Post-Main Sequence Evolution of Massive Stars

1. Post-Main Sequence Evolution of Massive Stars Stars of more than 8 solar masses leave behind neutron stars and black holes and typically explode as supernovae.

2. But first, NOVAE from WDs • Back to binary star evolution: • More massive star leaves MS first, becomes RG, then WD • Less massive star swells as it leaves MS, fills its Roche lobe • Mass then flows from companion star onto WD through inner Lagrangian point. • This mass forms an ACCRETION DISK around the WD • The mass stream hits the outer part of the accretion disk and makes a HOT SPOT

3. Accretion Disk in Close Binary w/ WD

4. Accretion Disks & Cataclysmic Variables Viscosity in the ACCRETION DISK (AD) causes its gas to: • lose its angular momentum and SPIRAL INTO THE WD; • as it does so, it gets very hot and EMITS ULTRAVIOLET RADIATION • often, more radiation comes from the disk than from the (very small) WD • INSTABILITIES in the AD can cause dramatic variations in rate of inflow • therefore, AD Luminosity varies a lot -- • CATACLYSMIC VARIABLES produced this way.

5. NOVAE • An explosion on the surface of a WD: Nova Herculis 1934 and typical light curve

6. What makes a Nova explode? As H gas from companion builds up on WD surface it gets hotter and denser • Eventually (typically after 103 or 104 yrs) it IGNITES • This THERMONUCLEAR DETONATION (of pp chains to He, mainly) • produces a huge burst of POWER -- this is a NOVA.

7. Novae and Recurrent Novae • Most of the gas is expelled in a rapidly expanding shell. • Luminosity rises between 5 and 12 magnitudes (100--63,000 times) in just a few days; • rapid decline over a couple of weeks is followed by slow decline to original low L in a few years. • While most of the accreted H is blasted off in nova explosions some (plus some created He) does remain on the WD surface and the WD's mass increases. • As long as mass continues to flow from the companion, many such explosions can occur -- RECURRENT NOVAE

8. Ejected Shells: Novae Persei & Cygni

9. Thought Question What happens to a white dwarf when it accretes enough matter to reach the 1.4 MSun limit? A. It explodes B. It collapses into a neutron star C. It gradually begins fusing carbon in its core

10. Thought Question What happens to a white dwarf when it accretes enough matter to reach the 1.4 MSun limit? A. It explodes B. It collapses into a neutron star C. It gradually begins fusing carbon in its core

11. Life Stages of High-Mass Stars • Late life stages of high-mass stars are similar to those of low-mass stars: • Hydrogen core fusion (main sequence) • Hydrogen shell burning (supergiant) • Helium core fusion (supergiant)

12. How do high-mass stars make the elements necessary for life?

13. Big Bang made 75% H, 25% He – stars make everything else

14. Helium fusion can make carbon in low-mass stars

15. CNO cycle can change C into N and O

16. Helium Capture • High core temperatures allow helium to fuse with heavier elements to make Oxygen, Neon, Magnesium, etc.

17. Helium capture builds C into O, Ne, Mg, …

18. Massive Stars Cook Heavy Elements • After MS, H shell burning, He core burning, He shell burning, stars w/ M>8M have so much gravity that • the C core crushed until it reaches T > 7 x 108 K so • Carbon can also fuse • 12C + 4He  16O +  some • 16O + 4He  20Ne +  also some • 12C + 12C  24Mg +  • These fuels produce less energy per mass, so each is burned up faster and faster • Most will fuse Oxygen and Neon too: • 16O + 16O  32S +  20Ne + 4He  24Mg + 

19. Multiple Shell Burning • Advanced nuclear burning proceeds in a series of nested shells • High Mass Star Evolution

20. Element Formation & Abundances The more common heavy elements have an even number of protons: built up by 4He nuclei (alpha process) • H and He alone were made in the BIG BANG. • All other elements (up to iron) are made in PRE-SUPERNOVAE stars. • Anything heavier than Fe (unstable) made in Supernovae Fe is endpoint of fusion; it has the minimum mass per nucleon; energy would be absorbed -- not given off -- to go to heavier nuclei

21. Alpha Process Builds Middle Elements

22. Elemental Abundances: H Rules!

23. How Does a High-Mass Star Die?

24. Approach to Supernova • The iron core collapses: excess neutrons build up • 16O + 16O  31S + n • Si fusion up to Fe takes < 1 day to complete! • Mass of Fe core grows: exceeds Chandrasekhar mass • Yielding CORE COLLAPSE • Key details: high energy photons are absorbed, causing a pressure drop:photodisintegration! • 56Fe +   13(4He) + 4 n • 4He +   2p + 2n • Only when density > 109g/cm3 (500 cars/teaspoon) • and T > 5x109K

25. Neutronization and Collapse • This PHOTODISINTEGRATION occurs in < 0.1 second! • Further NEUTRONIZATION (production of excess neutrons) occurs when electrons are crushed into protons: • p + e  n +  (weak nuclear reaction) • Atoms disappear and become nuclear matter, with density about 4 x 1014 g/cm3 ! • The core collapses! • (Whole sun into a city size -- a billion tons/teaspoon!)

26. Supernova Explosion • Core electron degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos • Neutrons collapse to the center, forming a neutron star

27. Core Collapse, continued • Once NEUTRONIZATION is nearly complete, • the core collapse is halted by a combination of NEUTRON DEGENERACY PRESSURE • and the REPULSIVE PART OF THE STRONG NUCLEAR FORCE. • The core, with radius about 10 km, becomes a NEUTRON STAR (NS -- to which we'll return shortly)

28. FORMATION of a TYPE II SUPERNOVA • Ca, Si, S, Mg, Ne, O, C layers continue to burn and collapse onto the NS core. • BUT huge NEUTRINO PRESSURES build up, and, in addition, the NS is so “stiff” that matter hitting it BOUNCES from a SHOCK. • Bounce works better if star is rotating (and has big magnetic fields) • EXPLOSIVE NUCLEOSYNTHESIS produces ELEMENTS HEAVIER THAN IRON • Also helps BLAST OFF MOST OF THE STAR'S ENVELOPE. • This rapidly expanding star gets very luminous, very fast since the radius is so big: A SUPERNOVA

29. Energy and neutrons released in supernova explosion enable elements heavier than iron to form, including Au (gold) and U

30. Type II (massive star) SN formation, illustrated

31. PROPERTIES of TYPE II SUPERNOVAE Luminosities equal more than that of 109 ordinary stars for a few days, while at peak power • Timescales: rise, 1 day; peak, 1 week; hump, 2 to 3 months; slow decline, 2 years (powered by 56Co).

32. More SN Type II Properties Most of the star's mass is EJECTED at velocities from 10,000-30,000 km/s (3-10% of the speed of light!!!). • Spectra are rich in H lines: much hydrogen expelled Crab nebula photos 14 years apart don’t line up; Illustrates fast outward motion

33. Supernova Remnants (bright nebulae) • The expelled gas interacts with the ISM to make a SUPERNOVA REMNANT, a BRIGHT NEBULA which glows for ~ 105 years: Crab Nebula Views • CRAB SN was seen in 1054 CE and its expanding SNR is beautiful now: Crab Nebula Movie

34. SN 1987A • The nearest, recent Type II SN in the Large Magellanic Cloud (50 kpc away) • Neutrinos (17) were detected; only ones not from the Sun: great confirmation of massive star evolution theory!

35. Rings Around Supernova 1987A • The supernova’s flash of light caused rings of gas (ejected from star earlier) around the supernova to glow

36. Impact of Debris with Rings • More recent observations are showing the inner ring light up as debris crashes into it

37. Type I (White Dwarf) Supernovae • Type I SN are usually even more luminous: • peak M = -19 This is about as luminous as an ENTIRE GALAXY! • Very close to a STANDARD CANDLE, I.e, all Type Ia (WD) SNe are nearly equally bright • Rise in ~1 day; fast decline, months; slow decline, over years • Spectra are devoid of H lines (no H envelope) • Often found in Pop II (low metalicity) regions, while Type II SN are associated only with Pop I (composition like sun) stars.

38. Type Ia SN formation, illustrated WD driven over the Chandrasekhar limit thanks to accretion in a binary system

39. Origin of Type I SNe • Majority, at least, arise from WDs in binary systems (Type Ia). • If WD starts out massive and close to Chandrasekhar limit, then if it accretes much mass from companion it can be pushed over the maximum mass electron degeneracy pressure can support. • This usually leads to a collapse and immediate DETONATION EXPLOSION, which usually COMPLETELY DISRUPTS the star (i.e. no NS left behind). • Some Type I SNe may have a WD core collapse to a Neutron Star and most (but not all) gas expelled. • Exploding layers shine even brighter since they don't have to push out overlying mass that high mass (Type II) SN have. No envelope also explains the missing H lines.

40. Explosive Nucleosynthesis • Nearly all elements heavier than iron are produced in SN explosions • The s-process (slow) adds neutrons to build elements via intermediate decays (AGB stars & SN): • 56Fe+n57Fe; 57Fe+n58Fe; 58Fe+n59Fe • These neutron-rich isotopes decay to elements with more protons that are stable: 59Fe59Co+e-+ • The r-process adds lots of neutrons very fast to produce the heaviest stable (and unstable) elements (those above Bismuth): only during SN explosions • These elements then pollute (enrich?) the ISM • Power for SN light curves comes from Ni-56 and Co-56 decays

41. Role of Mass • A star’s mass determines its entire life story because it determines its core temperature • High-mass stars with >8MSun have short lives, eventually becoming hot enough to make iron, and end in supernova explosions • Low-mass stars with <2MSun have long lives, never become hot enough to fuse carbon nuclei, and end as white dwarfs • Intermediate mass stars can make elements heavier than carbon but end as white dwarfs

42. Low-Mass Star Summary Main Sequence: H fuses to He in core Red Giant: H fuses to He in shell around He core Helium Core Burning: He fuses to C in core while H fuses to He in shell Double Shell Burning: H and He both fuse in shells 5. Planetary Nebula leaves white dwarf behind Not to scale!

43. Reasons for Life Stages • Core shrinks and heats until it’s hot enough for fusion • Nuclei with larger charges require higher temperatures for fusion • Core thermostat is broken while core is not hot enough for fusion (shell burning) • Core fusion can’t happen if degeneracy pressure keeps core from shrinking Not to scale!

44. Life Stages of High-Mass Star Main Sequence: H fuses to He in core Red Supergiant: H fuses to He in shell around He core Helium Core Burning: He fuses to C in core while H fuses to He in shell Multiple Shell Burning: Many elements fuse in shells 5. Supernova (Type II) leaves neutron star behind Not to scale!

45. Good Stars! (They Recycle) • ISM • Star formation • Stellar Evolution • Explosions and enrichment of the • ISM (do again!). • About 3 solar masses / year are recycled in the Milky Way