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The left hand of Aquarius: diverse deep-sky treasures lie in and near western Aquarius.

The constellation Aquarius, the Water Carrier, represents a man pouring water from a jar. On the all-sky chart at the center of this magazine, the stars labeled Alpha ([alpha]) and Beta ([beta]) mark his shoulders. His Water Jar is to the left of Alpha, and his left arm stretches over the back of neigh-boring Capricornus, the Horned Goat. The Water Carrier's left hand is usually shown holding part of his robe, adorned by the stars Mu ([mu]) and Epsilon ([epsilon]). In his 1822 atlas, Alexander Jamieson was the first to show Aquarius clutching Norma Nilotica, a graduated pole for measuring the rising of the river Nile during its annual floods. This is a fitting addition, as some tales claim that Aquarius triggers these floods when he plunges his Water Jar into the river.

The left hand of Aquarius is a great springboard for finding several celestial treats. Diving 3.6[degrees] southward from Mu brings us to the globular cluster Messier 72. Although it's the faintest globular cluster in Charles Messier's famous 18th-century catalog, I've managed to spot it in 14x70 binoculars as a very small and dim fuzzy patch. It occupies the right angle of the 1[degrees] triangle it makes with two 6th-magnitude stars.

My 130-mm (5.1-inch) refractor at 23x displays a moderately faint, hazy ball with a brighter center. A 9th-magnitude star sits near the cluster's east-southeastern side. A magnification of 63x shows two fainter stars guarding M72 from the east and south, while a third star is nestled in the southern fringe of the cluster's 5' halo. The outer halo is faint, but it begins to brighten at a diameter of about 3'. A broadly brighter 2' core dominates the cluster. At 117x several tiny stars fleck the globular, most in its halo and outer core. Through my 10-inch reflector at 299x, M72 is delightfully sprinkled with very faint to extremely faint stars across its entire face.

The meager asterism Messier 73 sits 1.3[degrees] east of M72. They share the field of view in my 130-mm scope at 23x, which shows M73 as a tiny grainy patch with one star distinguishable on its southern edge. At 63x I see four stars forming a petite checkmark only 1' long.

Astronomers debated M73's status as a cluster or asterism until the question was laid to rest in a 2002 journal paper by Michael Odenkirchen and Caroline Soubiran. The authors investigated the four checkmark stars, plus two stars of similar magnitude 4' to the south, and state that the group is "clearly not a physical stellar ensemble." The study indicates that these stars move in different directions through space and lie at different distances from Earth.

Charles Messier discovered M73 in 1780, but M72 was discovered earlier the same year by Pierre Francois Andre Mechain. Like Messier, Mechain discovered other celestial objects while sweeping the sky for comets. He passed his finds on to Messier, who included them in his catalog. Of the 110 deep-sky wonders we list as "Messier objects" today, 26 were original discoveries by Mechain--though some people dismiss M102 as a duplicate of M101.

The beautiful planetary NGC 7009 (widely know as the Saturn Nebula) also inhabits this part of the sky, handily resting 1.3[degrees] west of deep yellow, 4.5-magnitude Nu (v) Aquarii. I can pick out the nebula as an obvious "star" with a little blue halo through my 130-mm scope at 23x. At 37x the halo stands out better, but the color fades to blue-gray. At 68x the color is almost gone. The somewhat oval nebula is spanned by a brighter, greatly flattened oval that runs east-northeast to west-southwest and harbors a brighter center.

My 10-inch scope at 213x shows short spikes extending the long ends of the bright bar. It also teases out a small darker region at the nebula's heart. Despite the central star's visual magnitude of 12.7, it tends to be lost in the glow of this bright planetary nebula. High magnifications help, and oddly enough, I've seen a hydrogen-beta filter kill enough of the nebula's light to reveal the star.

On September 7, 1782, William Hershel discovered NGC 7009 with his 12-inch reflector, making this the first nebula to be discovered with a reflecting telescope. The extensions that give the nebula its Saturnian profile did not surrender to a telescope until Lord Rosse turned his 72-inch "Leviathan" toward it in 1848. In his observing notes the following year, Rosse coined the nebula's now famous nickname, and he later wrote, " ... it has ansae, which probably indicate a surrounding nebulous ring seen edgeways."

Springing northward, we come to the attractive double star 12 Aquarii. In my 9x50 finderscope, it lies at the western end of a rambling trail of fainter suns traversing 2.8[degrees]. Through my 130-mm refractor at 102x, the pale yellow primary star closely guards a considerably fainter alabaster attendant to the south-southwest. The colors show better when the stars are pulled farther apart at 234x.

Nearby 10 Aquarii dwells in the northern reaches of an interesting melange of bright and faint stars. Through my 130-mm scope at 37x, the 10 Aquarii Group is bejeweled with 25 stars down to 12th magnitude. It's elongated 1/2[degrees] north-south and widens as it tumbles southward. The double star Roe 148 (separation 12.9") stands out in the southern part of the group, while at 63x Roe 114 (separation 6.5") pops up in the southwest. Their components weigh in at magnitude 10.0 to 10.7. Edward Drake Roe, Jr. discovered these pairs in the late 19th century. He found most of the doubles that bear his name with the 6 1/2-inch Alvan Clark refractor at his private observatory during his tenure as a Syracuse University mathematics professor.

The HD 196944 Group is a more memorable sight in my 130-mm scope at 37x. It reminds me of a Christmas tree with its tip pointed south. The tree is lit with 20 moderately bright to faint stars in a 23' bunch. Its brightest bulb shines pale yellow, while the second brightest glows deep yellow. Named for one of the designations of its brightest star, this asterism lies 3.2[degrees] north-northwest of Epsilon Aquarii.

Large scope enthusiasts may enjoy hunting Abell 70 and PGC 187663, an unparalleled blend of a planetary nebula and a remote galaxy shining through its rim. They lie just over the border in eastern Aquila, forming a nearly equilateral triangle with Epsilon Aquarii and the orange star 3 Aquarii. Look for this strange pair 9.7' east and a bit north of a 9th-magnitude star, the brightest in the area.

My 14.5-inch reflector at 63x shows a small, round nebula 3 1/2' west of an 11th-magnitude star. At 170x it becomes a subtle, wide ring crowned with a brightening on its northern rim. If I didn't know about the galaxy, I could easily mistake this brightening for part of the nebula. A 14th-magnitude star sits a scant 1/2' southeast of the ring's edge. At 276x the annularity is easier to see, and the galaxy behind its northern edge looks a bit too straight to be part of the ring. This duo is nicknamed the Diamond Ring because of the bright spot on its rim, an effect that is quite striking on astrophotos.

You don't necessarily need a big scope to try for Abell 70. Experienced observers under dark skies have been able to discern the planetary nebula's ring with scopes as small as 8 inches in aperture.

Abell 70 has an unusual binary central star composed of a white-dwarf star and a dwarf or subgiant barium star. A barium star is enriched in barium and other elements that it's not evolved enough to produce. These elements were presumably transferred to the barium star by a stellar wind from its companion when it was in the late stages of its life as a red-giant star.

Treasures in and near Western Aquarius

Object   Type       Mag(v)  Size/Sep    RA       Dec.

Messier  Globular      9.3      6.6'    20h        -12
72       cluster                      53.5m  [degrees]

Messier  Asterism      9.7      1.4'    20h        -12
73                                    58.9m  [degrees]

NGC      Planetary     8.0  44"x 23"   21 h        -11
7009     nebula                       04.2m  [degrees]

12       Double       5.8,      2.5"    21h         -5
Aquarii  star          7.5            04.1m  [degrees]

10       Asterism      5.8     29' x    21h         -5
Aquarii                          15'  00.5m  [degrees]
Group                                              35'

HD       Asterism      6.6       23'    20h         -6
196944                                40.9m  [degrees]
Group                                              49'

Abell    Planetary    14.5     42" x    20h         -7
70       nebula                  37"  31.6m  [degrees]

PGC      Galaxy         16     42" x    20h         -7
187663                           18"  31.6m  [degrees]

Angular sizes and separations 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.

Sue French welcomes your comments at
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Title Annotation:Deep-Sky Wonders
Author:French, Sue
Publication:Sky & Telescope
Geographic Code:1USA
Date:Oct 1, 2012
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