Messier 101, the Pinwheel Galaxy is a very large spiral galaxy with a diameter of 184,000 light years, which we can look at almost exactly from above. The Galaxy was discovered by the French astronomer Pierre Méchain on March 27, 1781. The numerous, bright HII star formation regions with masses of up to over 10 million solar masses are immediately noticeable in the spiral arms. These regions are so bright that some have been assigned their own NGC numbers.

Interesting are also the asymmetries of the spiral arms and a core, which seems to be shifted out of the center. Interactions of the small galaxies NGC 5474 and NGC 5477 in the vicinity of Messier 101 are generally accepted as causes. This would also explain the numerous, large star formation areas in the spiral arms.

In total, M 101 has five prominent accompanying galaxies: NGC 5204, NGC 5474, NGC 5477, NGC 5585 and Holmberg-IV.