Chapter 20Chapter 20

Stellar EvolutionStellar Evolution(Transparencies)(Transparencies)

Evolution of Low-Mass StarsEvolution of Low-Mass Stars1. 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.

ANNOUNCMENT

This Friday (April 12, 2002) we will meet in Kennedy Auditorium again.

Please remind anyone who is not here today.

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!

Nucleosynthesis

Fused Products Time Temperature H 4He 107 yrs. 4 X 106 K

4He 12C Few X 106 yrs 1 X 108 K 12C 16O, 20Ne,

24Mg, 4He 1000 yrs. 6 X 108 K

20Ne + 16O, 24Mg Few yrs. 1 X 109 K 16O 28Si, 32S One year 2 X 109 K

28Si + 56Fe Days 3 X 109 K 56Fe Neutrons < 1 second > 3 X 109 K

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 SystemEach star can be pictured as being surrounded by a “zone of

influence” or Roche lobe.

What can happen?

What can happen?

H-R Diagram Questions1. 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 Questions9. 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-

sequence stars?

15. Where are the oldest stars?

16. Which stars along the main-sequence

live the longest?

End of Chapter 20