Explanation: This stunning vista represents the highest resolution image ever made of the Andromeda Galaxy (aka M31) at ultraviolet wavelengths. Recorded by NASA’s Swift satellite, the mosaic is composed of 330 individual images covering a region 200,000 light-years wide. It shows about 20,000 sources, dominated by hot, young stars and dense star clusters that radiate strongly in energetic ultraviolet light. Of course, the Andromeda Galaxy is the closest large spiral galaxy to our own Milky Way, at a distance of some 2.5 million light-years. Just slide your cursor over the image to compare the appearance of this gorgeous island universe in optical light with its ultraviolet portrait.