Triangulum Galaxy

A face-on spiral spread wide open, it looks in photographs like a "pinwheel," its loosely unwound arms studded with pink knots of star birth. Because its core is small and its light is spread evenly across the whole disk, its surface brightness is low, so to the naked eye it shows only as a very faint glow, hard to make out.

The Triangulum Galaxy (M33) is the third-largest spiral galaxy in the Local Group, after Andromeda and the Milky Way. At about 60,000 light-years across it is a little over half the size of our galaxy, yet new stars are being born vigorously along its spiral arms. It is home to "NGC 604," one of the largest star-forming regions known — dozens of times bigger and brighter than the Orion Nebula. It is very likely a gravitationally bound companion of the Andromeda Galaxy.