fisher@dvinci.DEC (07/17/85)
The Stardate about the shockwave was very interesting...I had never heard
that theory before, but it raises some questions:
1) What kind of wave is the shockwave? Matter? EM? If matter, it must have
a devil of a job propagating through near vacuum!
2) How does the shockwave move? Circularly around the galaxy? If so, why?
(If it is matter, I suppose gravity curves its path, but if not...?)
3) How come non-spiral galaxies don't have shockwaves?
4) The statement that gaps between the spiral arms of the galaxy are caused by
stars "burning out" implies to me that the shockwave's period is on the order
of (distance between arms)/(average star lifetime). Does this work out to be
a reasonable number (e.g. less than c, but large enough to have propagated
around the galaxy since the universe began)? I suppose this is an elementary
thing that any respectable astronomer would have worked out before expounding
the theory, but I am curious anyway.
Burns
UUCP: ... {decvax|allegra|ucbvax}!decwrl!rhea!dvinci!fisher
ARPA: fisher%dvinci.dec@decwrl.ARPAethan@utastro.UUCP (Ethan Vishniac) (07/18/85)
> > 1) What kind of wave is the shockwave? Matter? EM? If matter, it must have > a devil of a job propagating through near vacuum! The shockwave is matter, perhaps mediated by magnetic fields. "vacuum" is a relative term. > 2) How does the shockwave move? Circularly around the galaxy? If so, why? > (If it is matter, I suppose gravity curves its path, but if not...?) The shockwave is a standing wave pattern. A standing wave pattern in a stable disk will, of course, curve around the disk. "Gravity curves its path" is a reasonable way of thinking of it. > 3) How come non-spiral galaxies don't have shockwaves? Insufficient gas has been retained in elliptical galaxies. Dwarf irregulars are too small to support stable wave patterns. However, they do sometimes show signs of irregular shock waves associated with star formation. > 4) The statement that gaps between the spiral arms of the galaxy are caused by > stars "burning out" implies to me that the shockwave's period is on the order > of (distance between arms)/(average star lifetime). Does this work out to be > a reasonable number (e.g. less than c, but large enough to have propagated > around the galaxy since the universe began)? > Burns Actually the time it takes the stars to move between the shock waves is a little longer since the stars are moving in almost circular orbits around the galaxy at a different speed from the pattern speed of the shock wave. However, the important point here is that the lifetime of the stars that matter is only on the order of a few tens of millions of years. That's how long *very* bright stars live. Faint stars (like our sun) formed in a shock wave move in and out of the spiral arms many times in their lifetimes. -- "Don't argue with a fool. Ethan Vishniac Borrow his money." {charm,ut-sally,ut-ngp,noao}!utastro!ethan Department of Astronomy University of Texas