M101 is a relatively large galaxy compared to the Milky Way. With a diameter of 170,000 light-years it is nearly twice the size of the Milky Way. M101 can be seen to be asymmetrical on one side. It is thought that in the recent past (speaking in galactic terms) M101 underwent a near collision with another galaxy and the associated gravitational tidal forces caused the asymmetry. In addition, this encounter also amplified the density waves in the spiral arms of M101. The amplification of these waves leads to the compression of the interstellar hydrogen gas, which then triggers strong star formation activity. If you click on the image to magnify it, you can see the magenta colored star forming regions. This is a 5 hour exposure, taken the nights of April 16 and 18, 2010, through my Hyperion Astrograph and SBIGSTL11000m camera. The sessions were guided by CCDAutopilot, completely automating data acquisition.