Stellar Evolution (Transparencies) Chapter 20 Stellar Evolution (Transparencies)

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.

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4. When the temperature reaches 10 million Kelvin in the core, fusion begins and transforms 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.

6. Hydrogen 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 an event called the Helium Flash. 8. Stars can then become unstable and turn into pulsating stars like RR Lyrae Variables or Cephied Variables.

9. As a star burns helium into carbon the radiation pressure pushes the star's outer atmosphere away from the core creating a Planetary Nebula. 10. This leaves an exposed core called a White Dwarf. These have about the same diameter as the Earth.

Evolution of High-Mass Stars 1 to 5. Same as before… intersteller cloud  dense core   protostar  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.

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

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!

11. The core pressure then surges and lifts the outer layers from the star in a titanic explosion - a supernova!

Evolutionary Time Scales for a 15 M Star Nucleosynthesis Evolutionary Time Scales for a 15 M Star

Changing H-R diagram of a hypothetical star cluster.

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

Glodular cluster 47 Tucanae. ~ 11 billion years old

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

What can happen?

What can happen?

H-R Diagram Questions 1. What property is measured along the horizontal axis? 2. … along the vertical axis? 3. Where are the red giants? 4. … the white dwarfs? 5. … the hottest stars? 6. … the coolest stars? 7. … the largest stars? 8. … the smallest stars?

H-R Diagram Questions 9. Where are O class stars? 10. … M class stars? 11. … G class stars? 12. Where is the Sun? 13. Where are the high-mass main- sequence stars? 14. Where are the low-mass main- 15. Where are the oldest stars? 16. Which stars along the main-sequence live the longest?

End of Chapter 20