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