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Issue 5.02

The Astrophysics Spectator

January 30, 2008

Stars are born deep within molecular clouds.  When the newborn stars begin generating heat, briefly at the beginning through their gravitational contraction and later through the thermonuclear burning of hydrogen, they dispel the clouds and reveal themselves.  Their appearance—their masses, their composition, their clustering—depends on the characteristics of the molecular clouds that generated them.  To understand how stars form, one must first understand the molecular clouds.

The molecular clouds are the coldest and densest regions of interstellar space, with temperatures a only several degrees greater than the 2.7Copyright °K temperature of the microwave background.  As the name implies, molecular clouds are composed of molecules rather than atoms, which is a consequence of the cold temperatures and the exclusion of optical and ultraviolet starlight from the cloud interiors.  The most common molecule is the hydrogen molecule (H2), but many other molecules, including some rather complex hydrocarbon molecules, are found in these clouds. The densities within a molecular cloud exceed 105 times the average density of the interstellar gas.  This environment provides the right conditions for the gravitational collapse these clouds into stars and star clusters.

The pages added in this issue describe the basic properties of molecular clouds and present the concept of the Jeans length, which is the length characterizing the gravitational collapse of a group of stars or of a cloud of gas.

Next Issue:  The next issue of The Astrophysics Spectator is scheduled for February 13.

Jim Brainerd

Milky Way Galaxy

Molecular Clouds. The coldest regions of the Galaxy are the dark nebulae that block our view of the Galactic disk.  These clouds of dust and hydrogen molecules shield their interiors from starlight, which allows their interiors to cool to temperatures only several degrees higher than the 2.7Copyright ° Kelvin of the microwave background.  These clouds can become unstable to gravitational collapse, which leads to the formation of stars.  (continue)

Newtonian Gravity

Gravitational Stability and Collapse.  The gravitational stability of groups of stars and of clouds of gas is measured by a length scale called the Jeans length.  A system that is larger than the Jeans length will break up into clumps that are a Jeans length in size.  These clumps will collapse into gravitationally-stable systems in a timescale called the Jeans time.  All one needs to know to calculate these scales is the average mass density and the average random velocity of the stars or gas in space.  (continue)

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