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Galaxy NGC4414 from HST Astronomy 162:
Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30

Lecture 21: Testing Stellar Evolution

Readings: Ch 21, section 21-3

Key Ideas

H-R Diagrams of Star Clusters

Ages from the Main-Sequence Turn-off

Open Clusters
Young clusters of few 1000 stars
Blue Main-Sequence stars & few giants

Globular Clusters
Old clusters of a few 100,000 stars
No blue Main-Sequence stars & many giants

Testing Stellar Evolution

The Problem:

The Solution:

Star Clusters

Groups of 100's to 1000's of stars moving together through space.

All stars in a cluster

Each cluster thus provides a snapshot of what stars of different masses look like at the same age (and composition)!

The Main Sequence, Revisited

The properties of stars along the Main Sequence give us clues as to how to "read" a cluster H-R diagram.

As we have seen in previous lectures on the Main Sequence and Star Formation:

  1. The Main Sequence is a Mass Sequence:

  2. A star's Main-Sequence Lifetime depends on Mass:

  3. Low-Mass stars take longer to form than High-Mass stars.

Progressive Evolution

As a cluster ages:

Effect is that stars peel off the Main Sequence from the top (high-mass end) down as the cluster ages from 1 Myr to 10 Gyr.

H-R Diagrams of a Simulated Star Cluster

This simulated star cluster has 30,000 stars with masses from 0.3 to 30Msun. Stellar evolution calculations from the MIST v1.1 database were used.

The plots below show snapshots of the cluster at 1 Myr through 10 Gyr. The green line in each shows the location of the zero-age main sequences (ZAMS).

H-R Diagram, T=1 Myr H-R Diagram, T=10 Myr
H-R Diagram, T=100 Myr H-R Diagram, T=1 Gyr
H-R Diagram, T=10 Gyr 
Click on an individual H-R diagram above to view the full-resolution version.

Watch this animation of the full cluster simulation showing the evolution of the H-R diagram from an age of 100 kyr to 19 Gyrs (YouTube video):

Main Sequence Turn-off

Point where the Main Sequence "turns off" towards giant stars.

Color of the turn-off is an indicator of the cluster age:

Types of Clusters

Open Clusters:
Image of the young Open Cluster NGC 2266 (300k jpeg)
Credit: Sven Kohle & Till Credner, image from Calar Alto.

Globular Clusters:

Image of the Globular Cluster M80 (422K jpeg)
Credit: Space Telescope Science Institute

Open Clusters

H-R Diagrams of Open Clusters show: [H-R Diagrams for 2 open clusters (The Pleiades & Praesepe) with different ages (7Kb GIF)]

Globular Clusters

H-R Diagrams of Globular Clusters show:
Globular Cluster H-R Diagram


Cluster H-R Diagrams give us a snapshot of stellar evolution.

Observations of clusters with ages from a few Million to 15 Billion years confirms much of our picture of stellar evolution.

Piecing together the story of Stellar Evolution has been a triumph of observational and theoretical work. We still have many questions to answer, but the big picture is secure.

Return to [ Unit 3 Index | Astronomy 162 Main Page ]
Updated: 2017 October 17
Copyright © Richard W. Pogge, All Rights Reserved.