The Pinwheel Galaxy

Degree of difficulty 3 (of 5)

Minimum aperture 50mm

Designation NGC 5457

Type Galaxy

Class Sc

Distance 21.8 Mly (H2000) 24.1 Mly (2006) 23.5 Mly (2001) 21.9 Mly (Cepheids, 1996)

Size 184,000 ly

Constellation Ursa Major

R.A. 14h 3.2min

Decl. +54° 21′

Magnitude 7.7

Surface brightness 23.7mag/arcsec2

Apparent diameter 28.8′×26.9′

Discoverer Méchain, 1781

History M 101 was discovered on the 27th of March 1781, by Pierre Méchain. He described the new find as “Nebula without star, very obscure and pretty large, 6′ to 7′ in diameter.” This galaxy was found just before the final version of Messier's catalog was going into print. In a last-minute effort, M 101 made it into an appendix, together with two more objects found by Méchain that night (M 102 and M 103).

On the 14th of April 1789, William Herschel observed M 101 and discovered three nebulous spots within its glow. These are now known as NGC 5461, 5462, and 5447. William Smyth got very inspired by this object, although misled about its nature, when he wrote in the 1830s: “It is one of those globular nebulae that seem to be caused by a vast agglomeration of stars, rather than by a mass of diffused luminous matter; and though the idea of too dense a crowd may intrude, yet the paleness tells of its inconceivable distance, and probable discreteness.” Lord Rosse finally noticed and sketched the spiral structure of M 101 with its knots, and he made out a diameter of 14′.

To d'Arrest, who had a much smaller telescope at his disposal, this galaxy looked much different: “The outer margin is very faint and difficult to perceive. The nebula is not quite round, its nucleus diffuse. This is a complex object with two centres linked to each other; that, however, is not easily recognized.”