1. Stellar Evolution Astrophysics Lesson 12

2. Learning Objectives To 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.

3. Homework  Remember to bring the completed open book exam this Friday.

4. 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.

5. Pillars of Creation

7. 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.

8. 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.

9. Protostars in Orion I

10. Protostars in Orion II

11. Protostars in Orion III

12. Observe this with naked eye…

13. 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”

14. 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.

15. 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.

16. 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.

17. 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.

18. 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.

19. 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…

20. Planetary Nebula I

21. Planetary Nebula II

22. Planetary Nebula III

23. The Whole Story

24. The HR Diagram Evolutionary Track for a 1 M o star