- What Happens after a Significant Iron Core Develops?
The Iron core is supported by degeneracy but very quickly the
Chandrasekar Limit is exceeded.
- The iron core collapses and within about 0.1 seconds the temperature
in the core reaches K.
- 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 ( knocked clear of the
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).
- After another 0.1 second goes by another strange phenomenon
occurs called neutronization. The density is so high that
the combine with to form neutrons. This only happens in this
regime where degeneracy has been exceeded. This process releases
a flood of neutrinos.
- By 0.25 seconds after the initiation of the collapse the core is
solid neutrons and the density has reached
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.
- 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 .
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 .
- Is there any reason to accept this theory?
- 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.
- The emission lines of SNII are seen to be Doppler shifted
by between 10,000 and 20,000 km/sec as predicted.
- The peak luminosity of a single SNII is comparible to the
luminosity of all the stars in a small galaxy combined as
- 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 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
- For the first time we had information about the star that
exploded. It was a 20 supergiant - Bingo!.
- In the Galaxy, if we point our telescopes to the positions
of historical supernovae, we see the chemically enriched, still-expanding
shells of gas.
- 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
- 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.