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Swimming with stars

Map 15: Equatorial Region 6
Aquila to Pisces
m72_noao_as
M72 is a globular cluster located in the constellation Aquarius.
REU program/NOAO/AURA/NSF

This star map includes part of the sky's so-called watery region, showing Delphinus the Dolphin and the northern parts of Aquarius the Water-bearer, Capricornus the Sea Goat, and Cetus the Whale. Not shown — but lying nearby — are Piscis Austrinus the Southern Fish and Eridanus the River. Combined, these six constellations account for more than 10 percent of the sky.

In Delphinus, the standout deep-sky object is globular cluster NGC 6934. This object shines at magnitude 8.7. Find it by dropping 4° south from Epsilon Delphini. NGC 6934 measures more than 8' across, but to see it to that extent, you'll need a 16-inch telescope. More modest instruments show it as 3' in diameter. You may pick out a few outlying stars, but its central region remains unresolved.

Apart from a few double stars, Equuleus the Little Horse contains no deep-sky objects of interest. But the other horse — Pegasus — boasts quite a few. The best is globular cluster M15. This magnitude 6.3 object measures more than 10' across. You can spot M15 with your naked eyes, but don't confuse it with the star SAO 107195, which lies only 17' to the east. Through a 6-inch or larger telescope, you'll see several hundred stars in a variety of patterns scattered about M15's dense, unresolved core.

Aquarius boasts three Messier objects. M72 lies nearly 3.5° south-southeast of magnitude 3.8 Epsilon Aquarii. M72 glows at magnitude 9.2 and measures approximately 5' across. The cluster is so highly concentrated that you won't be able to resolve any stars near its core.

Only 1°20' east of M72 lies M73. Most lists classify M73 as an open cluster, but it consists of only four stars: a nearly equilateral triangle of 10th- and 11th-magnitude stars with a fainter companion to the west. Check out M73, cross it off your Messier list, and move on.

If you observe Aquarius' third Messier object next, you may think it's in a different class than M72 and M73. Globular cluster M2 is a showpiece. To find it, scan roughly 4.5° due north of Beta Aquarii. If you have sharp eyes, you'll see this magnitude 6.3 cluster without optical aid from a dark site. A superb object in any telescope, M2 displays hundreds of stars through 10-inch and larger instruments.

Slightly more than 1° west of Nu Aquarii lies the Saturn Nebula (NGC 7009). Its name arises from the extensions, or ansae, at either end of the planetary nebula's disk that roughly resemble Saturn's rings. The extensions measure 15" past the ends of the 25"-long oval disk. At the end of the extensions are fainter bulbs you'll have trouble seeing through a 10-inch scope. Whether you see NGC 7009 as mainly blue or mainly green depends on your color perception.

While you're observing in Aquarius, don't miss the Helix Nebula (NGC 7293), which lies on Map 21. This magnitude 7.3 planetary nebula measures 13' across. Counteract its low surface brightness by using a nebula filter. With a filter in place, you'll see the ring structure through a telescope as small as 4 inches in aperture. Through a 12-inch scope, you may see slightly brighter concentrations on the north and south edges.

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