Messier 51, the Whirlpool Galaxy, is a face-on grand-design spiral in Canes Venatici, a short step from the handle of the Big Dipper. Two bright arms sweep out of a compact core and reach a smaller companion, NGC 5195, which is passing behind the disk. Tides from that long encounter keep the arms sharp and light them with young star clusters and pink knots of hydrogen.
Charles Messier recorded the galaxy on 13 October 1773. In the spring of 1845, Lord Rosse traced its spiral with the great reflector at Birr Castle, the first time spiral structure was recognized in any galaxy. The pair is also known as Arp 85; the main galaxy is NGC 5194.
NASA places M51 about 31 million light-years away. Catalog and Cepheid estimates put it nearer, around 23 to 25 million light-years. On that nearer scale the bright disk is about 77,000 light-years across, a little smaller than the Milky Way. At magnitude 8.4 the glow is within reach of binoculars from a dark site. The arms themselves appear in a modest telescope and in long-exposure photographs such as these, including a finished portrait of the pair and a frame with the field stars recomposed over the galaxy.
Facts
- Face-on grand-design spiral (NGC 5194) with companion NGC 5195, in Canes Venatici
- About 31 million light-years (NASA); nearer estimates are 23–25 million light-years
- Bright disk about 77,000 light-years across on the nearer distance
- Apparent size 11.2′ × 6.9′; visual magnitude 8.4
- Spiral structure first recognized by Lord Rosse in 1845
- J2000 coordinates: 13h 29m 53s, +47° 11′ 43″
