نگرهی ِ موج ِ چگالی
negare-ye mowj-e cagâli
Fr.: théorie des ondes de densité
One possible explanation for → spiral arms,
first put forward by B. Lindblad in about 1925 and developed later by
C.C. Lin and F. H. Shu. According to this theory, spiral arms are not material
structures, but regions of somewhat enhanced density, created by
→ density waves. Density waves are perturbations amplified by
the self-gravity of
the → galactic disk. The perturbation results from natural
non-asymmetry in the disk and enhanced by environmental processes, such as galaxy encounters.
Density waves rotate around the → galactic center and periodically
compress the disk material upon their passage. If the spiral arms were
rigid structures rotating like a pinwheel,
the → differential rotation
of the galaxy would wind up the arms completely in a relatively
short time (with respect to the age of the galaxy), → winding problem.
Inside the region defined by the → corotation radius,
density waves rotate more slowly than the galaxy's stars and gas; outside that
region they rotate faster.
As the density waves rotate, they are overtaken by the individual stars and
nebulae/molecular clouds that are rotating around the galaxy at a higher rate.
The molecular clouds passing through the density wave are subjected to compression because
it is a region of higher density. This triggers the
formation of clusters of new stars, which continue to move through the
The short-lived stars die, most likely as supernovae, before they can
leave the spiral density wave. But the longer-lived stars that are
formed pass through the density wave and eventually emerge on
its front side and continue on their way as a slowly dissipating
cluster of stars.
Density wave theory explains much of the spiral structure that we see,
but there are some problems. First, computer simulations with density
waves tend to produce very orderly "grand design" spirals with a
well-defined, wrapped 2-arm structure. But there are many spiral
galaxies that have a more complex structure than this
(→ flocculent spiral galaxy).
Second, density wave theory assumes the existence of spiral density waves and then
explores the consequences.
→ stochastic self-propagating star formation.
→ density; → wave;