Lecture 16 Clusters of Stars - Texas A&M Universitypeople.physics.tamu.edu/depoy/astr101TR/Notes/lecture16.pdf · stars reach the M‐S ... the low‐ mass stars sll approaching.

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?