Harold Corwin

NGC 4571 = IC 3588. Found by William Herschel, and reobserved by John Herschel and d'A, the galaxy's NGC position is pretty well determined. Nevertheless, Schwassmann measured the 14th magnitude star superposed about an arcmin west of the nucleus and called it N4571 in his 1902 survey of the Virgo Cluster. He also picked up the galaxy itself at its correct position, and included it in his list as a new nebula. Thus, the equality of the numbers.

This galaxy (also = GC 3113) has also occasionally been mentioned as a possible candidate for M 91, first by Dreyer himself in the NGC. See his note there where he equates NGC 4571 with GC 3120 = h1367 (more below) and actually favors the idea the M 91 was a comet. He must have realized that NGC 4571 is too faint (it has V_T = 11.3 in RC3) to have been seen by Messier. M 91 is almost certainly NGC 4548; see that and NGC 4579 = M 58 for more.

John Herschel was curious about M 91, too. He gave it a place in his 1833 catalogue (it is h1367) using Messier's position and saying

A bright * 9m, and 2 or 3 smaller; close by the B star and sp it, is a small well defined [sic] body which may be a close double star, and np it is also a F neb. The place set down is that of Messier's 91st neb, but I do not think this can be that object, whose existence even seems questionable.

In the GC (it is GC 3120), he is even more definite: "np this place is a F neb; not [John Herschel's italics] M. 91, whose existence ?."

The idea of M 91 as a comet was eventually taken up by Flammarion in his lists of Messier's objects (Bull. Soc. astr. France, 31-35, 1917-21), and by Shapley and Davis (PASP 29, 177, 1917; they refer back to NGC 4571). See also Glyn Jones, "The Search for the Nebulae", Alpha Academic, 1975; and his "Messier's Nebulae and Star Clusters", Cambridge, 1991 for more on M 91.

― NGC Notes by Harold Corwin