Pinwheel galaxy (M101), optical image. This is a large spiral galaxy that lies around 22 million light years from Earth in the constellation Ursa Major. It is one of the largest spiral galaxies known, with a width of over 170,000 light years. Its spiral arms contain a very large number of starbirth regions (pink), immense clouds of gas lit up by the hot young stars within them. The arms appear blue as they are largely populated by young stars, which are white or blue in colour. The small, tight central nucleus is yellower as it contains older, redder stars and a higher concentration of dust.
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