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Discovered by Pierre Méchain on March 20, 1781.
M51 discovered by Charles Messier on October 13, 1773.

Messier: M51.

January 11, 1774. 51. 13h 20m 23s (200d 05' 48") +48d 24' 24"
Very faint nebula, without stars, near the eye of the Northern Greyhound [hunting dog], below the star Eta of 2nd magnitude of the tail of Ursa Major: M. Messier discovered this nebula on October 13, 1773, while he was watching the comet visible at that time. One cannot see this nebula without difficulties with an ordinary telescope of 3.5 foot [FL]: Near it is a star of 8th magnitude. M. Messier reported its position on the Chart of the Comet observed in 1773 & 1774. Memoirs of the Academy 1774, plate III. It is double, each has a bright center, which are separated 4'35". The two "atmospheres" touch each other, the one is even fainter than the other. Reobserved several times.

[Handwritten note in Messier's personal copy of the 1780 version of the catalog:] M. Méchain has seen that nebula double. -- On March 20, 1781, saw this nebula: effectively it is double; the center of the two is brilliant or clear, distinct: and the light of the one and the other touch each other. [Included is a sketch of the two "nebulae" and a star]

Bode: Bode 25.

A small nebula.
On January 5, 1774, I found below (S) the last star Eta in the tail of the Great Bear (UMa), or at the neck of Asterion [the northern Hunting Dog, Canis Venaticus], west and in a triangle with the 23rd and 24th star (after Flamsteed), a small, faintly luminated nebulous patch of slightly oblonged shape. It was only visible with the 7-foot (FL) telescope, and forms a trapezium with 3 small stars west of it, the separation to which I measured with the heliometer, as shown in the first figure of table IV (*).
(*) This and the other figures show the relative positions of the nebulous stars as seen with an astronomical telescope, or reversed.

William Herschel:

[From Phil. Trans. 74 (1784), p. 437-451; mis-interpretation as cluster]
.. For instance, [29 Messier objects including M51], all which are said to be nebulae without stars, have either plainly appeared to be nothing but stars, or at least to contain stars, and to shew every other indication of consisting them entirely.

[1811: PT Vol. 1811, p. 226-336; here p. 285]
8. Of double Nebulae with joined Nebulosity In addition to the instances referred to in the preceding article [Of Nebulae which are brighter in more than one Place], of nebulae that have more than one centre of attraction I give the following list of what may be called double nebulae. (*) See [15 nebulae, including M51]

H I.186 (M51B = NGC 5195):
H I.186. May 12, 1787. cB. pL. R or lE. vgbM. 3' np. the 51st of the Conn.des Temps.
Considerably bright. Pretty Large. Round or little elongated. Very gradually brighter toward the middle. 3' north preceding [NW] of the 51st of the Connoissance des Temps [M51].

[Unpublished Observations of Messier's Nebulae and Clusters. Scientific Papers, Vol. 2, p. 654]
1783, Sept. 17. 7 feet, 57. Two nebulae joined together; both suspected of being stars. Of the most north [H I.186, M51B (NGC 5195)] I have hardly any doubt. 7 feet, about 150. A strong suspicion next to a certainty of being stars. I make no doubt the 20 ft. will resolve them clearly, as they want light and prevent my using a higher power with this instrument.
1783, Sept. 20. 20 ft., 200. Most difficult to resolve, yet I do no longer doubt it. In the southern nebula [M51 (NGC 5194)] I saw several stars by various glimpses, in the northern [M51B (NGC 5195)] also three or four in the thickest part of it, but never very distinctly. Evening very bad.
1787, May 12 (Sw. 734). Bright, 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 three or four places, forming altogether a most curious object. [H] I.186 [M51B (NGC 5195)] B. R. S. vgbM. [bright, round, small, very gradually brighter to the middle] just north of the former.
1788, April 29 (Sw. 836). vB. L. [very bright, large], surrounded with a beautiful glory of milky nebulosity with here and there small interruptions that seem to through the glory at a distance. [H] I.186 [M51B (NGC 5195)] cB. pL., a little E. [considerably bright, pretty large, a little elongated], about 3' p. [preceding, W] Mess. 51 and about 2' more north. (*)
(*: Dreyer's note: These are the only observations recorded of the great spiral.)

John Herschel (1833): h 1622.

h 1622 = M51.
Sweep 255 (April 26, 1830)
RA 13h 22m 37.1s, NPD 41d 56' 9" (1830.0) [Right Ascension and North Polar Distance]
A very bright round nucleus surrounded at a distance by a nebulous ring (See fig. 25, and the Note on this neb in the Appendix).

Sweep 140 (March 20, 1828)
RA 13h 22m 38.5s, NPD 41d 56' 13" (1830.0)
A most astonishing object, &c. (See Appendix as above).

Sweep 257 (May 13, 1830)
RA 13h 22m 38.9s, NPD 41d 54' 53" (1830.0)
A nucleus and double or divided ring, &c. &c.

Sweep 329 (March 7, 1831)
RA 13h 22m 40.0s, NPD 41d 55' 36" (1830.0)
Place of the nucleus. The rings barely discernible for a haze.

Sweep 138 (March 17, 1828)
RA 13h 22m 41.5s, .... (1830.0)
p B; E; v g b M; seen through cloud
Pretty bright; extended; very gradually brighter toward the middle; seen through cloud.

Sweep 256 (April 27, 1830)
RA 13h 22m 43.8s::, NPD 41d 57' 28"::: (1830.0)
(See Appendix)

[Appendix]

[Figure on Plate X, Figure 25, No. 1622, M. 51, RA 13h 22m 39s, NPD 41d 56']
Fig. 25. M. 51 - This very singular object is thus described by Messier: - "Nébuleuse sans étoiles." "On ne peut la voir que difficilement avec une lunette ordinaire de 3 1/2 pieds." "Elle est double, ayant chacune un centre brillant eloigné l'un de l'autre de 4' 35". Les deux atmosphères se touchent." By this description it is evident that the peculiar phenomena of the nebulous ring which encircles the central nucleus had escaped his observation, as might have been expected from the inferior light of his telescopes. My Father describes it in his observations of Messier's nebulae (which are not included in his catalogues,) as a bright round nebula, surrounded by a halo or glory at a distance from it, and accompanied by a companion; but I do not find that the partial subdivision of the ring into two branches throughout its south following limb was noticed by him. This is, however, one of its most remarkable and interesting features. Supposing it to consist of stars, the appearance it would present to a spectator placed on a planet attendant on one of them excentrically situated towards the north preceding quarter of the central mass, would be exactly similar to that of our Milky Way, traversing in a manner precisely analogous the firmament of large stars, into which the central cluster would be seen projected, and (owing to its distance) appearing, like it, to consist of stars much smaller than those in other parts of the heavens. Can it, then, be that we have here a brother-system bearing a real physical resemblance and strong analogy of structure to our own? Were it not for the subdivision of the ring, the most obvious analogy would be that of the system of Saturn, and the idea of Laplace respecting the formation of that system would be powerfully recalled by this object. But it is evident that all idea of symmetry caused by rotation on an axis must be relinguished, when we consider that the elliptic form of the inner subdivided portion indicates with extreme probability an elevation of that portion aboce the plane of the rest, so that the real form must be that of a ring split through half its circumference, and having the split portions set asunder at an angle of about 45deg each to the plane of the other.

Smyth: CCCCLXXXV [485]. M51.

CCCCLXXXV. 51 M. Canum Venaticorum.
AR 13h 23m 06s, Dec N 48d 01'.7
Mean Epoch of Observation: 1836.69 [Sep 1836]
A pair of lucid white nebulae, each with an apparent nucleus, with their nebulosities running into each other, as if under the influence of a condensing power. They are near the ear of Asterion, the northern hound; and the smaller nebula, or northern one, having the brightest nucleus, was differentiated by the wire micrometer; they are 3deg southwest of Alkaid [Eta UMa], where the place is indicated by a line from Dubhe [Alpha UMa] through Megrez [Delta UMa], extended nearly twice that distance into the southeast beyond. There are three telescopic stars following, and a bright 7th-magnitude about as far beyond them as they are from the nebulae. but the preceding part of the field is quite clear. Sir John Herschel has given a beautiful representation of this extraordinary object, [fig.] No. 25, in the illustrations to his Catalogue of 1830.
This fine field was discovered by Messier in 1772 [actually 1773], and described as a faint double nebula whose centres are 4'35" apart, but with "the borders in contact." The southern object is truely singular, having a bright centre surrounded with luminosity, resembling a ghost of Saturn, with his ring in vertical position. They form Nos. 1622 and 1623 of [J.] H.'s catalogue, who terms the southern, or halo nebula, a most astonishing object, probably a similar system to our own, the halo representing the Galaxy. "Supposing it," he remarks, "to consist of stars, the appearance it would present to a spectator placed on a planet attendant on one of them, excentrically situated toward the np quarter of the central mass, would be exactly similar to that of our Milky Way, traversing, in a manner precisely analogous, the firmament of large stars, into which the central cluster would have been projected, and (owing to its greater distance) appearing, like it, to consist of stars much smaller [fainter] than those in other parts of the heavens. Can it then be that we have here a brother-system, bearing a real physical resemblance and strong analogy of structure to our own?"
We have then an object presenting an amazing display of the uncontrollable energies of the OMNIPOTENCE, the contemplation of which compels reason and admiration to yield to awe. On the outermost verge of telescopic reach we perceive a stellar universe similar to that to which we belong, whose vast amplitudes no doubt are peopled with countless numbers of percipient beings; for those beautiful orbs cannot be considered as mere masses of inert matter. And it is interesting to know that, if there be intelligent existence, an astronomer gazing at our distant universe, will see it, with a good telescope, precisely under the lateral aspect which theirs presents to us. But after all what do we see? Both that wonderful universe, our own, and all which optical assistance has revealed to us, may be only the outliers of a cluster immensely more numerous. The millions of suns we perceive cannot comprise the Creator's Universe. There are no bounds to infinitude; and the boldest views of the elder Herschel only placed us as commanding a ken whose radius is some 35,000 times longer than the distance of Sirius from us. Well might the dying Laplace explain: "That which we know is little; that which we know not is immense."

Lord Rosse

[Phil. Trans. 1844, p. 321-324, on his observation with his 3-feet (36-inch) aperture telescope]
Fig. 25 [of JH (1833)] abounds in stars mixed with nebulosity; I have not yet seen it on a very fine night, but it was observed by my assistant, and a gentleman who was with him, and they had no doubt but that the centre was completely resolved.

[Phil. Trans. 1850, p. 499-514, drawing on plate XXXV, Fig. 1; on his observation with his 6-feet (72-inch) aperture, 53-ft FL "Leviathan" telescope]
[p. 505] The spiral arrangement of 51 Messier was detected in spring 1845.
[p. 509] Plate XXXV. figs. 1 and 2 are seen on a scale of half an inch to a minute; ...
Plate XXXV. fig. 1, H. 1622. - This object has been observed twenty.eight times with the 6-inch instrument; it has been repeatedly observed with the 3-feet instrument.
September 18, 1843. - Observed with the 3-feet instrument; power single lens, 1-inch focus; a great number of stars clearly visible in it, still Herschel's ring not apparent, at least no such uniformity as he represents in his drawing.
April 11, 1844. - Observed with the 3-feet instrument, two friends assisting; both saw the centre clearly resolved.
April 26, 1848. - 6-feet instrument. Saw the spirality of the principal nucleus very plainly; saw also spiral arrangement in the smaller nucleus.
The following measurements were taken by my assistant, Mr. Johnstone Stoney, in the spring of 1849 and 1850.

                                    Greatest                            Greatest
            Mean of the   No. of   difference   Mean of the   No. of   difference
            observations  obser-   between ob-  observations  obser-   between ob-
            of position.  vations  servations   of distance   vations  servations
                                  and the mean                        and the mean
  

                d  '                  d  '        '  "                    "
  N. n.        16 34         4        3 27        4 22.2         4        9.6
  N. 1.        52  4         1        ....        2  6.6         1
  N. 2.        54  0         4        1 57        5  0.0         4        5.4
  N. 3.       104 20         2        2  3        2 45.6         2        3.6
  N. 4.       111 57         2        0 40        4  3.6         2        0.6
  N. 5.       165 35         2        0 31        1 43.2         2        1.1
  N. 6.       191 42         1        ....        3 54.0         1
  N. 7.       211  2         1        ....        2 36.6         1
  N. 7, 8.    270 42         1        ....        0 34.8         1
  N. 9.       231 32         4        3 35        1 23.4         3        6.6
  N. 9, 10.   197 57         1        ....        0 27.0         1
  N. 11.      279 21         4        4 18        1 49.8         3       22.2
  N. 11,12.   225 27         1        ....        0 12.6         1
  N. 13.      281 37         2        0 22        3 59.0         1
     14, 15.  297 15         1        ....        
  N. 15.      310 34         4        4 17        2 55.8         4       13.8

  N. Alpha                                        3 22.8         2        0.1
  N. Beta       5  7        ...       ....        1 28.2         3        3.0
  N. Gamma                                        2 37.8         3        2.4

  N. Delta                                        1 46.2         1
  N. Epsilon                                      2 46.7         1
  N. Zeta      95  7        ...       ....        1 40.8         1
  N. Eta                                          3 15.6         1
  

Observations. - There is a great discrepancy between the measured position of 11 and 12 and the rough diagram made at the time of observation.
N. 13 is twice noticed in the observing-book.
Once N. 11,13 is taken as one position; the other times N. 11 and 13 are taken separately, N. 13 being made 1d 40' less than N. 11; hence 270d 31' is a more probable position for N. 13 than that given in the Table.
The Greek letters are perpendiculars from N. on tangents to the outside of the convolutions, the tangents from Alpha, Beta and Gamma being vertical, that is, parallel to the position 95d 7', and those for Delta, Epsilon, Zeta, Eta horizontal, i.e. parallel to position 5d 7'.
The greater part of the observations were made when the eye was affected by lamp-light, which made it difficult to estimate correctly the centre of the nucleus; it was of importance that no time should be unnecessarily spent, and after the lamp had been used a new measure was taken, as it was judged that the object was sufficiently seen. With the brighter stars this would frequently happen before the nucleus was well defined, as all impediments to vision seem to affect nebulae much more than stars the light of which would be estimated as of the same intensity. In the foregoing list the greatest discrepancies are in the measures of bright objects, and this is probably the proper account of it. No stars have been inserted in the sketch which are not in the table of the measurements. The general appearance of the object would have been better given if the minute stars had been put in from the eye-sketch, but it would have created confusion.

John Herschel, General Catalogue: GC 3572.

GC 3572 = h 1622 = M51.
RA 13h 23m 55.4s, NPD 42d 5' 4.0" (1860.0). [Right Ascension and North Polar Distance]
!!!; nucl & ring (h); spiral (R). 10 observations by W. & J. Herschel.
Magnificient; nucleus and ring (JH); spiral (Rosse)
Remark: Figures in P.T. 33 [JH 1833], plate ii, fig. 25; P.T. 50 [Lord Rosse 1850], plate xxxv, fig. 1; woodcut diagram in Rosse 1861.

GC 3574 (NGC 5195):
GC 3574 = h 1623 = H I.186.
RA 13h 24m 4.4s, NPD 42d 0' 50.7" (1860.0).
B; pS; R; vgbM; f of 2. 6 observations by W. & J. Herschel.
Bright; pretty small; very gradually brighter toward the middle; following [Eastern component] of 2.

Huggins

[Further Observations on the Spectra of some Nebulae, with a Mode of determining the Brightness of these Bodies. Phil. Trans. Roy. Soc., Vol. 156 (1866), p. 381-397; here p. 388-389]
[No. [GC] 3572. 1622 h. 51 M. R.A. 13h 23m 55s.4. N.P.D. 42d 5' 4". Remarkable; nucleus and ring (h); spiral (R).]
"The outer nucleus unquestionably spiral with a twist to the left." - Lord Rosse
"Both nuclei resolved; brighter parts of spiral branches suspected to be resolvable. Stars innumerable, though I feel satisfied that it is not a cluster." - Lord Oxmantown [Rosse]
"Nos observations n'accusent aucun changement dans la position relative des deux têtes dans l'intervalle de 13 ans." [Our observations don't accuse any change in the relative position of the two heads [nuclei] in an interval of 13 years.] - O. Struve
Each of the bright centres brought successively upon the slit. Spectrum continuous. A suspicion that some parts of the spectrum were abnormally bright relatively to the other parts.

Lassell

[Memoirs of the Royal Astronomical Society, Vol. XXXVI (36)]
[Drawing on Plate VI, Fig. 27]

Dreyer (1877)

GC 3572, h. 1622 [M 51]. Drawings in Lassell, Plate VI, Fig. 27.

Dreyer: NGC 5194.

NGC 5194 = GC 3572 = h 1622; M 51.
RA 13h 23m 58s, NPD 42d 4.9' (1860.0). [Right Ascension and North Polar Distance]
!!!, Great Spiral neb; = M51
Magnificient, great spiral nebula.
Remark: Figures (together with NGC 5195) in P.T. 33 [JH 1833], plate X, fig. 25; P.T. 50 [Lord Rosse 1850], plate XXXV, fig. 1; Lass 2 [Lassell, Memoirs R.A.S. vol. xxxvi], plate VI, fig. 27; Ld R [Observations of Nebulae and Clusters at Birr Castle, 1848-78 (Transactions Royal Dublin Society, vol ii, 1880)], plate IV (also [woodcut] diagram); Vogel 2 [Publicationen des astrophys. Observatoriums zu Potsdam, vol. iv, Part I (Observations with the 27-inch Vienna Refractor)], plate III.

NGC 5195:
NGC 5195 = GC 3574 = h 1623 = H I.186.
RA 13h 24m 5s, NPD 42d 0.6' (1860.0).
B, pS, lE, vgbM, inv in M51.
Bright, pretty small, little extended [elongated], very gradually brighter toward the middle, involved in M51.
Remarks see above, M51.

Curtis

[Descriptions of 762 Nebulae and Clusters photographed with the Crossley Reflector. Publ. Lick Obs., No. 13, Part I, p. 9-42]
NGC 5194 and 5195, RA=13:25.7, Dec=+47:43. [Publ. Lick Obs.] Vol. VIII, Plate 47. The beautiful spiral M.51 in Canes Venatici. Including very faint matter to the north of [NGC] 5194, scarcely visible in any of the very numerous published reproductions, it covers an area about 12'x6' in p.a. approximately 30deg. A sharp stellar nucleus in [NGC] 5194, and the whorls show a multitude of stellar condensations. The satellite nebula, [NGC] 5195, has a bright, elongated nucleus; its nebulosuty is of a more diffuse type, without discernible spiral structure, and with several rifts which suggest absorption effects. See Abs. Eff. 22 s.n.