Issue 5.03

The Astrophysics Spectator

February 18, 2008

The disparity between the high densities of the stars and the low densities of the interstellar gas is striking.  The average density of the Galactic disk is only around 1 atom per cubic centimeter.  The Sun, on the other hand, has an average density of 1.4 grams per cubic centimeters, or about 6×10²³ atoms per cubic centimeters.  How does nature go from one extreme to the other?  Not only must gravity overcome the pressure of the interstellar gas, the energy liberated as gravity acts on a gas must be radiated away if the gas is to binds itself with with its own gravity.

The step that nature takes to go from dispersed gas to stars is to create molecular clouds.  As the name implies, molecular clouds are composed principally of molecules rather than of atom or ions.  What makes a molecular cloud the precursor of stars is that its gravitational field is strong enough to cause its gravitational collapse and its infrared emission is copious enough to radiate away the liberated gravitational potential energy.  From this collapse, new stars are born.

With this issue of the web site, I add a page that describes the two theories for how a molecular cloud collapses.  Also with this issue, I revise the page about the Jeans length to correct an error on  the timescale for gravitational collapse and to bring the definition of Jeans mass more in line with  common usage (more than one definition of Jeans mass exists in the scientific literature).

Next Issue:  The next issue of The Astrophysics Spectator is scheduled for February 27.

Milky Way Galaxy

Molecular Cloud Collapse.  The gas pressure inside a molecular cloud is too weak to prevent the collapse of the cloud into numerous stars.  However, other sources of pressure—magnetic fields and supersonic turbulence within the molecular cloud—are capable of slowing this collapse.  The debate over how these sources of pressure act on a cloud has lead the theoretical community to two very different theories for the collapse of molecular clouds.  Under one theory, static regions in a turbulent cloud collapse rapidly into stars.  Under the second theory, turbulence and magnetic fields slow, but do not stop, the collapse of the whole cloud.  (continue)