Joe, Excellent topic :-) I will throw in my 2-cents. It is a pressure wave, but pressure waves and sound waves are the same phenomena. They both work by compression and rarification of a material. It is also true that most gas clouds are empty space, as you said, but in truth nearly everything is completely empty space (space between atoms, covalent bonds, shared orbitals, etc). The phenomena occurs, because the pressure wave is formed by the actual collision of atoms along the path of propagation. Hence, because of the laws of thermo-dynamics, the impact is propagation is nearly perfect with relation to the speed of impact. If a single hydrogen is a accelerated, and eventually collides with another hydrogen, the resulting acceleration will be nearly perfect to the original velocity. This entire process is how star systems are made. As we all know, everything starts with the large Mag. clouds we watch in the sky each night. As one of these pressure waves passes through the cloud, areas condense, causing an increase in local gravity. Enough of these density pockets occur, then gravity is yet increase further. Eventually, this is the catalyst that creates every star and planet, the gas cloud becomes dense enough and hot enough to pass into proto-star phase, radiating nearly exclusively infrared. So, hopefully this is enough info to answer the quandary. If not, just call on good old Dr. science james (still student at the U with contacts in the Physics department) Another interesting note, on passing, this "sound wave" stuff is the same reason we have spiral arms. The stars in the arm don't move as is thought, instead, the pressure wave eminating from the center of the galaxy, causes a spiral to form from the compression and rarfication on a 100-light-year wide scale. Gotta love the unknown. Cheers, James Helsby Quoting Chuck Hards <chuckhards@yahoo.com>:

This should make it easier for Joe to believe that it's truly a pressure-wave phenomemon. I was merely speaking off the cuff, not attempting to truly quantify the interstellar density. Hard to be precise when at work with my mind wrapped around composites instead of astro-physics.

Thanks

C.

--- Paul Gettings <gettings@mines.utah.edu> wrote:

...

On 10-Sep-03 8:25, Chuck Hards wrote:

...

The process is mechanical, the same as sound in the human range, even though the individual molecules are kilometers apart. You are thinking correctly about In space, the density of molecules is closer to tens to millions per cubic meter, not km. In the region of a gas cloud, the density would be many thousands to millions of particles per cubic meter. Only in the deepest part of intergalactic space could the density drop near the particle/km^3.

Hence, it is not very difficult to propogate a pressure wave by mechanical interactions. The propogation speed (speed of sound) is _very_ low compared to a normal atmosphere; very easy to go Mach 1! :)

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