Hot, young stars in galaxy M101. Coloured ultra- violet image showing the distribution of hot, young stars within the galaxy M101 (Pinwheel Galaxy, NGC 5457). These stars form many clusters within the arms of the galaxy, and are often surrounded by vast clouds of glowing, ionised hydrogen gas (HII regions). Stars which emit mainly ultraviolet light are intensely hot, massive and short-lived compared to stars like the Sun. M101 is about 12 million light years from Earth. This image was made using the Ultraviolet Imaging Telescope, part of the Astro-2 mission of Shuttle Endeavour in March 1995.