davidl@tekecs.UUCP (David Levine) (08/31/83)
As I understand it, a "just-barely star" which ignites but then burns out
before all its hydrogen fuel is consumed is called a white dwarf.
The fate of a star is determined by its mass. All stars on the Main Sequence
begin as clouds of cold hydrogen. If the mass of the cloud is insufficient to
cause its collapse and ignition, the cloud remains a cloud. If the mass is
great enough, pressure and gravitational potential energy force the atoms in
the center together into fusion (actually a very complex process, not just
bashing two hydrogens together to make one helium). At this point the star
"flashes", which blows the more tenous outer nebula away. The mass of the star
itself is that portion of the nebula which was close enough to the center that
its gravitational attraction is greater than the pressure caused by fusion.
This balance of pressure versus gravity determines the star's size throughout
its life.
The star burns until the hydrogen in the core (the core is the part where
pressures are high enough to sustain hydrogen fusion) is all turned into
helium. Note that the outer parts of the star, where pressures are too low for
hydrogen fusion, remain hydrogen. At this point the star goes out. Pressure
drops and gravity begins to take over. The star begins to collapse.
Again the mass of the star determines what happens. A star with sufficient
mass collapses until the pressure at the center is great enough to fuse helium.
Again the star ignites in a "helium flash," which blows away a portion of the
outer atmosphere of the star. Now the star is a red giant. (When this happens
to the sun, the Earth and all the inner planets will be absorbed.) For stars
smaller than a certain mass, there is insufficient gravity to cause helium
fusion and the star simply fades away as a white dwarf -> black dwarf.
It takes a long time for a white dwarf to cool off. In fact, any which exist
in our galaxy, even the very oldest, are still cooling.
After the red giant phase, this scenario repeats again and again: burn out
core, collapse, re-ignite with a new form of fusion. At each iteration stars
with mass too small to ignite the next phase cool off and die. More massive
stars continue building up shells of unburned material (hydrogen on the
outside, then helium, etc.), resembling onions. Eventually the most massive
stars reach the point that the core is fusing into iron.
Iron is funny. All elements lighter than iron release energy when they fuse,
but iron and the heavier elements require energy input to cause fusion. When
the progenitor element of iron in the star's core is all fused away, the star
burns out and collapses again. This time, when fusion of iron begins the
process takes energy away rather than adding it, hastening the collapse. As
the star collapses faster and faster, strange things start to happen. Hydrogen
and other unburned fuels begin to fuse as pressures rise, and fusion of iron
increases as well (accelerating the process further). Now neutrinos
(a byproduct of the fusion process) are generated in vast numbers. Normally
these neutrinos escape the star (this is happening all the time), but at
this point the star is so dense that they are stopped (!!). The energy carried
by each is re-absorbed by the star, hastening fusion and collapse still
further. This results in a chain reaction: a supernova! The star explodes
when pressures exceed the gravity of such a massive star, scattering the
elements manufactured to the four (solar) winds. Eddies of phenomenal pressure
in the process cause fusion to create the heaviest elements, which are not
otherwise produced in stars.
In this way are all the elements in the Periodic Table produced from hydrogen.
A modern just-so story from
-- David D. Levine (...decvax!tektronix!tekecs!davidl) [UUCP]
(...tekecs!davidl.tektronix@rand-relay) [ARPA]