Object of the Week March 17th, 2019 – Messier 91 A missing Messier for 188 years!

M91
NGC 4548

Galaxy SBb(rs)

Coma Berenices

RA 12 35 26.4
DEC 14 29 47

Size: 5’.4” x 4’x3”

Mag: 11.0

Located at the southern end of Coma Berenices, approximately half way between Denebola (Beta Leonis) of Leo and Vindemiatrix (Epsilon Virginis) of Virgo, lies M91. Discovered by Charles Messier on March 18th, 1781 (238 years to the day after this should have been posted!), M91 is a beautiful barred spiral in images yet a rather challenging object in the eyepiece but more on that later.

The night Messier discovered this object would also prove to be his most productive night ever. On this night, Messier cataloged 9 objects – 8 galaxies of the Virgo Cluster and M91, a globular cluster in Hercules. In the process of cataloging his discoveries from this evening, Messier may have inadvertently logged M91’s coordinates as those of M58. Until this error was rectified years later, many believed that M91’s coordinates was either that of a comet or of M58.

William Herschel cataloged M91 as HII.120 on April 8th, 1784 but it wasn’t until 1969 when amateur astronomer William C. Williams of the Fort Worth Texas area, uncovered the true identity/location of M91. Williams discovered that when cataloging M91, Messier must have meant that M91’s location was relative to M89. Williams then applied the difference between the two objects, using Epoch 1950, and was able to determine the true coordinates of M91 to within .1’ – effectively verifying that NGC 4548 is actually M91. Williams then wrote to the 1969 December issue of Sky & Telescope magazine explaining the following…

“It can be simply demonstrated that the lost Messier object M91 is very probably the galaxy NGC 4548, at right ascension 12h 32m.9, declination +14d 46′ (1950 coordinates).
As Owen Gingerich pointed out in his article “The Missing Messier Objects” (Sky and Telescope, October, 1960, page 196), M91 was the last of a group of eight nebulae observed by Charles Messier on the night of March 18, 1781. The French astronomer described it as a “nebula without stars” and fainter than M90. He gave its position as 12h 26m 28s, +14d 57′ 06″; precessed to 1950, this is 12h 35m.0, +14d 02′. In what follows, 1950 coordinates are used.

My solution of the puzzle assumes that Messier determined the position of NGC 4548 by measuring its right ascension and declination relative to those of the nearby galaxy M89 (since there are no suitable reference stars in the vicinity):

NGC 4548 12h 32m.9 +14d 46′
M89 12h 33m.1 +12d 50′
difference -0m.2 +1d 56′

It is further assumed that, in calculating the coordinates of the new object, by mistake he applied the observed differences to M58, a 9th-magnitude galaxy Messier had recorded two years earlier:

M58 12h 35m.1 +12d 05′
difference -0m.2 +1d 56′
“M 91” 12h 34m.9 +14d 01′

It reproduces the Messier position to 0m.1 in right ascension and 1′ in declination.

The Skalnate Pleso Atlas Catalogue gives the visual magnitudes of NGC 4548 and M90 as 10.8 and 10.0, respectively. This checks with Messier’s statement that M91 was the fainter of the two. The same source gives the size of NGC 4548 as 3.7 by 3.2 minutes of arc.”

So after 188 years, it is now universally recognized that M91 is the same as NGC4548 and is at the coordinates I list above in my header.

Visually, M91 can be a rather difficult object, particularly in smaller telescopes and/or poor sky conditions. Scopes in the 8” realm reveal a round, diffuse glow with a condensed core. Scopes north of 18” tend to reveal a brighter oval glow with a bar orientated NE to SW with apparent arms surrounded by a diffuse halo. Most observations will reveal a brighter core easily visible in all manner of aperture.

So, there you go, a 200 year mystery solved. Sort of. I’m sure I don’t do the history justice and someone like Steve Gottlieb and his expertise can definitely elaborate even further as to the mystery surrounding M91. But until then, be sure to give this object a fair shake. It’s a beautiful springtime barred spiral worthy of a look.

And as always….

“Give it a go and let us know! Good luck and great viewing!”

Messier-91 _Adam Block.jpg
Copyright Adam Block UA