Charles Messier discovered M51 = NGC 5194 = h1622 on 13 Oct 1773 with a 3.5-inch refractor. Johann Bode made an independent discovery on 5 Jan 1774, along with the first drawing using his 3-inch refractor. Using his 12-inch (20-ft focal length) on 20 Sep 1783, William Herschel commented (clearly interested if it was a cluster), "most difficult to resolve; yet I no longer doubt. In the southern nebula I saw several stars by various glimpses, in the northern also 3 or 4 in the thickest part of it, but never very distinctly." Viewing with his 18.7-inch on 29 Apr 1788, he described "Two, the most south vB, L, surrounded with a beautiful glory of milky nebulosity with here and there small interruptions that seemed to show the glory at a distance." On 12 May 1787 he recorded "B, a very uncommon object. Nebulosity in the center, with a nucleus surrounded by detached nebulosity in the form of a circle; of unequal brightness in 3 or 4 places; forming altogether a most curious object." He apparently never observed M51 with his 48-inch reflector (40-foot focal length). John Herschel described M51 as "A very bright round nucleus surrounded at a distance by a nebulous ring" and as a "double ring, or rather one-and-half rings rather like an armillary sphere."

M51 was the first galaxy in which spiral structure was clearly seen. The discovery was made by Lord Rosse (William Parsons) using his newly completed 72-inch Leviathan in the spring of 1845 (replacing the earlier 36-inch scope). No observing logs were kept of the earliest observations, so the exact date in unknown, but spirality was not reported by Romney Robinson (director of the Armagh Observatory) and James South (double star observer) during their observation on 5-6 March 1845, which focused on resolvability. In early April 1845 (possibly the 6th), William Parsons observed alone and "discovered" the spiral structure, producing a stunning sketch that was circulated at the British Association for the Advancement of Science in Cambridge in June 1845. By 1850, M51 had been observed at least 28 times and a second, more dynamic sketch implying motion, was published in LdR's 1850 "Observations of the Nebulae" (plate XXXV, figure 1).

Probably using LdR's drawing as a "guide", William Lassell sketched the spiral structure and connecting arm on 12 May 1846 (in his notebook) with his 24" equatorially mounted reflector. He made two detailed sketches using his 48" in 1862 from Malta, showing slightly variations in the spiral structure. Jean Chacornac made an excellent pencil sketch (unpublished) in 1862 using the 31" silvered-glass reflector of the Paris Observatory. Using only a 9.6" refractor at Rome, Father Angelo Secchi claimed "even in our telescope it is easy to recognize the spiral and the two branches can be followed very well." (he often compared his telescopic views favorably with LdR's and Lassell's).

A bitter debate between Wilhelm Tempel and Dreyer began in 1878 after Tempel was highly critical of Lord Rosse's and William Lassell's sketches of spiral structure. Tempel had only observed M51 through the 11" refractor at Arcetri, and although he sketched curving arcs in the halo, he felt Rosse and others were interpretating these features as spiral forms. He wrote "one cannot fend off the thought that these forms and shapes are only figments of the imagination...". On 29 Apr 1889 Isaac Roberts obtained the first photograph of M51, ending the debate.

As far as the origin of the nickname "Whirlpool Galaxy", Ormsby Mitchel's Sidereal Messenger column in November 1847 (Vol. 2, No. 4) announced "Lord Rosse's Whirlpool Nebula" and included a copy of his sketch. The following year Romney Robinson described spiral nebulae "... resemblance to bodies floating on a whirlpool is, of course, likely to set imagination at work..."

200/250mm - 8" bright, large, hint of spiral arms.

300/350mm - 13.1" (4/24/82): very bright, very large, bright nucleus. Two winding spiral arms are obvious with a dark gap between the arms on the west side. The connecting arm to NGC 5195 is definite although near my visual threshold. There is a sharp bend in the outer arm at the south end of the galaxy. After this point, the arm trails faintly north to NGC 5195 located 4.6' from center.

400/500mm - 17.5" (3/28/87): stunning spiral structure, connecting arm visible with direct vision. First observation with 17.5" on 23 Mar 1985.

900/1200mm - 48" (5/12/12, 4/7/13, 4/30/19, 5/3/19): during these observations through Lowrey's 48" I focused on some of the stellar clusters and brighter knots in the spiral arms, which have a very high contrast with the large, dusty, darker regions between the arms. The "southern" arm nearly merges with the core on the northeast side. Two clusters and a cluster/HII region were seen as faint "stars" very close east and northeast of the core. [BGG2008] A1 is a mag 17.5 "star" 30" NE of center and similar [BGG2008] 3cl-b is 30" E of center. The designations are from a 2008 study by Bastian et al. A stellar HII region ([CCM69] 37A) is only 10" NE of 3cl-b.

As this arm unwinds clockwise to the west, two close very small, fairly bright knots ([CCM69] #77 in Carranza, Crilon and Monnet's "Kinematic Study of Ionized Hydrogen in M51" in A&A, 1, 479) are visible 1.6' W of center, just SE of a star. A small bright knot (#71) is further out on this arm, 2.3' SW of center. The section of the arm between these knots is quite bright and mottled. The southern arm then continues to swing around on the east side and heads north, with a lower surface brightness bridge to NGC 5195.

The "northern" arm begins on the south or southwest side of the core, and a series of small knots (#60/67) are along the inner south side, roughly 1' from center. As the arm unwinds on the east side of the core, a large bright knot (#52) resides ~1.4' ESE of center. The arm is quite bright in a large, clumpy region (#27/29) as it heads north in the direction of NGC 5195. At the closest point to NGC 5195 are 4 very small knots; the brightest is #10, 2.6' NNE of center. Very close southwest is #8 and just northwest is #5. This arm has a sharp bend and shoots west on the north side and includes a moderately large, brighter patch (#90/91) 2' NNW of center. The arm fades somewhat as it unwinds further along the western edge of the halo.

48" (4/2/11): during this observation I focused on the attached companion NGC 5195. The entire connecting arm was always a prominent direct vision feature of the pair with variations in brightness and width along its length. The arm brightened as it connected to NGC 5195. The slightly oval core was extremely bright but nevertheless a very bright stellar nucleus punctuated the center. The spiral arms had an extremely high contrast and appeared etched in the eyepiece like a photograph with a great deal of knotty structure. The outer arm that sweeps clockwise to the north and then around the west side extended much further south (~5' from center) than I had ever previously seen and separated quite a bit from the main spiral. Sprays of nebulosity looped off of NGC 5195 to the north (see notes).

Notes by Steve Gottlieb