Star Clusters

Stellar Evolution We 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: Mass Gravity Mass Gravity Of course, much of this is theory

How do we check our ideas? As always, we OBSERVE But 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.

How do we check our ideas? 1. Runner 2. Cyclist 3. Roller Blader 4. 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 started If 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? 1.Purple 2.Red 3.Green 4.White (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 Betelgeuse Mintaka Rigel Which star has reached the end of its evolutionary path? Betelgeuse, Mintaka or Rigel? It’s hard to say, since the stars in Orion are all different ages and distances it’s difficult to say which has been evolving the longest. Don’t confuse oldest with most evolved.

Star Clusters The best candidates for observing evolution are located in the Star Clusters These are groups of stars that are –close together in space –bound together by gravity –formed at the same time, from the same cloud They 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

Which 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.

M13 – Globular Cluster

47 Tucanae – Globular Cluster

Omega Centauri-Globular Cluster

NGC 4755 The Jewel Box – an Open Cluster

The Double Cluster Open Clusters

M7 – Ptolemy’s Cluster Open Cluster

M45 - Pleiades

M11 – The Wild Duck Cluster

Hyades The Hyades, in Taurus, is an open cluster. Let’s remove Aldebaran and just look at the cluster stars.

Comparisons TypeGlobularOpenAssociation Sizelargemediumsmall Diameter (Ly) less than Mass (M sun )10 4 – Number of Stars10 4 – Colorredred or blueblue Luminosity (L sun )10 4 – AgeVery oldOldyoung Number in Galaxy 's ExampleM13Jewel BoxPleiades

How 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.

Theoretical 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 Evolution

A cluster age of 10 Million years.

Testing 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.

Testing Stellar Evolution Cluster age = 1 billion years.

Testing Stellar Evolution Cluster age = 10 billion years.

The H-R diagram for NGC 2264, a nearby cluster. Estimated age is 1 million years. Testing Stellar Evolution ZAMS

The H-R diagram for the Pleiades Estimated age of 100 million years. Testing Stellar Evolution

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

H-R diagram for M13, a globular cluster. Estimated age is 13 billion years. Testing Stellar Evolution

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 Evolution