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

Lecture 21: Testing Stellar Evolution

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

• 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

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.

The Main Sequence, Revisited

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

1. The Main Sequence is a Mass Sequence:
   • Massive stars are hot and high luminosity.
   • Low-mass stars are cool and low luminosity.

2. A star's Main-Sequence Lifetime depends on Mass:
   • Massive stars have short M-S lifetimes
   • Low-mass stars have long M-S lifetimes.

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

Progressive Evolution

• 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 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 30M_sun. 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).

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.

• 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 (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

• 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

Globular Clusters

• 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

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.