Hydrogen in M51 . ... Explanation: Perhaps the original spiral nebula, M51 is a large galaxy, over 60,000 light-years across, with a readily apparent spiral structure . Also cataloged as NGC 5194, M51 is a part of a well-known interacting galaxy pair, its spiral arms and dust lanes clearly sweeping in front of companion galaxy NGC 5195 (top). ... Reddish hydrogen emission regions, called HII regions , are the regions of intense star formation seen to lie mainly along M51's bright spiral arms. ...