Explanation:
Why do many galaxies appear as
spirals?
A striking example is
M101,
shown above,
whose relatively close distance of about 27 million
light years allows it to be studied in some detail.
Recent evidence indicates that a
close gravitational interaction with a neighboring galaxy created
waves of high mass and condensed gas which
continue to orbit the galaxy center.
These
waves compress existing gas and cause star formation.
One result is that
M101, also called the
Pinwheel Galaxy, has several extremely
bright star-forming regions
(called HII regions) spread across its spiral arms.
M101 is so large that its
immense gravity distorts
smaller nearby galaxies.
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