Sky and Telescope - May 2012 - (Page 58)

OBSERVING

Deep-Sky Wonders

Left: NGC 4516 is a spiral with a pronounced S-shaped bar. Center: IC 3476 is an irregular galaxy whose blue stars suggest ongoing starburst activity. Right: NGC 4571 is an unusual ringed spiral galaxy.

the accepted solution to this puzzle, ﬁrst published in this magazine’s Letters column for December 1969. William C. Williams found that NGC 4548 ﬁts Messier’s description and position for M91 if you assume that Messier mixed up which object he’d used as his reference point. Thus Messier’s oﬀsets should be applied to M88, not M58. The little galaxy NGC 4516 sits one-third of the way from M88 to M91 and 8′ north of an imaginary line connecting them. With my 105mm scope at 87×, I can see NGC 4516 intermittently with averted vision as a very small glow elongated north-south. (Averted vision is the practice of looking a bit oﬀ to one side of a faint object to allow its light to fall on a more sensitive area of your eye’s retina.) The view through my 10-inch reﬂector at 192× is fascinating. At ﬁrst glance, I was astonished when the galaxy’s tilt seemed to change with averted vision. A more careful look showed a bright region about 1′ long and one-quarter as wide, tipped west of north, that spans a slender, larger oval running nearly north-south. Only the former stood out with direct vision. The bright area is lovely, and consists of a small core with a distinct nucleus at the center of a shallow S curve. Averted vision wraps it all in a delicate cocoon. Let’s drop 24′ south-southeast from M88 to a northsouth pair of 12th-magnitude stars 2.7′ apart. The intriguing galaxy IC 3476 rests 4.4′ east of the northern star. It’s readily visible in my 105-mm refractor at 87× as a hazy blot about 40″ across. In the 10-inch scope at 192×, three faint suns join the 12th-magnitude pair to outline

a candle snuﬀer ready to put out the light of IC 3476. The galaxy displays a lumpy brightness distribution and peculiar shape. IC 3476 is about 40″ wide at its northeastern end and tapers down toward the southwest, giving it a candle-ﬂame proﬁle 1¼′ long. The wide end contains a relatively large, blotchy, bright area, and some additional clumpy enhancements line the galaxy’s southeastern side. Strange little IC 3476 has been assigned a morphological type of IB(s)m — a barred, Magellanic-type irregular galaxy with traces of spiral structure. IC 3476 surprised astronomers with a supernova in 1970. Our ﬁnal target is NGC 4571, found 28′ southeast of M91 with a 9th-magnitude star guarding its northeastern edge. The galaxy appears roundish with a small brighter core in the 105-mm scope at 87×. Its low-surface-brightness halo improves with averted vision and is roughly 2′ to 2½′ across. Although NGC 4571 is more obvious in my 10-inch scope, it discloses no further secrets. NGC 4571 is an unusual galaxy of type SA(r)c. Its ﬂeecy spiral arms reach inward to a well-deﬁned ring that closely circles the galaxy’s core. Such ring structure is rare in a late-type spiral galaxy, that is, one that exhibits a small central bulge and loosely structured arms with prominent condensations. All these wonderful world-isles dwell in the core of the vast Virgo Cluster, possessing whole heavens within themselves that have long inspired the wondering skygazer’s amazement. ✦

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SLOAN DIGITAL SKY SURVEY (3)

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