Russell Lecture: Dark Star Formation and Cooling Instability

Abstract

Optically thin cooling gas at most temperatures above 30 K will make condensations by pressure, pushing material into cool, dense regions. This works without gravity. Cooling condensations will flatten and become planar/similarity solutions. Most star formation may start from cooling condensations, where gravity is only important in the later stages. The idea that some of the dark matter could be pristine white dwarfs that condensed slowly onto planetary-sized seeds without firing nuclear reactions is found lacking. However, recent observations indicate 50 times more halo white dwarfs than have previously been acknowledged, enough to make the halo fraction observed as MACHOs. A cosmological census shows that only 1% of the mass of the universe is of known constitution.