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Best Bang Since The Big One
StarKinder, I'll warn you. This might make your brain hurt a bit.
Illustration. Credit: NASA/CXC/M.Weiss
In September of 2006, Robert Quimby, an astronomer at the University
of Texas, was looking for exploding stars. This sounds tricky but is
really pretty straightforward. Just pick an easy to observe galaxy,
point a camera (telescope) at it and wait. Eventually a star in it will
go Boom! On September 18th, Dr. Quimby was observing a ordinary galaxy
named NGC1260 when he noticed his nova - now named SN2006gy. Dr. Quimby
notified the other nova hunters around the world and they pointed bigger
ground based and space based scopes at it. 40 days later, it was still
getting brighter. This was really strange. No type of nova ever acted
like that and the peak brightness of SN2006gy was at least 10 times
brighter than any supernova ever recorded. What could this thing be?
Now, we need to do a bit of refreshing. Do you remember our
discussions about how stars explode? Go to:
https://solar-center.stanford.edu/activities/jeff/ and refresh your
memory. Basically a star explodes when it uses up all its fuel and the
central core collapses or one star in a binary (2 star) system steals
the outer atmosphere of its companion and goes boom. Pretty much all
nova are variations on these two ideas. SN2006gy didn't look like
either of these types.
E=mc˛ Remember that? It is an important part of the Laws of
Relativity. Energy equals mass and mass equals energy. We can "make"
particles by smashing other particles together at very nearly the speed
of light and we can turn mass into energy by putting a particle and its
anti-particle together (there are other ways of making the mass-energy
magic but these are good examples). You can even turn an photon - one
of those particles of "light" - into real solid particles of matter.
Sometimes doing this can create a disaster. Here's how.
If you have REAL high energy photon - more energetic than violet,
or ultra-violet, or even x-rays - called a gamma ray and you tangle it
up in an electromagnetic field, you can get the photon to turn into an
electron and a positron both traveling in the same direction that the
gamma ray was traveling. Normally an electron-positron pair would
instantly smack together and form a gamma ray again (kind of a waste of
time). Well, since we are in a electromagnetic field and electrons and
positron are electrically charged, one gets bent one way and the other
the other and they don't smack together.
Turns out that if you have a HUGE star, one that has about 150 times
the mass of our sun, it uses up its fuel really really fast. It turns
all its core hydrogen to helium, and helium to beryllium, and beryllium
to oxygen, all in just a few hundred thousand years (maybe a couple
million). Each one of those steps is hotter than the last and the star
gets hotter and puffed up. Hot gas expands - as in a hot air balloon -
and the high energy photons steaming out of the core also expand the
star (this is much like the solar wind we talked about a few days ago
but this is WAY more powerful) by "light pressure". It turns out that
the photons that are created when the star starts "burning" oxygen have
just enough energy to turn into an electron-positron pair as we
discussed above. Suddenly the outer atmosphere of our star is no longer
held up by light pressure and the outer atmosphere crashes down on
the star's core. All that gravitational energy crunches the hydrogen
and helium still in the atmosphere and causes lots of it to fuse and
there is a big explosion. Now this is a big explosion but not BIG BIG.
Two things happen. The outer atmosphere gets blown off in a sphere of
moderately fast expanding hot gas and the core of the star uses up lots
of fuel in this "bounce". In just a few years the oxygen starts fusing
into sulfur and the core gets REALLY hot and the whole thing happens
again only this time the outer atmosphere gets blown off moving very
fast. This new shell of rapidly expanding super hot gas smashes into
the older, slower shell of gas from the first boom and almost all of the
total energy in both shells turns into photons. This is probably the
huge explosion that Dr. Quimby found.
There is a possibility that the size of the huge star was just right
that the entire core of the star blew up and left nothing but debris -
no black hole, no neutron star, no white dwarf!
The reason why this is a very rare event is that stars of 150 solar
masses are extremely rare. We may not even have any in our galaxy.
When the universe was very young and there was nothing but lots of
hydrogen and helium about, these kinds of huge stars were common but they
ran out of fuel a blew up in just a few thousand years and the stars we
see now are made from the debris left over from these titanic
precursors. This debris is made up of all the elements from hydrogen to
uranium and it is very difficult to build a huge mass star out of it
(there are laws of physics that make it hard to do).
When Robert Quimby found this nova, it was the record largest nova
ever found. It is quite a surprise that it no longer holds that
record. But that's another story.