1. PHYS390 (Astrophysics) Professor Lee Carkner Lecture 14
Stellar Evolution
PHYS390 (Astrophysics)
Professor Lee Carkner
Lecture 14

2. Questions
How does the size of the star change as it moves along the Hayashi track? Explain.
L decreases while T stays the same, the only way this can happen is if the size decreases
How does the size of the star change as it moves a long the Henyey track? Explain.
L stays the same while T increases, the only way this can happen is if the size decreases

3. Evolution
The greater and more rapid the core changes, the greater and more rapid the surface changes
When stars run out of hydrogen to burn in the core, they move off the main sequence
Become giants

4. Clusters
But we can observe a cluster of stars, all born at the same time
but, they will all be in different stages of evolution
High mass star evolve faster than low mass stars

5. Types of Clusters
Everything other than hydrogen and helium is called a metal
Z ~ 0.03 maximum
stars that have formed more recently have more metals
No metals =
First stars, not observed
Metal poor =
Metal rich = Population I
Found in the disk in open clusters

6. Cluster Diagrams Can identify the main sequence
Get a well determined distance to the cluster

7. Cluster Age
High mass stars use fuel up fastest and will leave the main sequence first
The star that is currently leaving the main sequence is called the turn off point
Main sequence lifetime of that star is the cluster age

8. Color-Magnitude Diagram
Make an HR diagram by plotting:
Brightness
Temperature or spectral type
Find the turn off point, look up the spectral type corresponding to its color, look up its main sequence lifetime
Can also look up its absolute visual magnitude and use to find distance

9. Questions Blue stragglers Hertzsprung gap
Some clusters show high mass stars that are “late” leaving the main sequence
Hertzsprung gap
Very few stars just above main sequence

10. Core Changes
When the core becomes largely He, H burning in the core stops
Shell burning produces a lot of energy
Some energy expands envelope
Becoming a giant

11. Isothermal Core Shonberg-Chandrasekhar limit (Mic/M) ~ 0.37(menv/mic)2
Called an isothermal core
Shonberg-Chandrasekhar limit
(Mic/M) ~ 0.37(menv/mic)2
Mass in the isothermal core can’t be greater than Mic or else the core cannot support the envelope

12. Degeneracy Exert degeneracy pressure Pe = Kr5/3
They can’t all be in the same state
Exert degeneracy pressure
Pe = Kr5/3
Many stars have partially degenerate cores at the end of the main sequence and thus can have a larger isothermal core than you would expect from the SC limit

13. High and Low Mass Stars Burn fuel slowly and are fully convective
Main sequence lifetimes very long
For some, longer than Hubble time
High mass stars have convective cores and so have some mixing

14. Next Time
Read 13.2
Homework: 13.6, 13.11, 13.13