Aquarius the Water Carrier
"Crystalline brother of the belt of heaven,
Aquarius! to whom King Jove was given
Two liquid pulse streams 'stead of feather'd wings,
Two fan-like fountains,- thine illuminings
For Dian play".
Keats (Endymion)
Aquarius the ‘Water Carrier’ has been associated with water since time immemorial, possibly because the Sun passed through its regions during the rainy season. Many cultures depicted this group of stars as a man or boy pouring water from a receptacle of some kind, usually a bucket or urn. The Arabians saw it as a mule carrying two barrels of water upon its back. Aquarius lies between Capricornus and Pisces, an inconspicuous though interesting area of the heavens. On the 1st of October 1672, Mars occulted the star psi Aquarii. Flamsteed predicted it and observations by Richer in France allowed later astronomers to calculate the mean motion of Mars and hence determine the mass of the Earth and our distance from the Sun.
It was also near gamma that the Capuchin Friar of Cologne, Schyraelus de Rheita, observing Jupiter in 1643, thought he saw five new satellites. However, his excitement was soon crushed when Jupiter left his ‘attendants’ behind, these stars being a little grouping in front of the ‘Urn’.
The brightest star Alpha of magnitude 2.93 is also known as Sadalmelik from Al Sa'd al Malik ‘The Lucky One of the King’. Lying at a distance of about 1100 light years, this class G2 supergiant has a similar surface temperature as our Sun and with a diameter some 80 times, its luminosity is about 6000 times that of our Sun.
Beta is a magnitude 2.86 class G0 type star and shines with the light of 5800 Suns. Also called Sadalsuud, it is derived from Al Sa'd al Su'ud ‘The Luckiest of the Lucky’. The probable distance of this star is 1030 light years. There are a couple of faint companions lying 35.5" and 58.6" away. Both are of 11th magnitude, one being discovered by Sir John Herschel, the other by S.W. Burnham.
Epsilon (Al Bali) of magnitude 3.77 comes from Al Sa'd al Bula ‘The Good Fortune of the Swallower’. This white Sirian A-type main sequence star lies about 108 light years away.
Gamma (Sadachbia) shines at magnitude 3.84 and comes from the Arabic Al Sa'd al Ahbiyah ‘The Lucky Star of the Hidden Things’. Lying fairly close to the Sun at 95 light years, this class B9 giant shines 20 times brighter.
Zeta, of magnitude 3.66, is in reality a double star composed of two subgiant F-type stars of magnitudes 4.4 and 4.6 separated by 1.7" which corresponds to about 100 astronomical units. It was discovered by C. Mayer in 1777 and rediscovered by Sir William Herschel around 1779. If the distance of this system is 75 light years, the luminosities of these stars are 8 times and 7 times that of the Sun.
Delta (Skat) of magnitude 3.28 comes from Al Sak ‘The Shinbone’. Delta is a Sirian A-type main sequence star some 85 light years distant. The expected luminosity is 28x that of the Sun.
The variable star R Aquarii is an interesting pulsating red giant and fluctuates between magnitudes 5.9 and 11.4. With a class of M7, its diameter is 100x and its luminosity about 230 times that of the Sun.
There are 4 globular clusters known in Aquarius, one of them discovered photographically and too faint for amateur telescopes. The best of these starry balls is NGC 7089. First seen by Maraldi in 1746, Charles Messier catalogued it as M2 in 1760. Partial resolution is found in a 20cm telescope and larger apertures do it greater justice. Globular clusters are rated in classes, 1 being extremely compressed and 12 extremely loose and easily resolved into stars. M2 has been designated as class 2 so its stars seem incredibly packed together and it needs a large telescope to start to resolve this cluster to any great degree.
Sir John Herschel compared it to a heap of fine sand, and thought its brightest stars were of 14th to 15th magnitude. Most of these stars are red and yellow giants, at least 100,000 stars in all.NGC 6981 was discovered by M Mechain in August 1780 and catalogued by Messier in October 1780 as his 72nd entry. M72 is not very impressive, lying at a distance of approximately 60,000 light years. The brightest stars are slightly fainter than 15th magnitude and a 25cm telescope struggles to even achieve partial resolution of the edges, despite this being a class 9 object.
Herschel used high magnifications to great success and using a power of 280x on this cluster, saw it as:
"...a very bright object...a cluster of stars of a round figure but the very faint stars on the outside of globular clusters are generally a little dispersed so as to deviate from a perfectly circular form...it is very gradually extremely compressed in the centre, but with much attention even there the stars may be distinguished".
NGC 6994 (M73) is a group of 4 faint stars, appearing nebulous in Messier's telescope. Messier described it as:
"three or four small stars which look like a nebula at first sight; it contains a little nebulosity.."
Modern photographs show no sign of nebulosity at all. The magnitudes of these stars are 10.5, 10.5, 11.0 and 12.0. I'll leave the final word on M73 to Admiral Smyth:
"A trio of 10th magnitude stars in a poor field - that is M73. I give it out of respect to Messier's memory".
The next object is a remarkable planetary nebula with a high surface brightness. NGC 7009 is commonly called the Saturn Nebula, first coined by Lord Rosse, because of the faint ansae or rays extending from the main mass on either side. Large telescopes will bring up the superb blue-green colour, the ansae can be detected in as small an aperture as 25cm, though it needs steady seeing. Use a high power on a steady night for the best views.
NGC 7293, also commonly called the Helix Nebula, has a size about half that of the Moon, therefore its surface brightness is rather low. Indeed, any sky glow can render this object almost invisible. However, don't despair as, in a dark sky, it can be seen easily in binoculars. A 25cm telescope will show the central hole well and low powers work well on this ring of gas, the contrast with the background making it easier to see. The central star that gave rise to the tenuous ring of gas is a 13th magnitude, extremely hot, bluish dwarf star. The temperature exceeds 100,000 K and excites the surrounding gas (doubly ionised oxygen) into fluorescence. The Helix is thought to lie at the close distance of 450 light years, making it the nearest planetary nebula to our Sun.
Aquarius hasn’t the treasures held within the vault like those of the winter constellations but the Helix Nebula more than makes up for it. Careful exploration of this region will reward the patient observer.
Clear skies and good hunting ~ CosMos
CREDITS
NGC 7293 (Helix Nebula); NGC 7089 (M2):
Sketches by the late Scott Mellish