M101: The Pinwheel Galaxy
Why do many galaxies appear as spirals ? A striking example is M101, shown above , whose relatively close distance of about 22 million light years allow 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 circle the galaxy . 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.