22
Density
Wave Theory
–SPIRAL
WAVE rotates with galaxy, but slower than individual stars
•Like
moving traffic jam after an accident has been cleared
–Gas
(and stars) catch up with wave, move through it, eventually reach front
•Just
like cars catching up with moving traffic jam, eventually get through it
–Gas
is more crowded in wave – clouds collapse to form new stars
•More
collisions in the traffic jam
–There
are slightly more old stars in the arm too, because they speed up slightly coming
into it
and slow down slightly moving out of it.
–But
the best tracers are the things that mark recent cloud collapses: O,B stars, etc.
From our text: Horizons, by Seeds