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Lecture 19. The fate of massive stars: supernovae. Massive stars. Helium burning continues to add ash to the C-O core, which continues to contract and heat up. Carbon is ignited, forming. Shell structure.

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lecture 19

Lecture 19

The fate of massive stars: supernovae

massive stars
Massive stars
  • Helium burning continues to add ash to the C-O core, which continues to contract and heat up.
  • Carbon is ignited, forming
shell structure
Shell structure
  • If each reaction has time to reach equilibrium, the stellar interior will consist of shells of different composition and reactions
  • Oxygen is ignited next producing a Silicon core.
silicon burning
Silicon burning
  • Silicon burning produces numerous elements near the iron peak of stability
  • The most abundant:
  • Further reactions are endothermic and thus do not provide stellar luminosity.
  • As the iron peak is approached, the energy released per unit mass of reactant decreases. Thus the timescale becomes shorter and shorter
  • During Silicon burning the core has reached extremely high temperatures and densities:
  • The photons produced are so energetic they can destroy heavy nuclei, reversing the process of fusion. In particular:
core collapse
Core collapse
  • The inner core collapses, leaving the surrounding material suspended above it, and in supersonic free-fall at velocities of ~100,000 km/s.
  • The core density increases to 3x the density of an atomic nucleus and becomes supported by neutron degeneracy pressure.
  • The core rebounds somewhat, sending pressure waves into the infalling material
stalled shocks
Stalled shocks
  • As the shock wave propagates outward and encounters the infalling core, the high temperatures result in further photodisintegration.
    • This removes a lot of energy from the shock: it loses 1.7x1044 J of energy for every 0.1MSun of iron it breaks down.
    • If the iron core is too large, the shock becomes a stationary accretion shock, with matter accreting onto it.
instability growth
Instability growth
  • The rapid growth of long-wavelength mode instabilities may play a role
  • As the shock moves toward the surface, it drives the hydrogen-rich envelope in front of it.
  • When the expanding shell becomes optically thin, the radiation can escape, in a burst of luminosity that peaks at about 1036 W
light curves
Light curves
  • After the initial burst of luminosity, the supernova slowly fades away over a period of several hundred days.
  • As the shock wave propagates through the star, it creates a large amount of heavy, radioactive elements.
  • Each species decays exponentially with a unique timescale
radioactive decay
Radioactive decay
  • For example, the following beta-decay reaction occurs:
  • This decay is a statistical process: the rate of decay must be proportional to the number of atoms in the gas:
  • where l is the decay constant, and is characteristic of each radioactive element.
example radioactive decay
Example: radioactive decay
  • The energy released by the decay of one cobalt-56 atom is 3.72 MeV. Given 0.075 MSun of this isotope (this is how much was estimated to have been produced in SN1987A) how much energy does the decay release?
  • (for t measured in years)
  • The initial luminosity is 2.5x108 LSun. After one year it has decreased to 9.9x106 LSun.
  • If the star is relatively low mass, roughly M<25MSun, it can be supported by neutron degeneracy and becomes a neutron star.
  • For more massive stars, the gravitational attraction overcomes neutron degeneracy, and the core collapses to form a black hole.
supernova remnants
Supernova remnants
  • Crab nebula: believed to be the remnant of the supernova that went off in 1054 A.D.
    • Nebula is still expanding, at ~1450 km/s
    • The source of the luminosity and electrons is a pulsar in the centre of the nebula.
  • The Crab nebula is ~2 kpc away, with an angular size of 4x2 arcminutes. The expansion velocity is measured from the Doppler shift to be 1450 km/s. Estimate the age of the nebula. How bright would the supernova that gave rise to the Crab nebula have been?
supernova remnants17
Supernova remnants
  • Cygnus loop: this is a ~15,000 year old remnant.
    • The filaments are caused by shocks encountering the interstellar medium. These shocks excite the gas which then emits emission lines.

A small part of the remnant, expanding left to right

  • Occurred in the Large Magellanic cloud, a small galaxy near the Milky Way.
sn1987a progenitor
SN1987A progenitor
  • Progenitor was a much smaller star than usually responsible for Type II explosions.
  • Smaller stars are denser, so more energy was required to lift the atmosphere, and this resulted in a slower brightening and fainter peak luminosity.
sn1987a light curve
SN1987A light curve
  • The initial decay mostly tracks Co-56, followed by Co-57
  • This reaction produces high energy gamma rays which were detected for the first time, confirming the presence of this isotope.
  • Neutrinos produced in part by this decay were also detected: this was the first time neutrinos were detected from an astronomical source other than the Sun.
sn1987a the rings
SN1987A: the rings
  • The central ring is due to ejection by a stellar wind prior to the explosion.
  • Lies in the plane that contains the centre of explosion
    • Glows due to [OIII] emission, excited by radiation from the explosion
sn1987a the rings23
SN1987A: the rings
  • The central ring is due to ejection by a stellar wind prior to the explosion.
    • When the shock wave from the explosion reached this ring, in 2004, it excited the gas causing it to glow brightly.
sn1987a the rings24
SN1987A: the rings
  • The two other rings are not in the plane of the explosion, but in front of and behind the star
    • The explanation of these rings is still unknown