Star Clusters

Star Clusters

1Stellar EvolutionWe have spent considerable time in understanding the evolution of stars.At this point you should have realized that there are only 3 properties and 3 forces that are all-important:The properties:The forces:

MassGravityMassMassGravityGravityOf course, much of this is theory2How do we check our ideas?As always, we OBSERVEBut stars stay on the Main Sequence for most of their lives. It's hard to find the ones who are in a brief, transitory phase.We need to find a sample where the stars were born at nearly the same time and are at nearly the same distance away.

3

How do we check our ideas? Runner Cyclist Roller Blader Can't tell

How do we check our ideas?We can't tell who the best athlete is because we don't know where and when they startedIf they all started together in Philadelphia, we would have to think the runner is the best. On the other hand, perhaps the runner started from the marina several hours before the photo was taken.How do we check our ideas?

PurpleRedGreenWhite(Probably not, this is a Beer Garden)How do we check our ideas?

Since this is a Triathelon, and all triathletes start at the same place and the same time, the first one out of the water must be the best athlete.Observing Evolution

BetelgeuseMintakaRigelWhich star has reached the end of its evolutionary path?Betelgeuse, Mintaka or Rigel?Its hard to say, since the stars in Orion are all different ages and distances its difficult to say which has been evolving the longest.Dont confuse oldest with most evolved. 8Star ClustersThe best candidates for observing evolution are located in the Star ClustersThese are groups of stars that areclose together in spacebound together by gravityformed at the same time, from the same cloudThey contain stars of all different masses so we can see the end point of evolution in the massive ones, the beginning in the tiny ones and all variations in between

9Which Stars are most evolved?

The most evolved stars are the red ones. These are the Red Giants which have left the main sequence.

The blue and yellow are main sequence stars. Since all the stars are formed at the same time, the more massive have evolved faster. Thus a yellow G2 of the same age is less evolved than an M2 Red Giant. 10M13 Globular Cluster

1147 Tucanae Globular Cluster

12Omega Centauri-Globular Cluster

13NGC 4755

The Jewel Box an Open Cluster14The Double Cluster

Open Clusters15M7 Ptolemys ClusterOpen Cluster

16M45 - Pleiades

17M11 The Wild Duck Cluster

18Hyades

The Hyades, in Taurus, is an open cluster.

Lets remove Aldebaran and just look at the cluster stars. 19ComparisonsTypeGlobularOpenAssociationSizelargemediumsmallDiameter (Ly)50 - 300less than 30100 - 500Mass (Msun)104 108100 - 1000100 - 1000Number of Stars104 10850 - 10005 - 50Colorredred or blueblueLuminosity (Lsun)104 108102 - 106104 - 107AgeVery oldOldyoungNumber in Galaxy1501000's1000'sExampleM13Jewel BoxPleiades20How can we test stellar evolution models?H-R diagram for a group of stars all born at the same time: hot, massive stars evolve the most rapidly.

Over time, main sequence stars of progressively lower temps/masses peel away to the giant regions on the diagram.

21Theoretical H-R diagram for a star cluster with an age of 1 Million years. The red line is the ZAMS. Note that the lower mass stars are still evolving toward the MS, while some high-mass stars have already evolved off the MS.

Testing Stellar Evolution22Testing Stellar Evolution

A cluster age of 10 Million years.23Testing Stellar Evolution

Cluster age = 100 Million years. All lower mass stars have reached the MS, but the stars along the upper half of the MS have all ended their lives.24Testing Stellar Evolution

Cluster age = 1 billion years.25Testing Stellar Evolution

Cluster age = 10 billion years.26The H-R diagram for NGC 2264, a nearby cluster.

Estimated age is 1 million years.Testing Stellar Evolution

ZAMS27The H-R diagram for the Pleiades

Estimated age of 100 million years.Testing Stellar Evolution

28The H-R diagram for M 67, a cluster with an estimated age of 4 billion years.Testing Stellar Evolution

29H-R diagram for M13, a globular cluster.

Estimated age is 13 billion years.Testing Stellar Evolution

30

H-R diagrams of star clusters verify our models of stellar evolution.

We can then use the locations of cluster turn-off points to determine the ages of clusters.Testing Stellar Evolution31