The Whirlpool Galaxy owes its perfect spiral to a smaller neighbour
M51, the Whirlpool Galaxy, was the first object ever seen to have spiral arms, and its graceful shape is probably the work of a smaller companion. NGC 5195 appears to have punched through the Whirlpool's disk around 500 to 600 million years ago and again perhaps 50 to 100 million years ago, stirring up the arms we admire today.
Charles Messier found it on 13 October 1773 while cataloguing fuzzy objects that might fool comet hunters. In the 1840s William Parsons, the third Earl of Rosse, turned his 72-inch reflector at Birr Castle in Ireland on it and made out a spiral structure, the first such 'nebula' known. Only after Edwin Hubble measured Cepheids in spiral nebulae did astronomers accept these were separate galaxies, and radio maps of gas later proved M51 and its companion are physically linked.
It lies in Canes Venatici, about 3.5 degrees southwest of Alkaid, the star at the end of the Big Dipper's handle. The pair can be glimpsed with binoculars under dark skies; a 150 mm telescope begins to show the spiral, and instruments above 300 mm reveal glowing star-forming regions and the bridge to the companion. North of about 43 degrees latitude it never sets.
Estimates put it 23 to 31 million light-years away and about 76,900 light-years across, roughly 88 percent of the Milky Way's size but only about a tenth of its mass, at 160 billion suns. Its two prominent arms wind clockwise, one bending noticeably off a steady angle. At the centre sits a black hole, partly hidden by dust, feeding an active nucleus that shoots out a pair of ionisation cones. The interaction has also pulled out tidal streamers, the largest reaching 140,000 light-years from the core.
It remains a favourite for big telescopes. In 2005 the Hubble Heritage Project assembled an 11,477 by 7,965 pixel portrait of its arms, and in 2022 the James Webb Space Telescope examined its young star clusters in the mid-infrared.
Source: Whirlpool Galaxy
