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:

Stellar Evolution happens on billion-year time scales.
Astronomers only live for a few 10s of years.

The Solution:

Make H-R Diagrams of star clusters with a wide range of ages.

Star Clusters

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

All stars in a cluster

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.

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:

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.

Progressive Evolution

As a cluster ages:

Start with high-mass stars on the M-S, and low-mass stars still approaching.
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 stars.

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

Open 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 (300k jpeg)

Credit: Sven Kohle & Till Credner, image from Calar Alto.

Globular Clusters:

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 (422K jpeg)

Credit: Space Telescope Science Institute

Open Clusters

H-R Diagrams of Open Clusters show:

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 for 2 open clusters (The Pleiades & Praesepe) with different ages (7Kb GIF)]

Globular Clusters

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

Conclusions

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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