Rick's Cosmology Tutorial: Appendix B1 Abstract

The Entropy of a Collapsing Cloud of Gas: Why Does Star Formation Need a Cooling Mechanism?

One of the things which often surprises students is that a star can form from a cloud of gas only if a cooling mechanism is available. They tend to think that the formation of a star involves the collapsing gas cloud becoming very hot - hot enough to ignite nuclear fusion - so cooling is surely the last thing you would need.

A second conundrum is that a simple calculation shows that the entropy of a collapsed gas cloud is less than that of the original dispersed cloud. Why does this not violate the second law of thermodynamics?

The answer to both questions is the same. To collapse, the gas cloud must shed entropy (and hence energy) into the surroundings (inter-stellar space). So it is the second law of thermodynamics which requires the cooling mechanism, because 'cooling' also means removing entropy to another place. It is true that the collapsing gas cloud becomes hotter and will eventually trigger nuclear fusion. But only part of the released gravitational potential energy is used to heat the proto-star. The rest must be jettisoned into the surroudings as a vehicle to convey the balance of entropy. This is an overwhelmingly important issue since it provides the basic mechanism whereby ordered structures can form in the universe.

Read Appendix B1: The Entropy of a Collapsing Gas Cloud

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The Orion nebula, visible with the unaided eye near the belt in the constellation of Orion, is an immense nearby starbirth region and probably the most famous of all astronomical nebulas. The Orion Nebula lies about 1,500 light years distant and is located in the same spiral arm of our Galaxy as ourselves. Insets to the above mosaic show numerous proplyds, many of which are stellar nurseries likely harboring planetary systems in formation. Some proplyds glow as close disks surrounding bright stars light up, while other proplyds contain disks further from their host star, contain cooler dust, and hence appear as dark silhouettes against brighter gas. The latter is a valuable source of data on how planets are forming. Many proplyd images also show arcs that are shock waves - fronts where fast moving material encounters slow moving gas. [Credit: NASA,ESA, M.Robberto(STScI/ESA),HST Orion Treasury Project Team, L.Ricci(ESO)]