Post on 18-Dec-2015
Stellar Evolution
PHYS390 (Astrophysics)
Professor Lee Carkner
Lecture 14
Questions1) 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
2) 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
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
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
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
Cluster Diagrams
Can identify the main sequence
Get a well determined distance to the cluster
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
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
Questions
Blue stragglers Some clusters show high
mass stars that are “late” leaving the main sequence
1)
Hertzsprung gap Very few stars just above
main sequence2)
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
Isothermal Core
Called an isothermal core
Shonberg-Chandrasekhar limit
(Mic/M) ~ 0.37(env/ic)2
Mass in the isothermal core can’t be greater than Mic or else the core cannot support the envelope
Degeneracy
They can’t all be in the same state
Exert degeneracy pressurePe = K5/3
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
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
Next Time
Read 13.2 Homework: 13.6, 13.11, 13.13