25 August 2015 by Brian Jones
Walk’s resident stargazer Brian Jones introduces us to Sagitta, a tiny but distinctive constellation resembling a tiny dart, which is often linked to the hero Hercules or to Cupid, the god of love...
Although Sagitta, the arrow, is one of the smallest and faintest of the constellations, it does have quite a distinctive shape. Resembling a tiny dart, Sagitta is one of the few groups of stars that actually resemble the object that it is supposed to depict, this being the arrow with which Hercules slew the Stymphalian vultures. Stymphalus was a lake upon the banks of which lived a flock of dangerous brazen-clawed birds. Hercules managed to wipe out the entire flock with the help of a number of arrows that had been dipped in the venomous blood of Hydra, whom Hercules had slain as his second labour. Some stories link Sagitta to the Arrow of Cupid, an appropriate association in the minds of couples who ramble hand-in-hand under the warm summer skies!
How to find Sagitta
This tiny constellation can be found just to the north of the distinctive trio of stars Altair, Tarazed and Alshain in the neighbouring constellation of Aquila, and located almost on a line between these three stars and the bright star Albireo in Cygnus further to the north.
Altair is the southernmost of the three bright stars that make up the Summer Triangle, the others being Deneb in Cygnus and Vega in Lyra. All these stars are depicted on this chart, which shows the main constellations of the northern summer sky. Altair can be seen roughly half way between the southern horizon and the overhead point during summer evenings, from where you should have little trouble picking out Sagitta, provided the sky is dark and clear. If you do have problems tracking it down, a pair of binoculars will help.
Most of the stars in Sagitta are referred to by their Greek letters (find out more on star names), the only star in the constellation with a proper name being Sham. The name Sham comes from the Arabic word al-sahm which means ‘the Arrow’ and which centuries ago Arabic astronomers used to apply to the constellation as a whole.
Although the four main stars that form Sagitta appear to be close together in the sky, they actually lie at vastly differing distances from us and, if viewed from elsewhere in our galaxy, the arrangement of the stars forming Sagitta would bear little or no resemblance to the constellation we see from Earth. For example, the light from Sham set off on its journey towards us 425 years ago, putting it marginally closer than Beta Sagittae, which shines from a distance of around 440 light years. In contrast to these two stars is Gamma Sagittae, the light from which reaches us from a little over 250 light years ago and Delta Sagittae, which shines from a distance of 580 light years.
The brightest stars in Sagitta are both red giants, the more prominent of which is magnitude 3.51 Gamma Sagittae, which is slightly brighter than magnitude 3.68 Delta Sagittae (find out more on magnitude). Both these stars have orange tints that may be difficult to pick out, even with binoculars, except under exceptionally dark and clear skies. Sham and Beta Sagittae are of virtually the same brightness, both shining at magnitude 4.39 and both having slightly yellow tints that may also be difficult to pick out. The yellow giant Zeta Sagittae, located immediately to the north east of Delta, glows at magnitude 5.01 from a distance of around 250 light years.
Located in the southern-western reaches of Sagitta, close to the border with the neighbouring constellation, Aquila, and at a distance of around 470 light years, is Epsilon Sagittae. This star is quite faint and you may need the help of a pair of binoculars to pick it out. However, your search will be worthwhile because, if you look very closely at Epsilon, you will see that it is in fact a double star made up of two components which are fairly wide apart and which can be resolved in good binoculars or a small telescope. However, one of the stars is considerably fainter than its brighter companion, so you may need to look very carefully to pick both of them out.
If you happen to be trekking through a dark sky site, or even wandering around your own backyard, take a look at Hercules’ poisoned arrow! Happy stargazing!
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