Type II Supernovae

• What Happens after a Significant Iron Core Develops?

  The Iron core is supported by e^- degeneracy but very quickly the Chandrasekar Limit is exceeded.

  1. The iron core collapses and within about 0.1 seconds the temperature in the core reaches ~5 x 10⁹ K.

  2. The radiation field of the core is peaking out in the gamma ray regions and these photons are energetic enough to break apart the Fe nuclei in a process called "photo-disintegration"

     This is the higher-energy counterpart to photo-ionization where Ultra-violet photons can break apart atoms (e^- knocked clear of the atoms).

     The Fe atoms are rapidly blasted apart into neutrons, protons and electrons and this cools the core leading to further collapse (moving the wrong way down the binding energy curve).

  3. After another 0.1 second goes by another strange phenomenon occurs called neutronization. The density is so high that the e^- combine with p⁺ to form neutrons. This only happens in this regime where e^- degeneracy has been exceeded. This process releases a flood of neutrinos.

  4. By 0.25 seconds after the initiation of the collapse the core is solid neutrons and the density has reached 4 x 10¹⁷ kg/m³.

     This is ENORMOUS . Normal material is something like 99.999% empty space and even atoms are mostly empty space. The density of the neutron core is the same as the density of the nuclei of atoms. This is as closely packed as material can get.

  5. Once the core hits nuclear density, it becomes much stiffer than any brick wall. As the core collapsed, the outer layers of the star are also collapsing at up to 15% of the speed of light. The inner collapsing layers crash against the neutron core, BOUNCE back outward and set up a shock wave that, in combination with the neutrino flux blows the layers above the core into space at up to 20,000 km/sec (50 million miles/hour!).

     The core has collapsed to about 60 miles in radius with a mass between 1.4 and 3 M_Sun.

     It is during this explosive phase that all sorts of Non-equilibrium fusion reactions go on that build heavy elements far past Iron in the Period Table. (more later)

• This explosive end of the life of a massive star is called a Supernova Type II or SNII

  The models predict the luminosity of the explosion will exceed 10⁸ L_Sun.

• Is there any reason to accept this theory?
  1. We have seen many SNII (identified by the presence of hydrogen lines in their spectrum) in other galaxies in the past 100 years. They are always associated with the disks of galaxies and often with regions of recent star formation - SNII are observationally associated with young stars.

  2. The emission lines of SNII are seen to be Doppler shifted by between 10,000 and 20,000 km/sec as predicted.

  3. The peak luminosity of a single SNII is comparible to the luminosity of all the stars in a small galaxy combined as predicted.

  4. For the special case of SN1987a that exploded 165,000 years ago in the nearby companion galaxy to the Milky Way called the Large Magellanic Cloud, the neutrino burst was detected four hours before the optical brightening occured. This was beautifully consistent with the theory. The neutrino burst marked the neutronization event in the core and it took about 4 hours for the shock wave to travel to the surface of the star and start the optical brightening.

     Note, the 10⁸ L_Sun optical burst is only 1/10% of the luminous energy of a SNII outburst. The neutrino luminosity is the other 99.9% and it equals the entire optical luminosity of the Universe for each SNII...

  5. For the first time we had information about the star that exploded. It was a 20 M_Sun supergiant - Bingo!.

  6. In the Galaxy, if we point our telescopes to the positions of historical supernovae, we see the chemically enriched, still-expanding shells of gas.

  7. The final nail in the coffin comes from the detection of the neutron cores in the centers of the SNII remnants, but we will save that for next week...

• There have been ~1000 SN observed in other galaxies in the last 50 years.

  For big spiral galaxies like the Milky Way, there are about

  0.5/century for SNIa

  1.8/century for SNII

  But we miss many because of obscuration by dust.

• From Radio surveys for SN remnants we know of remnants in the entire Galaxy with an inferred rate of 3.4 SN/century

• There are some historical SN

  The 11th and 17th centuries were good centuries for SN:

  Bright "guest stars" were recorded in the years:

  1006, 1054 (the Crab), 1181, 1572, 1604 and 1658.

• For all of these cases, when we point our telescopes to the location of the "guest stars" we find a SN remnant. In a couple of cases, the remnant was discovered before the historical event was identified

  

• The explosion in 1054 A.D. was recorded by many people including the native american indians in the southwest deserts. It was reported to be bright enough to cast shadows during the day.

• The "Gum" nebula in the Southern Hemisphere is a remnant from a SNII that exploded around 9000 B.C. and was 4x closer than the 1054 A.D. event. Must have been 16x brighter, perhaps as bright as the full Moon.

  The second nearest SN remnant is called "Cass A". It must have gone off around 1600 A.D., must have been very bright, but somehow went unrecorded.

• Although there was a 400 year gap between 1181 A.D. and 1572 A.D., the 340 years since the last Galactic SN is unusual. We are long overdue for a very bright one.