Stellar Evolution

Astrophysics Lesson 12

Learning ObjectivesTo know:- How stars form from clouds of dust and

gas.

How main sequence stars evolve as they run out of hydrogen.

How this evolution appears on the HR diagram.

Homework Remember to bring the completed

open book exam this Friday.

Some terms… Radiation pressure - the pressure

exerted upon any surface exposed to electromagnetic radiation.

Hydrogen “burning” – actually refers to the fusion of hydrogen into helium not reacting with oxygen.

Pillars of Creation

Pillars of Creation

Star Formation Stars are formed from

great clouds of gas and dust, most of which is the remnants from previous supernovae.

The denser clumps collapse slowly contract under the force of gravity.

Protostars Hydrogen Fusion

When the clumps get dense enough, the cloud fragments into regions called protostars that continue to contract and heat up.

Eventually the temperature at the centre of the protostar reaches a few million degrees and hydrogen nuclei start to fuse together to form helium.

Protostars in Orion I

Protostars in Orion II

Protostars in Orion III

Observe this with naked eye…

On the Main Sequence The fusion of hydrogen

releases enough energy to create enough radiation pressure to stop the gravitational collapse.

The star has now reached the main sequence and will remain there while it fuses hydrogen to helium.

Core hydrogen “burning”

Leaving the Main Sequence

Stars spend most of their lives as main sequence stars.

As the star ages more and more helium builds up in the core.

Eventually all the hydrogen is gone and you are left with a core of only helium.

Shell Hydrogen Burning When the hydrogen in its core

runs out, the outward radiation pressure stops, gravity wins and the core starts to contract.

As the core contracts it heats up. This raises the temperature of hydrogen surrounding the core enough for it to fuse.

This is shell hydrogen burning – very low mass stars stop here.

Red Giant The core continues to

contract until it is hot and dense enough for helium to fuse into carbon and oxygen.

Core helium “burning”.

This releases a huge amount of energy which pushes the outer layers of the star outwards which then cool.

Red Giant.

Shell Helium “Burning”.When the helium runs out, the carbon-

oxygen core contracts again shell helium “burning”.

For stars with mass similar to the of the Sun, the carbon-oxygen core isn’t hot enough for fusion.

The core continues to contract until electrons exert enough pressure to stop it collapsing further.

Ejecting the Outer Layers The helium shell becomes

more and more unstable as the core collapses.

This causes the star to pulsate and eject its outer layers into space

planetary nebula.

A hot dense solid core is left behind

white dwarf.

White Dwarf Black Dwarf

Within a million years the nebula fades and the core will simply continue to cool and finally the star is said to be dead.

This is the fate that awaits our Sun in about 5 billion years.

Animations…

Planetary Nebula I

Planetary Nebula II

Planetary Nebula III

The Whole Story

The HR Diagram Evolutionary Track for a 1

Mo star