Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30
Lecture 21: Testing Stellar Evolution
Readings: Ch 21, section 21-3
- 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
- Stellar Evolution happens on billion-year time scales.
- Astronomers only live for a few 10s of years.
- Make H-R Diagrams of star clusters with a wide range of ages.
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)!
- are at the same distance, so it is easy to measure
their relative Luminosities,
- have the same age,
- have the same chemical composition,
- have a wide range of stellar masses.
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
- The Main Sequence is a Mass Sequence:
- Massive stars are hot and high luminosity.
- Low-mass stars are cool and low luminosity.
- A star's Main-Sequence Lifetime depends on Mass:
- Massive stars have short M-S lifetimes
- Low-mass stars have long M-S lifetimes.
- Low-Mass stars take longer to form than High-Mass stars.
As a cluster ages:
- Start with high-mass stars on the M-S, and low-mass stars
- High mass run out of hydrogen in their cores first, evolving
off into supergiants.
- As successively lower mass stars run out of hydrogen in their
cores, they too evolve off.
Effect is that stars peel off the Main Sequence from the top (high-mass end)
down as the cluster ages.
H-R Diagrams of a Model Star Cluster
Main Sequence Turn-off
Point where the Main Sequence "turns off" towards giant
- As cluster ages, the stars at the turn-off are lower mass
- Low mass stars have redder colors.
Color of the turn-off is an indicator of the cluster age:
- Older Clusters have redder and fainter turn-offs.
Types of Clusters
- Sparse clusters (few 100s - 1000s of stars)
- few parsecs in diameter
- Many blue Main-Sequence stars
- A few Giants
- Relatively Young ages (100's of Myr)
- Image of the young Open Cluster NGC 2266
- Credit: Sven Kohle & Till Credner, image from Calar Alto.
- Rich spherical clusters (105-106 stars)
- 10-30 parsecs in diameter
- No blue Main-Sequence stars
- Many Giants
- Old Ages (few Gyr)
- Image of the Globular Cluster M80
- Credit: Space Telescope Science
H-R Diagrams of Open Clusters show:
[H-R Diagrams for 2 open clusters (The Pleiades &
Praesepe) with different ages (7Kb GIF)]
- They are young to middle-aged
- Have blue Main-Sequence stars
- Few supergiants or giants
- Old Open clusters have more red giant stars
- Don't see Horizontal Branch stars
- Youngest Open Clusters still have gas clouds associated with them
H-R Diagrams of Globular Clusters show:
- Very old ages: 10-13 Billion Years
- Red turnoffs and no blue Main-Sequence stars
- Many Red Giants but no Supergiants
- A prominent Horizontal Branch
- Slightly bluer and fainter Main Sequence due
to having less metals than nearby stars
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.
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Updated: 2006 January 29
Copyright © Richard W. Pogge, All Rights Reserved.