Transcript Document
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 16 8 20 23 24 O, 10 Ne, 11 Na, 1223Mg , 12 Mg 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 produces numerous elements near the iron peak of stability The most abundant: 54 26 Fe 56 26 Fe 56 28 Ni Further reactions are endothermic and thus do not provide stellar luminosity. Timescales As the iron peak is approached, the energy released per unit mass of reactant decreases. Thus the timescale becomes shorter and shorter Core burning Lifetime H 10 million years 1 million years He C O Si 300 years 200 days 2 days 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: 56 26 4 2 Fe 1324 He 4n He 2 p 2n Tc 8 109 K c 1013 kg / m3 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 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 The rapid growth of long-wavelength mode instabilities may play a role 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 Break 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 For example, the following beta-decay reaction occurs: 56 56 Co Fe e e 27 26 This decay is a statistical process: the rate of decay must be proportional to the number of atoms in the gas: dN N dt where is the decay constant, and is characteristic of each radioactive element. 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? L 9.8 1034 e 3.26tW (for t measured in years) The initial luminosity is 2.5x108 LSun. After one year it has decreased to 9.9x106 LSun. 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. 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 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 SN1987A • Occurred in the Large Magellanic cloud, a small galaxy near the Milky Way. 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 • The initial decay mostly tracks Co-56, followed by Co-57 56 Co26 Fe e e 56 27 • This reaction produces high energy gamma rays which were detected for the first time, confirming the presence of this isotope. Neutrinos 56 Co26 Fe e e 56 27 • 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 • 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 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 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