The Pinwheel Galaxy - M101/NGC 5457

M101, called the Pinwheel Galaxy, is a giant spiral galaxy, approximately 170,000 light-years across.  Our Milky Way galaxy is approximately 100,000 light-years across.  It is estimated to contain at least one trillion (1012) stars. Look at the spiral arms for the very bright blue/white patches that represent areas where new stars are forming, and bursting into life.  In the cosmologically recent past, it is likely that M101 experienced a close encounter with another galaxy.  This may have caused the asymmetry seen in the photograph.  It would also amplify the density waves in the spiral arms leading to increased compression of the hydrogen causing the increase in star formation.  M101 is about 7.2 Mpc or 23.4 million light-years from Earth.  There is video of the Pinwheel Galaxy that shows its location in the sky, and zooms in to a close up view.