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

[Dragan]

Object of the Week March 17^th, 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 18^th, 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 8^th, 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

[Ivan Maly]

Here is my observation from 7 years ago (12" F/5, sky brightness 21.7 mag/arcsec^2 estimated):

"~63-190x with 8-24 mm Pentax. Roughly round overall, stars SE, E, and N. Small round nucleus, WSW-ENE bar about half the diameter of the halo. E end connects to a short concentric arc that extends slightly N of the bar end, and to the S almost to a small compact enhancement. The symmetrical arc is slightly separated outwards from the W bar end but similarly "overhangs" it with its S end. This arc starts concentrated and sharp in the NW sector, then gets dimmer and broadens into a sweeping arm that extends along the N and NE edge of the halo. It ends outwards from the ENE end of the bar, slightly overlapping with the beginning of the first arc."

I note that in the image in Dragan's post, S is to the left.

[Bertrand Laville]

Hi All,

Here is my drawing with a 25" Obsession, and my report:
http://www.deepsky-drawings.com/m-091-t635/dsdlang/fr

It's a galaxy more difficult than I had imagined; to G, it is only a round spot, blurred, view V1.

Ethos 13mm x240 and x390 Ethos 8mm

The two magnifications are equivalent. The bar, L4, was seen without the know and in the proper alignment. The direction of the turns was suspected without knowing him, and in the right direction.
After displaying the image of Capella Observatory, the arms remain very difficult, almost limit; there is only half isophote, L2.5, more than the bottom of halo, L2. A slight increase in the base of the arm E and an HII region, R1, very difficult at the base of the spire W. [Note 22 January 2013: This is actually a set of HII regions ranging from R1a to R1b

R1a NGC 4548 [BKB2006] 0051, and R1b NGC 4548 [BKB2009] 0074, with [BKB2006] BRADLEY TR; KNAPEN JH; BECKMAN JE; Folkes SL 2006A & A ... 459L..13B - Astron. . Astrophys, 459, L13-16 (2006) - 04.12.06 21.08.07 November (III) 2006; A composite HII area in luminosity function Ha of Unprecedented statistical weight.
In conclusion, M 91 is a tough galaxy T 635!

Clear skies
Bertrand
http://www.deepsky-drawings.com

M-91-T635-BL-2012-12-12.jpg

[Uwe Glahn]

Nice story indeed. Perhaps one of the first astronomical faults in history?

The Smallest aperture I caught M 91 was a 8x30 binocular. I noted "unsure observation, popping at the right position, not steadily visible". The 16-inch brings out much more detail like a bright halo around an even brighter core with hints of the spiral structure. The 27-inch shows a spectacular view with easy spiral arms and tow companions in the neighborhood.

sketch: 27", 366x, NELM 7m0+, Seeing I
M91.jpg