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ClustersofStars
Astronomy101
Tes4ngStellarEvolu4on
• TheProblem:– StellarEvolu4onhappensonbillion‐year4mescales.
– Astronomersonlyliveforafew10’sofyears.
• TheSolu4on:– MakeH‐RDiagramsofstarclusterswithawiderangeofages.
StarClusters
• Groupsof100’sto1000’sofstars.• Allstarsinacluster...– areatthesamedistance
easytomeasurerela4veLuminositydon’tneeddistancestoindividualstars
– havethesameage.
– havethesamechemicalcomposi4on.
TheMain‐Sequencerevisited
• TheMainSequenceisaMassSequence:– High‐massstarsareho#erandbrighter.
– Low‐massstarsarecoolerandfainter.
• MainSequenceLife4medependsonMass:– High‐massstarshaveshortM‐Slife4mes– Low‐massstarshavelongM‐Slife4mes.
• Low‐Massstarstakelongertoform.
ProgressiveEvolu4on
• Asaclusterages:– High‐massstarsreachtheM‐Sfirst,withthelow‐massstarss4llapproaching.
– High‐massstarsrunoutofhydrogenintheircoresfirst,evolvingintosupergiants.
– Assuccessivelylowermassstarsrunoutofhydrogenintheircores,theytooevolveoff.
• PeelofftheMain‐Sequencefromthetop.
40,000 20,000 10,000 5,000 2,500
106
104
102
1
10-2
10-4
Temperature (K)
Lum
inos
ity (L
sun)
Age: ~1 Myr
Zero Age Main Sequence
40,000 20,000 10,000 5,000 2,500
106
104
102
1
10-2
10-4
Temperature (K)
Lum
inos
ity (L
sun)
Age: ~10 Myr
B Stars
40,000 20,000 10,000 5,000 2,500
106
104
102
1
10-2
10-4
Temperature (K)
Lum
inos
ity (L
sun)
Age: ~100 Myr
A Stars
40,000 20,000 10,000 5,000 2,500
106
104
102
1
10-2
10-4
Temperature (K)
Lum
inos
ity (L
sun)
Age: ~1 Gyr
F Stars
40,000 20,000 10,000 5,000 2,500
106
104
102
1
10-2
10-4
Temperature (K)
Lum
inos
ity (L
sun)
Age: ~10 Gyr
G Stars
Main‐SequenceTurn‐off
• PointwheretheMain‐Sequence“turnsoff”towardsgiantstars.– Asclusterages,thestarsattheturn‐offarelowermass
– Lowmassstarshavereddercolors.
• Indicatoroftheclusterage:– OlderClustershaveredderturn‐offpoints.
Age: ~10 Myr Age: ~1 Gyr
T Blue Red T
B Stars
F Stars
TypesofClusters
• OpenClusters:– Sparseclusters(few100-1000stars)– fewparsecsindiameter
• GlobularClusters:– Richsphericalclusters(105-106stars)– 10-30parsecsindiameter
OpenCluster
100’s of stars
Many blue M-S stars
Young Ages (100’s of Myr)
Few giants
GlobularClusters
100,000’s of stars
Many giants
Old Ages (~13 Gyr)
No Blue M-S Stars
OpenClusters
• H‐RDiagramsofOpenClustersshow:– Theyareyoungtomiddle‐aged
– HaveblueMain‐Sequencestars– Fewsupergiantsorgiants– OlderOpenclustershavemoreredgiants
– Don’tseeahorizontalbranch– Youngests4llhavegascloudsassociated
GlobularClusters
• H‐RDiagramsofGlobularClustersshow:– Veryold:10-15BillionYears– RedturnoffsandnoblueMain‐Sequencestars– Lotsofredgiants– AprominentHorizontalBranch
– SlightlybluerandfainterMainSequenceduetohavinglessmetalsthannearbystars
TypicalGlobularClusterH‐RDiagram
40,000 20,000 10,000 5,000 2,500
106
104
102
1
10-2
10-4
Temperature (K)
Lum
inos
ity (L
sun)
Horizontal Branch Zero-Age
Main Sequence
ConclusionsoftheTests
• ClusterH‐RDiagramsgiveusasnapshotofstellarevolu4on.
• Observa4onsofclusterswithagesfromafewMillionto15Billionyearsconfirmsmuchofourpictureofstellarevolu4on.
• Remainingchallengesareinsmalldetails,butthebigpictureissecure.
Summary:
• H‐RDiagramsofStarClusters• AgesfromtheMain‐SequenceTurn‐off
• OpenClusters– Youngclustersoffew1000stars– BlueMain‐Sequencestars&fewgiants
• GlobularClusters– Oldclustersofafew100,000stars– NoblueMain‐Sequencestars&manygiants
Ques4ons:
• Whyaretheretwodifferenttypesofclusters?• Whatisthemaximumageseenforclusters?
• Arethereotherclusterproper4esthatscalewithage?
