Giant galaxies weren't assembled in a day. Neither was this Hubble Space Telescope image of the face-on spiral galaxy Messier 101 (the Pinwheel Galaxy). It is the largest and most detailed photo of a spiral galaxy beyond the Milky Way that has ever been publicly released. The galaxy's portrait is actually composed from 51 individual Hubble exposures, in addition to elements from images from ground-based photos. The final composite image measures a whopping 16,000 by 12,000 pixels.

The Hubble observations that went into assembling this image composite were retrieved from the Hubble archive and were originally acquired for a range of Hubble projects: determining the expansion rate of the universe; studying the formation of star clusters in giant starbirth regions; finding the stars responsible for intense X-ray emission and discovering blue supergiant stars. As an example of the many treasures hiding in this immense image, a group led by K.D. Kuntz (Johns Hopkins University and NASA) recently catalogued nearly 3000 previously undetected star clusters in it.

The giant spiral disk of stars, dust and gas is 170,000 light-years across or nearly twice the diameter of our Milky Way. The galaxy is estimated to contain at least one trillion stars. Approximately 100 billion of these stars alone might be like our Sun in terms of temperature and lifetime. Hubble's high resolution reveals millions of the galaxy's individual stars in this image.

The Pinwheel's spiral arms are sprinkled with large regions of star-forming nebulae. These nebulae are areas of intense star formation within molecular hydrogen clouds. Brilliant young clusters of sizzling newborn blue stars trace out the spiral arms. The disk of the galaxy is so thin that Hubble easily sees many more distant galaxies lying behind the foreground galaxy.