[TML] Jewelry was Re: Personal Armor Noise

[Timothy Little]

On Wed, May 07, 2008 at 04:18:47PM -0400, Joseph Paul wrote:
> Do we have stars that go boom and pretty much completely unravel ie
> leave no core?

Yes, there probably are.  It's hard to be absolutely sure, since such
events are expected to be rather rare and generally extremely far
away.


> Where is the Fe layer in a star?

In a sense, nowhere and never.

Within a matter of months, large-mass stars burn through their
lower-mass nuclei, turning them into silicon and sulfur.  When that
runs out the core collapses, heating up from gravitational energy
until further fusion reactions become likely.  These take silicon up
in mass to nickel-56 within a few days, building outward from the
centre where temperatures were highest.

Nickel-56 decays to iron over a timescale of months, but not much of
that happens in the core.  The star explodes long before a tenth of it
decays to iron.


> If stellar fusion only goes up to Fe how do we get the elements
> heavier than that?

Two main ways, that I know of.  In supernovas (the larger stars
mentioned above):

As the nickel core builds up, its pressure and density increase until
a point is reached where the gravitational energy climbs faster than
the repulsion from electrons being forced into the tighter space.  At
that point it collapses toward a neutron star state, and radiates an
insanely dense flux of neutrons for a very short time.

Those neutrons are absorbed by matter above, converting them to
heavier isotopes.  Many of those decay rapidly into elements with more
protons and fewer neutrons, absorb more neutrons, and so on right up
the atomic number chain to the point where they fission into smaller
elements again.

Most of those heavier elements fall back into the core again (and are
crushed into neutronium or a black hole), but some are blasted into
space.


In most moderately large stars (e.g. our Sun): in the very late stages
of the star's life, very high temperatures are reached that allow
fusion to nickel (and decay to iron).  Some fusion reactions create
neutrons, and these are captured by nearby nuclei such as iron.
Unlike the supernova case, this takes place over thousands of years
and so these tend to decay (if unstable) before absorbing more
neutrons.  These elements can leave the star via stellar winds and
nova/supernova explosions.


The difference in the mix of heavy elements and isotopes between the
two origin processes is quite noticeable.


- Tim