The Pinwheel Galaxy
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This is the nearby Sc spiral (in fact, perhaps the textbook example of a luminous 
so-called luminosity class I spiral) NGC 5457 or Messier 101, in Ursa Major. It has 
several extremely luminous star-forming (H II) regions in the outer spiral arms, 
some sporting their own NGC numbers. It dominates a small group of galaxies, with 
some of its neighbors such as NGC 5474 showing wear and tear attributed to the tidal 
effects of M101. M101 itself is further noteworthy for its extensive and lopsided 
distribution of neutral hydrogen gas, and for showing evidence of gas falling into 
its disk at high speeds. Cepheid variables suggest a distance of about 7 Megaparsecs 
(about 22 million light-years); at that distance some can just be picked up from the 
ground when the seeing cooperates, though the definitive study had to await the 
availability of HST.

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Big, beautiful spiral galaxy M101 is one of the last entries in Charles Messier's 
famous catalog, but definitely not one of the least. About 170,000 light-years across, 
this galaxy is enormous, almost twice the size of our own Milky Way Galaxy. M101 was 
also one of the original spiral nebulae observed by Lord Rosse's large 19th century 
telescope, the Leviathan of Parsontown. Recorded at infrared wavelengths by the 
Spitzer Space telescope, this 21st century view shows starlight in blue hues while 
the galaxy's dust clouds are in red. Examining the dust features in the outer rim of 
the galaxy, astronomers have found that organic molecules present throughout the rest 
of M101 are lacking. The organic molecules tracked by Spitzer's instruments are called 
polycyclic aromatic hydrocarbons (PAHs). Of course, PAHs are common components of dust 
in the Milky Way and on planet Earth are found in soot. PAHs are likely destroyed near 
the outer edges of M101 by energetic radiation in intense star forming regions. Also 
known as the Pinwheel Galaxy, M101 lies within the boundaries of the northern 
constellation Ursa Major, about 25 million light-years away.