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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

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Lecture 19 l.jpg

Lecture 19

The fate of massive stars: supernovae


Massive stars l.jpg

Massive stars

  • Helium burning continues to add ash to the C-O core, which continues to contract and heat up.

  • Carbon is ignited, forming


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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.


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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.


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Timescales

  • As the iron peak is approached, the energy released per unit mass of reactant decreases. Thus the timescale becomes shorter and shorter


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Photodisintegration

  • 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:


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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


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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.


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Instability growth

  • The rapid growth of long-wavelength mode instabilities may play a role


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Explosion

  • 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


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Break


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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


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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.


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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.


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Remnants

  • 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.


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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?


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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


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SN1987A

  • Occurred in the Large Magellanic cloud, a small galaxy near the Milky Way.


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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.


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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.


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Neutrinos

  • 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.


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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


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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.


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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


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