Hubble Space Telescope image of the face-on spiral galaxy Messier 101 (M101). It is the largest and most detailed photo of a spiral galaxy that has ever been released from Hubble. The galaxy's portrait is actually composed of 51 individual Hubble images, in addition to elements from images from ground-based photos. This giant galaxy is 170,000 light-years across or nearly twice the diameter of our own Milky Way galaxy. It is estimated to contain at least one trillion stars and is located about 25 million light-years from Earth in the constellation Ursa Major.