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

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

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

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 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

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

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