Sky and Telescope - May 2012 - (Page 57)

Sue French welcomes your comments at scfrench@nycap.rr.com.

Sue French

Messier 91 is a classic example of a barred spiral galaxy.

ern end. M88’s halo covers about 4½′ × 2′ and enfolds a small, brighter core with a stellar nucleus. I can see, but not quite hold constantly, an extremely dim star superposed on the galaxy’s southeastern tip. In my 10-inch reﬂector at 192×, M88 spans 5½′ × 2½′ and grows gradually brighter toward a small, oval, inner core. Brightness variations in the halo suggest spiral arms that unwrap clockwise. The superposed star now accompanies a fainter companion to its south-southwest. Deep images show that the companion is a snug (1.9″) double star. It might take a very large telescope to separate its 15th-magnitude components. If you give it a try, please let me know how well you succeed. The galaxy M91 is only 50′ east of M88, with the two visible in the same binocular ﬁeld of view. However, M91 is not an easy binocular object. The smallest binoculars through which I’ve spotted it are 18×50s with image stabilization. Even then it was simply a small, very faint smudge that took a little while to spot. Through a telescope, you can follow part of Smyth’s starry pageant from M88 to M91. The star hovering above M88’s northwestern end is the ﬁrst in an arc of three that curves northeast, each star brighter than the one before. Farther east, three more stars curve back down to M91, the ﬁnal one lying oﬀ the galaxy’s western side. My 105-mm refractor at 28× shows M88 and M91 in the same ﬁeld, with M91 as a faint spot. At 87× the galaxy grows a small round core with a stellar nucleus. The core has barlike extensions that run east-northeast to west-southwest. M91 covers about 2′, and its core is less than half that.

My 10-inch scope at 192× reveals a faint wisp reaching north from the western extension of M91 and another reaching south from the opposite extension. The wisps quickly fade to a very faint halo that’s somewhat oval and aligned perpendicular to the bar. The oval is about 2½′ wide, but its length is diﬃcult to judge because the halo is rather dim. My 14.5-inch reﬂector at 170× makes the halo appreciably easier and puﬀs it out to about 5′ × 3½′. There’s no object at the coordinates given for M91 by its discoverer, Charles Messier, so M91 was long considered a “missing” object. Various hypotheses were put forward in a celestial version of a Where’s Waldo? hunt. Astronomers and historians suggested that M91 might be NGC 4571, a passing comet, or a duplicate observation of M58. But it was an amateur astronomer from Texas who came up with

Galaxies in Southern Coma Berenices
Object
Messier 88 Messier 91 NGC 4516 IC 3476 NGC 4571

Mag(v)
9.6 10.2 12.8 12.7 11.3

Size
6.9′ × 3.7′ 5.4′ × 4.3′ 1.9′ × 0.8′ 2.1′ × 1.8′ 3.6′ × 3.2′
h

RA
12 32.0 12 35.4
h m

NOAO / AURA / NSF

Dec.
+14° 25′ +14° 30′ +14° 35′ +14° 35′ +14° 13′

12h 33.1m 12h 32.7m 12 36.9
h m

Angular sizes are from recent catalogs. Visually, an object’s size is often smaller than the cataloged value and varies according to the aperture and magniﬁcation of the viewing instrument. Right ascension and declination are for equinox 2000.0.

Sk yandTelescope.com May 2012 57

Table of Contents for the Digital Edition of Sky and Telescope - May 2012

Sky and Telescope - May 2012
Table of Contents
Spectrum
Letters
75, 50 & 25 Years Ago
News Notes
Saturn’s Raging Superstorm
Stars Above, Earth Below: Astronomy in National Parks
The Remarkable Case of Comet Lovejoy
In This Section
May’s Sky at a Glance
Binocular Highlight
Planetary Almanac
Northern Hemisphere’s Sky
Sun, Moon, and Planets
Celestial Calendar
Exploring the Solar System
Deep-Sky Wonders
Going Deep
S&T Test Report
New Product Showcase
Telescope Workshop
Planetary Imaging with Your DSLR Camera
Gallery
Focal Point

Sky and Telescope - May 2012