Stellar Evolution

Astronomy 315Professor Lee

CarknerLecture 13

Changes

This is an illusion due to the fact that stellar lifetimes are very long

We can’t watch as any one star changes, so we have to examine different stars at different stages

Keys to Evolution Stars change properties as they go through their

lives The evolution of a star is based on two basic things:

The star will change so that it can get back into hydrostatic equilibrium

The mass of the star determines the evolutionary path it will follow

Fusion and Evolution

The outward force for a star is the thermal pressure

If the rate of energy generation in the

core changes it will change the point at which hydrostatic equilibrium occurs

Each star follows an evolutionary path on the HR diagram (as T and L change)

Pre-Main Sequence Evolution

H.E. is not reached again until fusion begins

Starts above the main sequence and moves down

The Main Sequence

As hydrogen is converted to helium the core gets a little denser and reactions speed up raising the luminosity

What happens when all the hydrogen is gone?

Post Main Sequence

The star then begins a series of other types of fusion reactions

Star leaves the main sequence and becomes a giant

Mass and Evolution

Few million years

Billions of years Main sequence lifetime (T) is inversely

proportional to mass:T = 1/M2.5

Cluster Evolution

All members of the cluster were born at

the same time but have different masses

High mass stars first

Cluster Evolution

Cluster Ages

The higher mass the stars the lower the age

The point at which the cluster diverges from the main sequence is called the turn-off point

The Pleiades

NGC 3293

Evolution of a

Cluster

Extrapolation If A0 stars live for 440 million years and F0 stars live for

3 billion year, how long do A3 stars live?

3X109 – 4.40X108 = 2.56X109

2.56X109/10 = 2.56X108

(2.56X108)(3) = 7.68X108

(4.40X108)(7.68X108) = 1.21X109 = 1.21 billion years

Post Main Sequence Evolution

Core becomes denser and contracts

Called the shell hydrogen burning Star burns from the inside out

Above the Main Sequence

This energy expands the outer layers of the star

The expansion cools the outer layers as well

The star moves up and right above the main sequence becoming a giant

Becoming a Giant

Structure of a Giant

Helium Burning

In some stars this happens very rapidly in a helium flash

Star becomes hotter and less luminous as the core readjusts

Burning Other Elements Helium burning happens very rapidly and

soon the core is full of carbon and oxygen

If the star is massive enough it will burn C and O into other elements

This is where everything heavier than He comes from

The End

Number of elements a star processes depends on mass

Elements end up in layers around the core

A star spends only about 10% of its life as a giant (for solar mass star about 1 billion years)

7) Main Sequence

8) Red Giant

9) Helium Flash Star rapidly

burns He in core

Evolution of a Solar Mass Star

10)Horizontal Branch

11)Asymptotic Giant Branch C and O core

contracts, He and H burns in shell, star expands and cools

Evolution of a Solar Mass Star

Which Way Does the Star Go? Up and to the

right (8-9, 10-11) Contracting core

and shell burning cause move to higher L, lower T

Down and to the left (9-10)

Next Time

Read Chapter 21.1-21.5