Astro Fact of the Day

Sir James Jeans, the pioneering astrophysicist made many contributions but today we will consider the Jeans Length. A cloud of gas, he reasoned, would collapse if pressure forces were overwhelmed by the gravitational self-attraction. One way to think about it is that if the time for a pressure wave to move across the cloud is greater than the time required for the cloud to collapse in free fall, gravity wins. Pressure waves, we may recall, travel at the speed of sound.

In the early days of the universe, when we were all young warthogs, or, more precisely, a plasma of hydrogen and helium nuclei and electrons, the speed out sound was very high, perhaps 2/3 of light speed c. After a few hundred thousand years, the plasma, cooled by cosmic expansion enough for electrons and nuclei to recombine into atoms, and sound speed plummeted.

This caused the Jeans length to decrease by ten orders of magnitude. Before recombination, only clouds of mass of galaxy super cluster size could collapse. Afterwards, clouds as light as a globular cluster (10^5 to 10^6 solar masses, say) could collapse.


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