Expert: Philip Stahl - 8/27/2004

Question
Hello!
I saw an episode of Nova yesterday that covered black holes and how they're formed. It touched on fusion within a star and how it creates elements (helium up to iron). I'm a bit confused. Why does it stop with iron? Wouldn't a sufficently massive star be able to keep fusing away, delaying it's collapse into a singularity? Is it possible that there could be elements formed during the creation of a black hole that are not on the periodic table?
Anything you can do to alleviate my befuzzlement will be greatly appreciated!

Answer
Hello.

Beyond a given threshold, the mass of the star doesn't really dictate or portend how far fusion reactions can go.

At the stage of attaining  Ni-FE (nickel -iron) in the core, the reactions become *endothermic* rather than exothermic. This means they absorb more energy than generated.

The gist is that this results in a massive destabilization of the star's hydrostatic equilibrium (the weight of overlying layers balanced by outward gas pressure, e.g. from radiation etc)

In effect, the onset of endothermic reactions results in the star's outward radiation and gas pressure being *decreased*.

Gravity "wins" and so there must be collapse of the star's layers, with oxygent ignited in one of them - precipitating the incredible explosion recognized as a *supernova*.

The exact details of the physics of synthesis of heavy elements is still a matter of investigation, but one process that is probably applicable to isotopes in the range 70 < A < 209 is the "r-process" first proposed by Burbidge, Burbidge, Fowler and Hoyle.

In this process, neutron capture on very short time scales (0.01 - 10 seconds) leads to very heavy element buildup- with the free neutron released in the supernova stage.

Since there are energy limits (to do with the 'Fermi energy') and details of statistical mechanics too technical to go into here, it is unlikely that elements exist - from such a process or others- not already identified in the periodic table.