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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

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Post-Main Sequence Evolution of Massive Stars

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post main sequence evolution of massive stars

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
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 disks cataclysmic variables
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.
  • An explosion on the surface of a WD: Nova Herculis 1934 and typical light curve
what makes a nova explode
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
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
thought question
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 question10
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
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)
helium capture
Helium Capture
  • High core temperatures allow helium to fuse with heavier elements to make Oxygen, Neon, Magnesium, etc.
massive stars cook heavy elements
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
Multiple Shell Burning
  • Advanced nuclear burning proceeds in a series of nested shells
  • High Mass Star Evolution
element formation abundances
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

approach to supernova
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
neutronization and collapse
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
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
Core Collapse, continued
  • Once NEUTRONIZATION is nearly complete,
  • the core collapse is halted by a combination of NEUTRON DEGENERACY PRESSURE
  • The core, with radius about 10 km, becomes a NEUTRON STAR (NS -- to which we'll return shortly)
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)
  • 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

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
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

supernova remnants bright nebulae
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
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
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
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 (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
Type Ia SN formation, illustrated

WD driven over the Chandrasekhar limit thanks to

accretion in a binary system

origin of type i sne
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
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
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
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