Astronomy

Aquarius

The Water Carrier

This months featured constellation, though lacking in any significant number of bright stars, has nontheless formed part of our knowledge of the skies for millenia. It was listed in Ptolemy's list of 48 constellations, is of course in the modern list of 88 constellations, and is one of the constellations of the zodiac.

It should also be noted that even during the best month for Aquarius (October) it never rises very high, so we'll be looking for a good clear night to observe these targets as they will always be fairly close to the horizon (the constellation currently transits at around 6pm!)

Aquarius is classically thought of as a water bearer. Zeus took a fancy to a youthful Ganymede whom he took (in the guise of Aquila the Eagle) up to Mount Olympus to be (no doubt at minimum wage) the cup-bearer of the gods - the myths do not seem to address what Ganymede thought of this upheaval.... With a great deal of imagination, and considering some of the much fainter stars, Aquarius generally resembles the figure of a man with a bucket from which is pouring a stream of water.

Target Summary

Target Common Name Type Magnitude Further Info
Zeta Aquarii Binary Star 4.6 / 4.4 Wikipedia
M2 Globular Cluster 6.5 SEDS Page
M72 Globular Cluster 9.3 SEDS Page
M73 Asterism 9 SEDS Page
NGC7009 Saturn Nebula Planetary Nebula 8 SEDS Page
NGC7293 Helix Nebula Planetary Nebula 7.3 SEDS Page

Stellar Sights

The brightest star in Aquarius is the mag 2.9 Beta Aquarii known as Sadalsuud (which means Luck of lucks, apparently). Zeta Aquarii, Sadaltager (Luck of the Merchant) is a double star with very closely matched components - mag 4.42 and 4.59 respectively. These are both yellow stars though one is a main sequence dwarf and the other is a subgiant. They are located 103 light years from Earth and have an orbital period of 760 years.

Into the Deep

Aquarius contains three Messier targets for us to enjoy - two globular clusters and an open cluster

M2 is a bright, mag 6.5 globular cluster, and is thought to be one of the oldes globular clusters in the Milky Way with an approximate age of 13 billion years. It contains 150,000 stars and is thus also one of the largest globular clusters. Rich and compact it is very pleasant site through the telescope on a dark night. M2 can be found easily from Alpha and Beta Aquarii, and is also close to Epsilon Pegasi which was used as a guide star in our recent Iridium Flare challenge.

M72 is the second globular cluster in Aquarius identified by Messier in his catalogue. It is quite different from M2, being much younger, and lies far on the other side of the galactic center. It is one of the dimmer clusters at around mag 9 and will appear only as a faint, fuzzy disc in the telescope with individual stars being impossible to resolve. It is curious that this cluster appears to contain several bright blue (and therefore young stars) when it is generally accepted that globular clusters are amongst the oldest sections of a galaxy

M73 lies close to M72 and is most definitely not a globular cluster. It is actually an asterism of four stars which are physically unconnected. It is one of best-known asterisms in the sky and has been the subject of numerous scientific studies. It has been studied so intensely because it was once thought to be an open cluster of stars (when Messier observed it he thought that there appeared nebulosity around it) that had been disturbed by the gravitational forces in the Milky Way. Although M73 was determined to be only a chance alignment of stars (in a paper published in 2002), further analysis of such asterisms is important for the identification of sparsely populated open clusters which can tell us much more about cluster formation and evolution.

Finally, Aquarius contains two conspicuous Planetary Nebula. NGC 7009 is known as the Saturn Nebula for its superficial resemblence to the planet (with rings nearly edge on). It will take a good night to see it as the magnitude is listed as 11.5 but it is worth tracking down as it regularly appears in the 'best of' lists. The second PN is the Helix Nebula, one of the closest PNs to Earth at a distance of 650 light-years away and much brighter at a magnitude of 7.3. It is a result of the death of a sun-like star which blew off its outer gases into space where they appear from our vantage point as if we are looking down a helix. This will be a lovely Astrophotography target once we get a crisp, clear early evening, though it should be noted that this target is much lower in the sky than the others discussed here.

Conclusion

I hope this brief introduction to Aquarius has given you more of an appreciation of this somewhat overlooked constellation. We'll be aiming to look at it over the next couple of observing sessions so come along and check out some of these amazing deep sky sights.

Clear Skies

Nick Bramhall

November 2006