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.

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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.

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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.

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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.

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Age: ~1 Myr

106

Age: 1 Myr

106

104

sun)

102

osity

(Ls

1

102Lum

ino

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Zero Age

Main Sequence

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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

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40,000 20,000 10,000 5,000 2,500Temperature (K)

Age: ~100 Myr

106

Age: 100 Myr

106

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sun)

A St102

osity

(Ls A Stars

1

102Lum

ino

104

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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

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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

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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.

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A 10 M A 1 GAge: ~10 Myr Age: ~1 Gyr

BBStars F

TBlue Red T

FStars

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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

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Open ClusterOpen Cluster

100’s of starsMany blue M SMany blue M-Sstars

Y AFew giantsYoung Ages(100’s of Myr)

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( 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

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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

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Typical Globular Cluster H-R DiagramDiagram

106106

104

sun)

HorizontalBranch

Zero-AgeMain

102

osity

(Ls Main

Sequence

1

102Lum

ino

104

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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.

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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

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No blue Main Sequence stars & many giants