What happens next, I'm sure you're wondering! Well, the core of the star continues to  shrink and heat up until the core becomes electron degenerate.   What's electron degenerate, you ask?  Well, quantum mechanics (a branch of modern physics) says that no two particles can be in the same place at the same time doing the same thing.  Another rule of quantum mechanics, the Heisenberg uncertainty principle, states that simultaneous position and momentum of an electron cannot be known any more exactly than in the following equation:

which means that the uncertainty in x times the uncertainty in the momentum of x is greater than or equal to Planck's constant divided by 2 times pi.

Since its state cannot be known any more precisely, it is ‘fuzzy’.

So, now imagine  electrons like me floating around  in an ionized gas, where electrons with the highest energy have been taken away - leaving the rest of the atom with a positive charge.  As the temperature of that gas increases, the momentum range increases and the electrons have many energy levels to choose from.  However, what if, despite this abundant range of possible states, they are all occupied because the gas is so dense?  When all states are occupied, the electrons resist further crowding with great pressure.  These electrons are referred to as degenerate.  This pressure can be so great that it halts the contraction of a star.

The continuing temperature increases won't  raise the pressure because the core is degenerate, but it does speed up nuclear reactions which raises temperature even further.   It keeps shrinking and heating up until the core becomes electron degenerate.  The surrounding hydrogen runs out and adds to the core (just as another hydrogen layer ignites).  This chain reaction just keeps heating and contracting the degenerate core.  Temperature in the core is around 100 million K.

Eventually the temperature gets so high that Helium starts to fuse together.  Two He atoms don't form a stable nucleus, but if they survive long enough to fuse to a third, Carbon is formed.  This is called the triple alpha process.  The start of this process sends a wave of heat energy through the entire core.  This ‘helium flash’ releases the degeneracy and the core expands (this, in turn, causes a contraction of the star's outer layers)

At this point we have a non degenerate core that is fusing He to form C & O.  Just outside this core is a layer of H fusing to form He. Soon the He in the central region is used up and there is a C & O core, surrounded by He fusing to form C & O, surrounded by H fusing to form He.  Whew!  What a lot of fusion!! It takes a long time, too - anywhere from 10-100 million years!

Up to this point, all stars are about the same.  What happens next depends on the mass of the star.  First, I'll tell you about what happened to my star.  I suppose I should also describe the star I was in.   I was part of a B star - which if you remember from the HR diagram on the previous page - is the second most massive and luminous type of star.  For stars above 8-10 solar masses, when He  burning begins in the core, the core is not degenerate so no helium flash occurs.  He fuses to form C (Carbon) and O (Oxygen) which fuse to form Ne (Neon), more O, then Mg (Magnesium), then Si (Silicon), and finally Fe (Iron).  Fe nuclei are so tightly bound that energy is required to fuse it.  Thus it stays at that point.  At this point the star is separated into layers of different fusing gases.  Here they are in order of decreasing temperature:

Fe-> Si-> O-> Ne-> C-> He-> H

The following diagram shows the composition and layers of our star:

So these massive stars like mine end up with these layers and iron in the center.  The core keeps sucking mass from the surrounding shells.  Ultimately, the star is pushed over the Chandrasekhar limit (which means that after extinguishing all sources of nuclear energy, the star is more than 1.4 solar masses).  The force exerted by us degenerate electrons can no longer resist gravity.  We electrons merge with protons in the iron nuclei to form neutrons.  This removal of electrons causes the core to collapse in less than a second.  The collapse only stops when the core density reaches that of an atomic nucleus.  This core collapse sends shock waves through the other layers of the star causing them to be blown off in a HUGE explosion!!!  This is a supernova, and they can be up to  SEVERAL HUNDRED MILLION TIMES  the star's former brightness!

Below is a picture of the Lagoon Nebula, an example of a supernova.  It's not the nebula we formed when my star blew up - there aren't any pictures of that, but this is a nice picture all the same.

So there I was, flying through space again - this time as an iron atom.  In this explosion, the elements blown off are returned to the interstellar medium and become available to be used in succeeding generations of stars.  Along with planetary nebulae, supernovae help build the supply of chemical elements since all elements heavier than iron are formed in the intense heat of the explosion.

So what's left over?  An extremely dense chunk of neutrons remains at the center.  This is referred to as a neutron star or a pulsar.  A neutron star is an object that's larger than a white dwarf (which will be discussed on the next page) and no larger than 3 solar masses.  It's very dense - since all that mass must be packed into an area that's only about 20 kilometers in diameter.  The star rotates at a very rapid pace and emits radio waves that can be used to tell how fast it rotates - every third signal shows that one rotation has been completed.
 

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