 19
40,000 20,000 10,000 5,000 2,500Temperature (K)
Age: ~10 Myr
106
Age: 10 Myr
106
104
sun) B Stars
102
osity
(Ls B Stars
1
102Lum
ino
104
ASTR111 Lecture 19
40,000 20,000 10,000 5,000 2,500Temperature (K)
Age: ~100 Myr
106
Age: 100 Myr
106
104
sun)
A St102
osity
(Ls A Stars
1
102Lum
ino
104
ASTR111 Lecture 19
40,000 20,000 10,000 5,000 2,500Temperature (K)
Age: ~1 Gyr
106
Age: 1 Gyr
106
104
sun)
102
osity
(Ls
F Stars1
102Lum
ino
104
ASTR111 Lecture 19
40,000 20,000 10,000 5,000 2,500Temperature (K)
Age: ~10 Gyr
106
Age: 10 Gyr
106
104
sun)
102
osity
(Ls
1
102Lum
ino
G Stars
104
ASTR111 Lecture 19
40,000 20,000 10,000 5,000 2,500Temperature (K)
Main-Sequence Turn-offMain-Sequence Turn-off
• Point where the Main-Sequence “turns off” towards giant stars.g– As cluster ages, the stars at the turn-off are
lower mass– Low mass stars have redder colors.
• Indicator of the cluster age:Indicator of the cluster age:– Older Clusters have redder turn-off points.
ASTR111 Lecture 19
A 10 M A 1 GAge: ~10 Myr Age: ~1 Gyr
BBStars F
TBlue Red T
FStars
ASTR111 Lecture 19
TBlue Red T
Types of clustersTypes of clusters
• Open Clusters:– Sparse clusters (few 1001000 stars)Sparse clusters (few 100 1000 stars)– few parsecs in diameter
• Globular Clusters:• Globular Clusters:– Rich spherical clusters (105106 stars)
10 30 parsecs in diameter– 1030 parsecs in diameter
ASTR111 Lecture 19
Open ClusterOpen Cluster
100’s of starsMany blue M SMany blue M-Sstars
Y AFew giantsYoung Ages(100’s of Myr)
ASTR111 Lecture 19
( 00 y )
Globular ClustersGlobular Clusters
100,000’s of stars
Many giants
No blue M-S stars
Old ages
( 13 G )ASTR111 Lecture 19
(~13 Gyr)
Open ClustersOpen Clusters
• H-R Diagrams of Open Clusters show:– They are young to middle-agedThey are young to middle aged– Have blue Main-Sequence stars– Few supergiants or giantsFew supergiants or giants– Older Open clusters have more red giants– Don’t see a horizontal branch– Don t see a horizontal branch– Youngest still have gas clouds associated
ASTR111 Lecture 19
ASTR111 Lecture 19
Globular ClustersGlobular Clusters
• H-R Diagrams of Globular Clusters show:– Very old: 1015 Billion Yearsy– Red turnoffs and no blue Main-Sequence stars– Lots of red giants– A prominent Horizontal Branch– Slightly bluer and fainter Main Sequence due
to having less metals than nearby stars
ASTR111 Lecture 19
Typical Globular Cluster H-R DiagramDiagram
106106
104
sun)
HorizontalBranch
Zero-AgeMain
102
osity
(Ls Main
Sequence
1
102Lum
ino
104
ASTR111 Lecture 19
40,000 20,000 10,000 5,000 2,500Temperature (K)
Conclusions of the testsConclusions of the tests
• Cluster H-R Diagrams give us a snapshot of stellar evolution.p
• Observations of clusters with ages from a few Million to 15 Billion years confirmsa few Million to 15 Billion years confirms much of our picture of stellar evolution.
• Remaining challenges are in small• Remaining challenges are in small details, but the big picture is secure.
ASTR111 Lecture 19
Summary:Summary:
• H-R Diagrams of Star Clusters• Ages from the Main-Sequence Turn-offAges from the Main Sequence Turn off• Open Clusters
Young clusters of few 1000 stars– Young clusters of few 1000 stars– Blue Main-Sequence stars & few giants
• Globular Clusters• Globular Clusters– Old clusters of a few 100,000 stars– No blue Main-Sequence stars & many
ASTR111 Lecture 19
No blue Main Sequence stars & many giants