If a supernova (the explosion of a massive star) trig-
gered star formation from dense clouds of gas and dust,
and if the most massive star to be formed from the cloud
evolved into a supernova and triggered a new round of
star formation, and so on, then a chain of star-forming
regions would result. If many such chains were created
in a differentially rotating galaxy, the distribution of
stars would resemble the observed distribute in a
spiral galaxy.
This line of reasoning underlies an exciting new
theory of spiral-galaxy structure. A computer simulation
based on this theory has reproduced the appearance of
many spiral galaxies without assuming an underlying
density wave, the hallmark of the most widely accepted
theory of the large-scale structure of spiral galaxies.
That theory maintains that a density wave of spiral
form sweeps through the central plane of a galaxy,
compressing clouds of gas and dust, which collapse
into stars that form a spiral pattern.