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.

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

Accretion Disk in Close Binary w/ WD

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.

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

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.

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

Ejected Shells: Novae Persei & Cygni

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

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

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)

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

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

Helium fusion can make carbon in low-mass stars

CNO cycle can change C into N and O

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

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

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 + 

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

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

Alpha Process Builds Middle Elements

Elemental Abundances: H Rules!

How Does a High-Mass Star Die?

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 Start here on 3/17

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!)

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

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) Start here on 10/20/09

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

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

Type II (massive star) SN formation, illustrated

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).

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 Start here on10/31

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

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!

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

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

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.

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

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.

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

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

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!

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!

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!

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