Times to form Elements [was Re: Why Would Aliens Hide?]

Robert J. Bradbury (bradbury@www.aeiveos.com)
Wed, 24 Nov 1999 05:17:08 -0800 (PST)

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