1. Chapter 13
Star Stuff

3. Evolution of Low-Mass Stars
1. The Sun began its life like all stars as an intersteller cloud.
2. This cloud collapses due to gravity into a dense core.
3. In about a million years a small, hot, dense core called a protostar forms.

7. Rotating Cloud Fragment
Jets of high speed gas
Proto-stellar disk
Protostellar Wind
Artist’s Conception of Star Birth

9. Schematic Illustration of protostellar disk-jet structure

10. 4. When the temperature reaches 10 million Kelvin in the core, fusion begins transforming the protostar into a main-sequence star.
5. Low mass stars like the Sun remain on the main-sequence for about 10 billion years. Massive stars stay on the main-sequence for about 1 billion years.

11. Life track of a 1M star from protostar to main-sequence star

12. Life tracks from protostar to main sequence for stars of different masses.

13. 6. After all of the Hydrogen is depleted in the core, fusion begins in a shell around the core and the star expands into a Red Giant.
7. After most of the hydrogen is fused into helium, helium fusion begins in the hotter core in an event called the Helium Flash.
8. Stars can then become unstable and turn into pulsating stars like RR Lyrae Variables or Cephied Variables.

15. After a star ends its main-sequence life, its inert helium core contracts while a hydrogen shell begins fusion at a higher rate. This forces the star’s outer layers to expand outward.

16. Core Structure of helium burning star.

18. The onset of helium fusion
The onset of helium fusion. When helium fusion begins, the star’s surface shrinks and heats. The star’s life track therefore moves downward and left on the H-R diagram

19. 9. As a star burns helium into carbon the radiation pressure pushes the star's outer atmosphere away from the core creating a second (double shell) red giant stage and then a Planetary Nebula.
Electron degeneracy pressure of the Carbon core halts any further collapse. Fusion process in the core stops.
10. This leaves an exposed core called a White Dwarf. These have about the same diameter as the Earth.

20. The life track of a 1M star from main-sequence to white dwarf.
Core structure at key stages

21. Gaseous shells surround the remnant carbon core

24. Evolution of High-Mass Stars
1 to 5. Same as before…
intersteller cloud  dense core 
 protostar  zero-age main-sequence star  main-sequence star
6. When a high-mass star exhausts the hydrogen fuel in its core the star leaves the main sequence and begins to burn helium.

25. 7. The star becomes a Red Supergiant after millions of years of helium fusion.
8. When helium is depleted, fusion of heavier elements begins. This process is called nucleosynthesis.
H  He  C  O  Si  Fe

26. Stellar Nucleosynthesis
Evolutionary Time Scales for a 15 M Star

27. The Multiple Layers Of Nuclear Burning In The Core Of A High Mass Star During Its Final Days

28. Life Tracks On The H-R Diagram From Main-sequence Star To Red Supergiant For Selected High Mass Stars

29. 9. Fusion stops with iron (Fe) and a star with an iron core is out of fuel.
Reason: Iron atoms cannot fuse and release energy.
10. The core collapses due to reduced pressure converting the iron core into mostly neutrons.
11. The core pressure then surges and lifts the outer layers from the star in a titanic explosion - a supernova!

30. Iron Core degenerates into a neutron core (neutron degeneracy).
Electrons and protons combine to form neutrons with the release of neutrinos
Average mass per nuclear particle from hydrogen to iron decreases and then increases for atomic masses greater than iron.

31. Origin Of The Elements- Stellar Nucleosynthesis
Observed Relative Abundances Of Elements In The Galaxy In Comparison To The Abundance Of Hydrogen.

34. Changing H-R diagram of a hypothetical star cluster.

35. The Double Cluster “h and  Persei
Only 10 million years old

36. Glodular cluster 47 Tucanae.
~ 11 billion years old

37. Evolution of a Binary Star System
Each star can be pictured as being surrounded by a “zone of influence” or Roche lobe.

38. What can happen?

39. What can happen?

40. The sun’s core will eventually become
Iron/Nickel
Carbon
Neutrons only
Electrons only

41. In about 5 billion years the Sun will become
A Black Hole
A Neutron Star
A Red Giant
A Red Dwarf

42. A High Mass Star Will Eventually Become
A White Dwarf
Nothing, as it has exploded.
A Red Dwarf
A Black Hole or Neutron Star

43. End of Chapter