Aquarius is the night sky’s 10th largest constellation, and in terms of zodiac constellation is second in size only to Virgo. Its most luminous star is Sadalsuud, a yellow supergiant situated 540 light years from Earth with an apparent visual magnitude of +2.87. From the Northern Hemisphere, the constellation is best seen in Autumn, and can be found lying between the constellations of Capricornus and Pisces in a dark region of sky referred to as “The Sea” on account of the watery themed constellation that are found there.
These faint constellations include Capricornus (Sea-goat), Cetus (Whale), Delphinus (Dolphin), Eridanus (Great River), Hydra (Water serpent), and Pisces (Fishes). The one exception in terms of brightness, though, is the constellation of Piscis Austrinus (Southern Fish) whose most luminous star, Fomalhaut, is of magnitude +1.16 There are a number of interesting deep-sky objects to be found in Aquarius, the most notable of which are explored here in this list.
Messier 2 (M2, NGC 7089)
Spanning an area of 175 light years, and consisting of at least 150,000 stars that give the cluster an apparent visual magnitude of +6.3 from a distance of about 37,000 light years away, M2 is one of the biggest and finest globular clusters in the entire night sky. The cluster is also among the oldest clusters known, with an estimated age of about 13 billion years. M2 counts at least 21 variable stars among its population, which consists almost exclusively of old, red and yellow stars, the brightest of which are of apparent visual magnitude 13 or so. Look for M2 about 5 degrees to the north of the star Sadalsuud.
Messier 72 (M72, NGC 6981)
Located about 53,000 light years away on the other side of the galactic center as seen from Earth, M72 is a relatively young cluster, and it contains several bright, blue giant stars, as well as at least 42 RR Lyrae- type variable stars. Although M72 spans an area of 106 light years, and shines with an apparent visual magnitude of +9.3 about three degrees to the south of the star Aquarii, it is not a particularly easy target to observe. As a general rule, telescopes with 10-inch apertures show the cluster as a faint patch of light, while an instrument with a 20-inch aperture or larger is required to resolve individual stars in the cluster.
Messier 73 (M73, NGC 6994)
Up until 2002, it was thought by most observers that M73, located about 2,500 light years away, was a very sparse open cluster. However, detailed analysis of the grouping have subsequently revealed that the six most luminous stars in the asterism are not only at hugely varying distances from Earth, but also that all the stars in the group are moving away from each other in vastly different directions. Look for this attractive +9 magnitude asterism about 1.5 degrees to the westward of the cluster M72, with the best time to observe it during the summer time.
Saturn Nebula (NGC 7009, Caldwell 55)
It is said that when the 3rd Earl of Rosse, aka William Parsons, first observed this nebula in the 19th century, he thought that it sort of resembled the planet Saturn, so he named it after the planet. The Saturn Nebula is a typical planetary nebula that formed when a low-mass star blew off its outer layers, to end up as a white dwarf star that now shines with about 20 solar luminosities at a temperature of 55,000K, giving it an apparent magnitude of +11.5. Look for the Saturn Nebula about one degree to the westward of the star Nu Aquarii. While there is still some debate about what causes the garish green tint of the nebula, most investigators believe that it is caused by the energetic UV light emitted by the central star.
Helix Nebula (NGC 7293, Caldwell 63)
Also known as The Eye of God, this large planetary nebula that stretches across nearly 2.5 light years is an easy target for modest amateur equipment since it is among the closest bright nebulae to Earth, being located only about 700 light years away. At its centre is a stellar core remnant, known as a planetary nebula nucleus (PNN), which will eventually become a white dwarf star. Look for the Helix Nebula one degree to the westward of the star Upsilon Aquarii, but note that instruments of 6-inch apertures and larger are required to see the nebula as more than just a faint patch of light.
The Aquarius Dwarf (PGC 65367, DDO 210)
This dwarf galaxy is an outlying member of the Local Group of Galaxies, and shines at magnitude +14.0 from a distance of about 3.1 million lights away. The galaxy provies an excellent example of the peculiar motions galaxies within a cluster of galaxies can have. Furthermore, the Aquarius Dwarf galaxy’s light is shifted towards the blue part of the spectrum as a result of it approaching us at a velocity of 137 km/sec. While there have been stars found within PGC 65367 that are over 10 billion years old, the majority of its stars are significantly younger with an average age of around 6.8 billion years.
Atoms for Peace Galaxy (NGC 7252)
This +12.7 magnitude elliptical galaxy is located about 220 light years away, and faintly resembles a diagram of an atomic nucleus that is being orbited by an electron, hence its name, which was taken from a speech by Dwight Eisenhower, titled, “Atoms for Peace”. The inner portion of the galaxy contains the remains of a spiral galaxy that measures only about 10,000 light years across, but which rotates in a direction opposite to that of the rest of the galaxy. The inner region of the galaxy also contains in excess of 500 extremely bright star clusters, most of which contain hot, blue stars that are between 50 and 500 million years old.
Located about 76 million light years from Earth, this peculiar spiral galaxy likely got its appearance as the result of a merger between two normal spiral galaxies that is thought to have occurred about 1 billion years ago. The galaxy, that is also designated Arp 222 in Halton Arp’s Atlas of Peculiar Galaxies, now occupies an area of 4.7′ by 3.5′, but this is likely to change as the galaxy is expected to morph into a giant elliptical galaxy in the far distant future.
Other Notable DSOs
Other noteworthy, but not readily visible deep sky objects in Aquarius, include the peculiar galaxy NGC 7257, the lenticular galaxy NGC 7759, and a cluster of galaxies located about 1 billion light years away called Abell 2597. Also located in Aquarius is an exceedingly luminous galaxy designated SSA22-HCM1, that “shines” at magnitude +26.6 from a distance of about 12.66 billion light years from Earth.