Self-Sustaining Star Formation
We have already touched upon the basic idea for the second possible explanation of spiral structure
in our discussion of star birth. Instead of spiral waves triggering star formation, perhaps it is
the other way around: maybe star formation drives the spiral wave formation.
Waves of Star Formation
Compression of a molecular cloud can initiate protostar collapse and the formation of a cluster
of stars. The largest, hottest stars formed produce shock waves in the surrounding medium, both by
their initial ignition and associated mass outflow,
and by supernova explosions at the end of their short lives.
These shock waves
can then compress additional portions of the surrounding cloud, triggering a second round of star formation,
and so on. The adjacent animation illustrates.
By such a sequence, a wave of self-sustaining star formation can pass through a
molecular cloud (such a wave of star formation appears to be passing through the molecular clouds in
Orion now).
This sequence of star formation can then, under the right conditions, be drawn into a spiral structure
by the differential rotation of the galaxy. Detailed simulations indicate that self-sustained star formation
can account for at least part of the spiral structure, particularly in galaxies (like ours) where
the spiral structure is not very uniform because of
branches and segments in the spiral arms.
Density Waves Versus Self-Sustaining Star Formation
It is thought that both the density wave theory and the theory of self-sustaining star formation
play roles in the formation of spiral structure. Perhaps in some galaxies density waves are strong and
we obtain simple "grand-design" spiral structure. Perhaps in others the orderly
two-arm spiral structure
expected from density wave theory is strongly modified by formation of spiral segments through self-sustaining
waves of star formation.
It is difficult to make more definitive statements because in neither the density
wave theory nor the self-sustaining star formation theory do we have a full understanding of what initiates
them. In each we start with an unexplained assumption: the existence of the density wave in the first case,
and the initial perturbation that triggers the first round of star formation in the second.
Example: M83
The above left figure shows a European Southern Observatory image of the spiral galaxy M83.
It is about 15 million light years away in the constellation Hydra,
and is sometimes called the "Southern Pinwheel". (M83 is part of a small physical grouping of
galaxies that also contains the radio galaxy Centaurus A that we shall discuss in
Chapter 25.)
This galaxy has well defined spiral arms
that are traced both by bright blue star clusters and by dark dust
lanes. However, under closer examination we
see that there are also branches and spurs coming off the spiral arms in various places. Thus,
M83 appears to be somewhere in between a "grand design" spiral and the "wooly galaxies"
described in the right panel.