Most galaxies reside in dense swarms of up to a thousand,
zooming around at some 600 miles a second. Astronomers first
noticed some years ago that nearby galaxy clusters are populated
with dim reddish elliptical galaxies. But the most distant and
therefore oldest clusters are full of bright blue spirals, some
of which are slightly irregular. The observations suggest that
galaxies in clusters evolve from spirals to ellipticals over the
course of about 5 billion years. Theorists have proposed various
mechanisms to explain this--including collisions of galaxies--
but none fully jibe with the data, says University of Washington
astronomer George Lake. Lake believes he and some colleagues may
have solved the puzzle. They used a supercomputer to simulate
the effect that living in a giant cluster of galaxies has on the
shape and color of its members. Their computer model tracks a
typical spiral galaxy as it orbits through a cluster consisting
of a thousand other galaxies. Direct collisions aren’t needed to
destroy a