The giant spiral disk of stars, dust, and gas is 170,000 light-years across or nearly twice the diameter of our galaxy, the Milky Way. M101 is estimated to contain at least one trillion stars. Approximately 100 billion of these stars could be like our Sun in terms of temperature and lifetime.
The galaxy's spiral arms are sprinkled with large regions of star-forming nebulae. These nebulae are areas of intense star formation within giant molecular hydrogen clouds. Brilliant young clusters of hot, blue, newborn stars trace out the spiral arms.
Messier 101 - Click for 800×625 image
More information: here
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