Deaths of Stars

1. Deaths of Stars Low Mass Intermediate Mass High Mass

2. Recap • At the end of its main sequence life, the core of any star depletes its hydrogen supply. • Hydrogen burning continues in a shell around the core. • This layer burns fuel more rapidly than the core did. • The atmosphere expands to a huge radius and the star becomes a Red Giant. • The core shrinks and becomes more dense.

3. RGB Stars • During hydrogen shell burning the star moves up and to the right on the HR diagram. • We say that the star ascends the Red-giant Branch

4. The Helium Core • Eventually the core will get dense and hot enough to start burning helium to carbon and oxygen. • This will happen differently depending on the mass of the star • In more massive stars (2-3 solar masses) the helium burning begins gradually. • For low mass stars like the Sun, the helium turns on rapidly in a Helium Flash.

5. Horizontal Branch Stars • Stars will burn helium in their cores for a short time (about 1% of the time they spent on the main sequence) • At this time the star moves down and to the left in the HR diagram • It gets smaller, a little fainter, and hotter. • We call these Horizontal Branch Stars

6. Helium Shell Burning • Soon core helium is used up, leaving carbon and oxygen ashes behind. • Now there are two burning shells, helium and hydrogen. • Again the atmosphere expands and the star becomes a red giant for the second time.

7. AGB Stars • The second ascent is onto the Asymptotic Giant Branch. • It parallels the previous ascent. • But the star is larger and more luminous.

9. I. Low Mass Stars • If AGB stars have less than ~ 4 solar masses their cores will not get hot enough to fuse carbon or oxygen. • The shells grow thin and eventually become dormant. The core cools. • The dormant helium shell compresses and heats until it is hot enough to ignite again in a short-lived fusion outburst called a helium shell flash. • Bursts of energy from the helium shell flash are called thermal pulses.

10. Mass Loss • Thermal pulses eventually drive off the outer envelope in a series of ejections, exposing the very hot core! • UV radiation from the hot core ionizes the gas in the expanding atmosphere causing them to glow • This ionized shell of gas is called a Planetary Nebula. The Ring Nebula

11. Planetary Nebula • About 30,000 in the Milky Way alone. • Expanding at 10-30 km/s. • Can reach 1 lightyear in diameter • Can only exist for about 50,000 years before dispersing. The Dumbbell Nebula

12. Helix Nebula

14. HST Images of Planetary Nebula

15. M2-9 in Ophiuchus

16. Cat’s Eye Nebula

17. The Hourglass Nebula

18. What about the Core? • The exposed core is called a White Dwarf • About the size of the Earth • The star will continue to glow for millions of years as it cools down. • Eventually it will fade away, becoming a Black Dwarf. Dumbbell Nebula White Dwarf

20. Electron Degeneracy • If the core could shrink and compress like it did at the horizontal branch phase, it would eventually heat up and fuse the carbon and oxygen. • But for stars with masses less than 4 times solar a property called electron degeneracy kicks in. • The white dwarf acts like a giant atom with electrons in unique energy states. • This keeps the nuclei separated and the core cannot shrink further.

21. The Chandrasekhar Limit • Subrahmanyan Chandrasekhar, possibly the greatest theoretical astronomer ever, found an upper limit to the mass that could be supported by electron degeneracy. • That limit, known as the Chandrasekhar Limit, is 1.4 Solar Masses.

22. White Dwarf Masses vs Original Star Mass

23. II. High Mass Stars • If AGB stars have more than ~ 4 solar masses, their cores will get hot enough to fuse carbon and oxygen. • Stars with masses between 4 and ~8 times solar still end up as planetary nebulae/white dwarfs. • If AGB stars have more than ~ 8 solar masses, their cores will get hot enough to fuse elements up to iron • Stars with masses between 8 and ~15 times solar are called intermediate mass stars. They end as neutron stars. • Stars with masses greater than ~15 times solar are called high mass stars. They end as black holes. • (There is confusion in the books on these values and names…)

24. Fusion Stages • Carbon burning fuses carbon into oxygen, neon, sodium, and magnesium • Neon burning fuses neon into oxygen and magnesium • Oxygen burning produces sulfur and silicon • Silicon burning produces ironwhich does not burn.

25. J J J J J T T T T T T IG IG Elemental Abundances in Solar System Revisited…

26. Fusion Times for a 25 Solar Mass Star During these phases the star, a supergiant, drifts back and forth on the top of the HR diagram.

27. End of the Line • The iron core is held up by electron degeneracy pressure as noted before. • But with denser nuclei per atom, the core mass can exceed the Chandrasekhar limit. • When this happens, electron degeneracy pressure fails. • The electrons are driven into the nuclei, combining with protons, creating neutrons and neutrinos! • With no support pressure, gravity suddenly pulls all the material violently to the center.

28. Violent Death • The temperature jumps to 5 billion °K in less than 1/10th of a second, generating blackbody photons - mostly high energy gamma rays. • The gamma rays “photodisintegrate” the core turning the iron atoms into a soup of protons, neutrons, alpha particles, and electrons.

29. Nuclear changes • The electrons are pushed into the protons and alpha particles creating neutrons. • The neutrinos from this process leave in a pulse, taking support energy with them. • The core reassembles in ¼ second as a giant “atomic nucleus” of neutrons. • The neutron ball will not compress further and becomes rigid.

30. The Supernova Explosion • The collapse of the core creates a perfect vacuum outside of it. Material above this falls in toward the core at 15% of the speed of light! • When it reaches the rigid core it bounces back! • This explosive rebound blasts the material into space as a Supernova Explosion.

31. Supernova • The explosive energy brightens the star by a factor of 108 - about 20 magnitudes. • It outshines the entire galaxy for a few weeks. • Up to 96% of the star is blasted into space. • We call these Type II Supernovae. • (So, yes these is another way of blowing up a star, the Type Ia Supernovae).

32. Supernova 1987a, Feb 23 1987 • The only supernova close enough to study the progenitor star occurred in the Large Magellanic Cloud in 1987.

33. A Blue Supergiant • The progenitor star was found in an archival study by researchers at CWRU. • It didn’t match theory! • should have been a red supergiant. • was instead a B3 I – a blue giant! • Soon it was realized that evolved different because it was metal poor. It also probably lost outer material • Estimated to be 20 Solar Masses.

34. SN1987a Remnant

35. Neutrinos • Neutrino detectors on earth recorded a significant neutrino burst 3 hours before the light rise was detected. • It takes a while for the light to work its way out. Neutrinos fly out unimpeded. • This was a great test of supernova theory that fortunately worked!

36. Other Supernovae • Collapsing high mass stars are of types II, Ib, and Ic. • Types II have their outer envelopes intact. • Types Ib are missing hydrogen from the envelope but not helium. • Types Ic are missing both hydrogen and helium.

37. Type Ia • The second type of supernova, Type Ia, is formed in binary star systems where one star is a white dwarf and the other is an evolving red giant. • The red giant fills its Roche Lobe and dumps material onto its companion.

38. Smaller WDs  Nova • The pressure on the surface material builds until hydrogen fusion ignites in a nuclear explosion! • The entire layer is blasted into space. • The star brightens by 10,000 to 1,000,000 times! • Neither the white dwarf nor its companion are destroyed. So this process will repeat. • This is a Nova. GK Persei

39. Type Ia Supernovae • On larger white dwarfs, the pressure of the incoming material combined with the internal pressure causes the white dwarf core to suddenly begin to fuse carbon. • The process quickly runs away and a thermonuclear explosion occurs blasting the white dwarf to pieces.

40. Supernova Remnants

41. SN Remnant in LMC

42. Cassiopeia A

43. Supernovae are Useful • The universe started out as mostly hydrogen and helium. All heavier elements come from supernovae. • Elements up to iron are formed in massive star cores. • Most of that iron was destroyed before the supernova explosion. • So then where do all the heavier elements including iron come from?

44. Element Production • A supernova has • A tremendous amount of energy • a lot of neutrons running around • The outer shells of the core contain all elements lighter than iron. These are now targets for the neutrons. • These elements capture neutrons until they are swollen up to isotopes like 250Fe! Then they decay into copper, gold, lead, etc. - all the remaining elements in the periodic table.

45. From elements formed inside massive stars! • These elements drift through interstellar space creating dust! • Some will sweep together during star formation to form terrestrial-like planets!