M51 is actually two galaxies. The larger galaxy (M51A) is designated NGC 5194 and the smaller galaxy (M51B) is designated NGC 5195. M51 is located near the end star (Alkaid) in the Big Dipper's handle at an estimated distance of 37 million light years.
M51A and M51B have interacted gravitationally and this interaction has distorted the structure of the larger galaxy. From our point of view M51B, the smaller galaxy, is actually behind M51A and is receding from the larger galaxy. Some astronomers believe the smaller galaxy has been captured by and is now in an elongated orbit around the larger M51A. Computer simulations of the interaction of these two galaxies support this conclusion.
This image was assembled with data obtained from the Hubble Legacy Archive. Three wide band filtered images were used for the red, green and blue channels. These were imaged using filters of f814w and f658n for the red, f555w for green and f435w for blue. The color image was created as a standard (R+Ha)-G-B image.
1Based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA).