Astro Mid-Term Sem. 2 (Book 3/7) Flashcards

1
Q

Star Clusters: Evidence of Evolution

- What are the two kinds of clusters?

A

Open and Globular

- They are similar in the way their stars evolve

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2
Q

Star Clusters: Evidence of Evolution

- What are the two kinds of clusters?

A

Open and Globular

- They are similar in the way their stars evolve

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3
Q

What are Globular Clusters?

A
  • Old clusters of a few 100,000 stars

- No blue Main-Sequence stars and many Giants

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4
Q

Testing Stellar Evolution: What is the Problem? What os the Solution?

A

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

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5
Q

What are Star Clusters?

  • How many planets?
  • What similarities do ALL the stars in a cluster share?
A

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

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6
Q

Star Clusters: Evidence of Evolution

- What happens when a star clusters “freeze frames”?

A

It makes visible a moment in stellar evolution

- The differences you see among stars in one cluster must arise from differences in their masses

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7
Q

How is the Main-Sequence a Mass Sequence?

A
  • Massive stars are Hotter and Brighter

- Low-mass stars are cooler and fainter

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8
Q

How does the Main-Sequence Lifetime depend on Mass?

A
  • Massive stars have SHORT M-S lifetimes

- Low-mass stars have LONG M-S lifetimes

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9
Q

How does the Main-Sequence Lifetime depend on Mass?

A
  • Massive stars have SHORT M-S lifetimes

- Low-mass stars have LONG M-S lifetimes (low-mass stars take longer to form)

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10
Q

How do Cluster H-R Diagrams indicate age?

A
  • 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
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11
Q

Progressive Evolution: What happens as the cluster ages?

A
  • 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
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12
Q

What happens when you compare clusters of different ages?

A

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.

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13
Q

What is the M-S Turn-Off? As the cluster ages what happens?

A
  • 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
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14
Q

How is the colour of the turn-off an indicator of the clusters age?

A

OLDER clusters have REDDER turn-off points

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