Astro Mid-Term Sem. 2 (Book 3/7) Flashcards
Star Clusters: Evidence of Evolution
- What are the two kinds of clusters?
Open and Globular
- They are similar in the way their stars evolve
Star Clusters: Evidence of Evolution
- What are the two kinds of clusters?
Open and Globular
- They are similar in the way their stars evolve
What are Globular Clusters?
- Old clusters of a few 100,000 stars
- No blue Main-Sequence stars and many Giants
Testing Stellar Evolution: What is the Problem? What os the Solution?
The Problem:
- Stellar Evolution happens on a billion-year time scales.
- Astronomers only live for a few 10s of years
The Solution:
- Make H-R Diagrams of star clusters with wide range of ages
What are Star Clusters?
- How many planets?
- What similarities do ALL the stars in a cluster share?
Groups of 100s to 1000s of stars
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
Star Clusters: Evidence of Evolution
- What happens when a star clusters “freeze frames”?
It makes visible a moment in stellar evolution
- The differences you see among stars in one cluster must arise from differences in their masses
How is the Main-Sequence a Mass Sequence?
- Massive stars are Hotter and Brighter
- Low-mass stars are cooler and fainter
How does the Main-Sequence Lifetime depend on Mass?
- Massive stars have SHORT M-S lifetimes
- Low-mass stars have LONG M-S lifetimes
How does the Main-Sequence Lifetime depend on Mass?
- Massive stars have SHORT M-S lifetimes
- Low-mass stars have LONG M-S lifetimes (low-mass stars take longer to form)
How do Cluster H-R Diagrams indicate age?
- All stars arrived on the M-S at about the same time
- The cluster is as old as the most luminous (massive) star left on the M-S
- All M-S stars to the left have already used up their H fuel and are gone
- The position of the hottest, brightest star on a cluster’s M-S is called the M-S Turnoff Point
- M-S Turnoff Point of a cluster tells us its age
Progressive Evolution: What happens as the 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 super giants
- As successively lower mass stars run out of Hydrogen in their cores, they too evolve off
- Effect is that stars peel off the M-S from the top (high-mass end) down as the cluster ages
What happens when you compare clusters of different ages?
It allows you to visualize how stars evolve - almost as if you were watching a film of a star cluster evolving over billions of years.
What is the M-S Turn-Off? As the cluster ages what happens?
- It is the point where the M-S “turns off” towards giant stars
- As a cluster ages, the stars at the turn-off are lower mass. Low mass stars have redder colours
How is the colour of the turn-off an indicator of the clusters age?
OLDER clusters have REDDER turn-off points