From: Robert J. Bradbury (bradbury@www.aeiveos.com)
Date: Wed Nov 24 1999 - 06:17:08 MST
On Wed, 24 Nov 1999, Liam. A. Chu wrote:
>
> If it takes two supernovae to get (e.g.) Fe through U, maybe we _are_ the
> Elder Gods, such as there are. 4 Billion + 4 Billion + 4.5 Billion adds up
> about right, in Fermi number terms.
It doesn't take two successive SN to generate heavy elements.
Stars > ~10 M_sun, burn successively through, C, Ne, O and Si, and end up
with an iron-group core. A type II supernova then results in the dispersal
of some fraction of these elements and the formation of a neutron
star or a black hole. Type-II supernova may also produce heavier
elements in the r(rapid)-process where a high neutron flux results
in the rapid production of heavier elements.
The more common path for heavy elements involves the s(slow)-process
and occurs in intermediate mass stars (1 M_sun < M_star < ~8 M_sun)
when it is in its asymptomatic giant branch (AGB) period, near the end
of its life (before the white dwarf stage). During this period it
goes through He-burning cycles where neutrons are produced, captured
to form heavy elements that are expelled in the form of stellar winds.
Rapid & slow refer to the neutron capture probability relative to the
half-life of radioactive isotopes.
The s-process is responsible for the production of most elements up
to Bi-209 (the heaviest nonradioactive element). The r-process is
required to produce heavier radioactive elements (including uranium).
Since a star like Sirus with ~2.3 M_sun will evolve to the AGB stage
in approximately a billion years, what *is* required for heavy element
abundance in a short time (hundreds of millions of years) is a collection
of stars with masses > ~ 3-4 M_sun.
See also
"How many generations of stars were needed to create the heavy elements
in our solar system?" at "Ask the Space Scientist":
http://image.gsfc.nasa.gov/poetry/ask/a11470.html
Robert
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