1. Ch 12--Life Death of Stars
9 Nov 2000
ASTR103, GMU, Dr. Correll

2. What do you think? Will the Sun end its existence? If so, how?
What is a nova?
What is the origin of the carbon, silicon, oxygen, iron, uranium, and other heavy elements on Earth?
What is a cosmic ray?
What is a pulsar?
9 Nov 2000
ASTR103, GMU, Dr. Correll

3. Low-Mass Stars Post Main Sequence hydrogen fusion shell
helium flash/helium fusion core
helium fusion shell (asymptotic giant branch)
9 Nov 2000
ASTR103, GMU, Dr. Correll

4. Post Main Sequence Giants
Core and interior shells condense giving off more energy causing the outer layers to expand and cool
9 Nov 2000
ASTR103, GMU, Dr. Correll

5. Planetary Nebula
Outer atmosphere of stars is ejected and the surrounding could of gas is called a planetary nebula
has nothing to do with planet formation
a wide variety of planetary nebula are seen!
9 Nov 2000
ASTR103, GMU, Dr. Correll

6. Bipolar Planetary Nebula
The Hourglass nebula about the star Eta Carinae is an example of a bipolar planetary nebula
about 8000 ly away
around a massive (~30Msun star)
9 Nov 2000
ASTR103, GMU, Dr. Correll

7. Remains of the Low-Mass Star
After outer layers ejected in planetary nebula and solar wind, all that remains is the hot, dense carbon-oxygen core--a white dwarf!
Fusion has ceased, and core cools down over billions of years
White dwarf mass < 1.4 Msun due to Chandrasekhar limit
Degenerate electrons support only this much mass before combining with protons to form neutrons
9 Nov 2000
ASTR103, GMU, Dr. Correll

8. Novas Consider what happens to a white dwarf in a binary system
As the larger star evolves and expands some of its outer layers fall onto the white dwarf companion
What happens when the hydrogen layer becomes dense enough?
Hydrogen flash of fusion--a Nova occurs
9 Nov 2000
ASTR103, GMU, Dr. Correll

9. High-Mass Stars Higher mass enables fusion of heavier elements
each successive shell burns in a shorter time
carbon for about 600 yrs
neon for about 1 yr
oxygen for about a month
silicon fusion for a day
final inert core is iron!
A star the size of Jupiter’s orbit is powered by a core the size of the Earth!
9 Nov 2000
ASTR103, GMU, Dr. Correll

10. High-Mass Stars
9 Nov 2000
ASTR103, GMU, Dr. Correll

11. Death of Massive Stars
As iron core grows, the immense weight is supported by electron degeneracy pressure--ie, electron gas
Eventually the electron gas is compressed to the point that the electron degeneracy pressure is overcome by gravity--electrons combine with protons forming neutrons and emitting gamma rays and neutrinos
As neutrons form the core collapses to form a neutron core. A rebounding shockwave creates a powerful explosion blowing off outer layers of the star --Supernova!
9 Nov 2000
ASTR103, GMU, Dr. Correll

12. Death of Massive Stars
9 Nov 2000
ASTR103, GMU, Dr. Correll

13. Supernova 1987A
9 Nov 2000
ASTR103, GMU, Dr. Correll

14. Supernova 1987A
9 Nov 2000
ASTR103, GMU, Dr. Correll

15. Supernova Types
Type Ia--begins as a carbon-oxygen rich white dwarf in a close binary system
giant companion enlarges, depositing gas onto white dwarf
as mass approaches the Chandrasekhar limit, carbon fusion ignites in the core
nuclear powered
9 Nov 2000
ASTR103, GMU, Dr. Correll

16. Supernova Types Type II--core collapse of massive stars
mass of core greater than Chandrasekhar limit
gravity overwhelms electron degeneracy pressure
core falls into the center
gravitational energy
Outer gas layers give many hydrogen lines in spectrum
9 Nov 2000
ASTR103, GMU, Dr. Correll

17. Neutron Stars What’s left after a type II supernova
for stars of mass of 8-25 Msun, when core collapses and electron combine with protons, we are left with a neutron core--a neutron star!
Core about km in diameter!
Gas layers completely blown away
Star now supported by neutron degeneracy pressure
9 Nov 2000
ASTR103, GMU, Dr. Correll

18. Neutron Stars Stars naturally seem to have magnetic fields.
What happens to the magnetic fields when stellar cores collapse?
9 Nov 2000
ASTR103, GMU, Dr. Correll

19. Pulsars
The magnetic field collapses also, creating a very high energy density electric dynamo
polar axis of magnetic field not always in line with rotation axis
field energizes and beam out charged particle
“lighthouse” model
9 Nov 2000
ASTR103, GMU, Dr. Correll

20. Pulsars We observe pulsars via radio waves 9 Nov 2000
ASTR103, GMU, Dr. Correll

21. Pulsars--SS433 SS433--a pulsar fed by a close binary companion
9 Nov 2000
ASTR103, GMU, Dr. Correll

22. X-Ray Bursters
Analogous to hydrogen flashes on white dwarves (novas), on a neutron star, the hydrogen slowly fuses into helium and then a helium flash can occur
rather than peaking in the optical range, helium flashes are hotter and in the x-ray range
x-ray burster
9 Nov 2000
ASTR103, GMU, Dr. Correll

23. Stellar Evolution Summary
9 Nov 2000
ASTR103, GMU, Dr. Correll

24. Stellar Evolution Summary
9 Nov 2000
ASTR103, GMU, Dr. Correll

25. What do you think? Will the Sun end its existence? If so, how?
The Sun will shed its outer layers as a planetary nebula in about 7 billions years. Its remnant white dwarf, with fusion ceased, will dim over the next several billion years
What is a nova?
A relatively gentle explosion of hydrogen gas on the surface of a white dwarf
What is the origin of the carbon, silicon, oxygen, iron, uranium, and other heavy elements on Earth?
These elements were generated during stellar evolution of massive stars and in supernova explosions
What is a cosmic ray?
High energy (speed) particles accelerated in supernova explosions
What is a pulsar?
A rotating neutron star with a magnetic field creating a “lighthouse” of radiation which sweep across space
9 Nov 2000
ASTR103, GMU, Dr. Correll

26. Questions for Thought
Describe the evolution of main sequence star with a mass of 20 Msun. Describe the various phases of energy generation, migration off the main sequence, and the eventual fate of the star to include what forces are at balance throughout.
9 Nov 2000
ASTR103, GMU, Dr. Correll