Aquarius is a dim constellation in a barren patch of sky far off the plane of the Milky Way. Just east of Capricorn, it marks the 11th constellation of the zodiac. This dim constellation lies near the other “watery” constellations including Cetus, the Sea Monster (or whale), Pisces, the Fishes, and Eridanus, the River. This ancient constellation was associated with water or water bearers since Babylonian times. Some representations have the water bearer pouring water into a stream that leads to the bright star Fomalhaut, the mouth of the southern fishes Piscis Austrinus. Like Capricorn, Aquarius has far fewer deep-sky sights than Sagittarius. But there are a handful of objects here of enduring interest including the famous Helix Nebula, one of the nearest planetary nebulae to Earth. Let’s take a short tour of some of the finer sights in this zodiacal constellation…
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Messier 2 – A Bright Globular Cluster
The globular cluster M2 is the finest Messier object in Aquarius and a near-twin of the splendid globular M15 in Pegasus, to the north. The cluster was cataloged by Charles Messier in 1760 as he searched for a comet in the area, though it was observed earlier by other French astronomers.
M2 has about the same brightness as M15 and it lies at the same distance, about 33,000 light years. But the core of M2 is less condensed and easier to resolve in larger backyard telescopes.
In a 4-inch scope at 30-40x, 6th-magnitude M2 is a dim, fuzzy ball. Increase magnification to 150-200x, and you’ll begin to resolve stars in the halo of the cluster. M2 also reveals a somewhat north-south oval shape at higher magnification under steady gaze. An 8-inch or larger scope fully resolves the halo and some stars towards the core.
M2 lies about 13º south of M15 in Pegasus and makes a nearly right-angle with alpha and beta Aquarii, also known as Sadalmelik and Sadalsuud. To find the cluster, locate the star epsilon Pegasi (Enif), the nose of the winged horse and the 4th-magnitude star alpha Equulei (Kitalpha) 5º to the southwest. From the halfway point between the two, look another 5º to the southeast to find three 6th-magnitude stars. South of those, you will see a string of stars that become increasingly bright to the southwest and lead directly to M2.
Messier 72 and Messier 73 – A Globular Cluster… and a Mistake!
Just north of Capricorn and over the border of Aquarius lies three deep-sky sights in a patch of sky just 3º wide. Let’s start with M72, the faintest globular cluster on Messier’s famed list. The cluster is a distant 55,000 light years away, which partly accounts for its low brightness of magnitude 9.3. The stars are hard to resolve in a telescope with objectives less than 8-inches in diameter. Although a 4-inch scope at high magnification may reveal some granularity in the halo. The core appears somewhat diamond-shaped in a small scope.
M72 is a fairly open globular cluster much like the more impressive and somewhat brighter and closer M71 in Sagitta. Look for M72 about 3.3º south-southeast of the 4th magnitude star ε (epsilon) Aquarii, less than a degree west of a 6th-magnitude orange star.
Now to a star cluster that’s not really a cluster at all, but rather a random alignment of four unrelated stars which Charles Messier mistook for a cluster. This is M73 and it’s located about 1.3º east of M72. M73 consists of just four stars of 10th and 11th magnitude in a tiny Y-shaped asterism about 1 arc-minute across. Use 100x to darken the background sky and bring out the best view of this tiny group. It looks like a stubby little rocket ship lost amidst a desert of background stars.
NGC 7009 – A Planetary Nebula
Two degrees northeast of M73 lies the small but fairly bright planetary nebula NGC 7009. At low power, it looks like a slightly swollen 8th-magnitude star. At 100x, the nebula appears as a pale green glow with a 12th-magnitude central star visible in even a 3-inch scope. A nebula filter will make the star disappear, but it may also reveal some detail, including a barely perceptible inner shell.
In a 10-12 inch telescope at 200x or more, the nebula reveals more detail, including two slender arms radiating in opposite directions from the center. It looks a little like the planet Saturn with nearly edge-on rings, so NGC 7009 is sometimes called the Saturn Nebula. In fact, the nebula spans about 45″x25″, a little larger than the apparent size of the planet Saturn.
Zeta Aquarii – A Closely-Spaced Double Star
Now for the challenging double star zeta (ζ) Aquarii, which lies in a small closed asterism in Aquarius known as the “Water Jar”. The components are separated by a very close 2.2”, so you need magnification and steady sky to split it. A 3-inch at 100x can just barely do the job. A 4-6 inch scope at 150x gives a better view. But if the seeing is poor, it may not be possible to separate this star at all. Each component is clearly white and nearly the same brightness. The two stars of zeta revolve around each other every 500 years. The pair is about 100 light years away.
Zeta Aquarii lies almost exactly on the celestial equator.
The Helix Nebula – A Large Planetary Nebula
The showpiece of the constellation Aquarius– in my humble opinion– is the grand and elusive Helix Nebula, NGC 7293. One of the closest and apparently largest of all planetary nebulae, the Helix is one of the few sights that’s easier to see in a small telescope than in a large one. In images, like the excellent creation by Terry Hancock at Grand Mesa Observatory, the nebula looks like an expansive eye in deep sky. Some refer to the Helix Nebula as the “Eye of God”.
Like all planetary nebulae, the Helix is a region of rarefied gas thrown off by a dying star of moderate mass. The star is essentially ejecting its outer atmosphere and exposing its blazing hot core to full view. The Helix is distinguished from other planetaries by its relative proximity to Earth, about 700 light years, and its very large apparent size, about 1/4 degree. By comparison, the Ring Nebula in Lyra is some 2,000 light years away and appears about 10-15x smaller in diameter. The Helix gets its name from its ring-like appearance which resembles the two coils of a spring seen on axis.
When you first see the Helix Nebula, you may be astonished to discover how large it appears. Most planetaries are so small as to appear star-like, even at moderate power, so happening upon a planetary nebula that’s half the size of the full Moon can be a little surprising, even for experienced observers. The apparent surface area of the nebula is about four times larger than M27, the Dumbbell Nebula, and more than 100x the area of the Ring Nebula (M57). Charles Messier missed this nebula, likely because his telescopes had a narrow field of view and did not enable him to notice such a large object with low surface brightness.
The Helix Nebula is located about 10º NW of the bright star Fomalhaut. It looks like a big moon-sized grey circle in a pair of binoculars. At low-to-medium power in a telescope, the nebula appears as a smokey oval set in a triangle of 10th-magnitude stars. At first it appears well defined and somewhat featureless. But keep looking. With averted vision, you’ll begin to see the ring-like structure of this ghostly object in a 3″ to 4″ scope. A UHC or OIII filter is usually a big help with this nebula.
The central region appears transparent at low power, and slightly brighter than the background sky at medium power. The 13th-magnitude central star is a challenge with a 4″ scope but a snap with a 6″ or 8″ reflector, especially at about 50x-80x. While higher magnification is useful for detecting the central star, anything more than 100x is wasted on the Helix Nebula.
Some observers at high altitudes and under dry sky claim to have seen the Helix without optics. If you have exceptionally clear and dry skies, give it a try. This will be much easier at southern latitudes where the nebula can be observed much higher in the sky.
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