Sagittarius

Transit Date of principal star:
13 July

Sagittarius has a muddled history. In ancient times the asterism of three bright stars in a curved line was seen as a bow to some, leading both Greek and Roman writers to confuse the constellation with Centaurus. As stated in "Centaurus", this constellation (in part) represents Cheiron, the king of the Centaurs. Sagittarius is also half-man, half-beast, said by some to have been placed in the heavens to guide the Argonauts in their travels. Others claim that the constellation was invented by the Sumerians, that Nergal (as the supreme god of war) is found on two cuneiform inscriptions. Yet this interpretation is open to debate, for Nergal is not necessarily linked with a bow. In the Gilgamech epic, Nergal is one of the "seven gods" to whom one sacrificed sheep and oxen. His name, in Sumerian, means "Lord of the Great Abode", that is, of the Underworld. Yet there are few extant stories that provide much of a picture of this god. Hammurabi, the great lawgiver (18 century BC) called him "the fighter without a rival who brought him victory" over those who would resist his laws. He was also seen as the god of plagues, and of destruction. However to consider Nergal as the prototype of The Archer seems to be stretching the evidence. For whatever reason, when the select group of twelve constellations was codified sometime in the third millennium BC, The Archer was one of them. It was the Romans who named the constellation Sagittarius ("sagitta" is Latin for `arrow'), although several stars carry Arabic names which identify just which portion of the constellation they represent: Alpha Sagittarii is named "Rukbat": (Rukbat al Rami=Archer's knee), and beta Sgr is "Arkab" (Tendon). The bow is outlined by three stars: Lambda Sgr: "Kaus Borealis" = the northern (part of the) bow Delta Sgr: "Kaus Meridionalis" = the middle (part of the) bow Epsilon Sgr: "Kaus Australis" = the southern (part of the) bow The arrow tip is gamma Sgr ("Al Nasl" = the point) While the asterism of the bow is quite apparent, it takes some imagination to see the half-man, half-beast pulling back on the string. Perhaps it helps to know that zeta Sagittarii is named "Ascella" (the armpit of the archer), while nu Sgr is "Ain al Rami": The Eye of the Archer. The Bayer stars are generally third and fourth magnitude. The brightest star is epsilon Sgr, while alpha Sgr is nearly fourth magnitude. In fact, there are fourteen stars brighter than alpha). The constellation has a number of fine binaries, and several superb deep sky objects. Double stars: Nu1 Sagittarii is a fixed binary with faint companion: 5.0, 10.8; PA 97º and separation 2.5". Note that nu1 and nu2 are not gravitationally bound, although they form an optical binary of some historical importance: these two stars caused Ptolemy to write about "a nebulous double star" long before Hershel coined the term "binary". 54 Sgr also catalogued as h 599 is a multiple system: AB: 5.4, 12; PA 274º, separation 38"; AC: 8.9; PA 42º, 45.6". The primary has a reddish tinge to it. Rho1 and rho2 form a nice triangle with h 2866: AB: 8.0, 8.3; 53º, 23.4" AC: 8.6; 137º, 24". Variable stars: Sagittarius has a variety of variables, some of which are suitable for small scopes, primarily cepheids but also one Mira-type long range variable. Upsilon Sgr is an eclipsing binary (beta Lyrae type: EB) with an unusually long period of 137.9 days. Its range will be undetectable to most observers, from 4.53 to 4.61, but what makes the system interesting is that it seems to be one of the most luminous systems known (with an estimated absolute magnitude of around -7.5). The brightest cepheids are: W Sgr (4.3-5.1 every 7.6 days) and X Sgr (4.2-4.9 every 7 days). R Sagittarii is a long-period variable fluctuating from 6.7 to 12.8 every 269.84 days. In 2000 the maximum should occur in the second week of July. The star is found two degrees NE of pi Sagittarii, or just past the midpoint of a line between pi and rho Sgr. Deep Sky Objects: Sagittarius has fifteen Messier objects, far more than any other constellation. However these fifteen are of varying quality. Three are spectacular, and a number of others are bright and impressive but a number are quite disappointing. While they are all included here, due to space limitations the less interesting objects have been omitted from the constellation graphic. M8 (NGC 6523) is a marvellous diffuse nebula known as the "Lagoon Nebula". This naked eye object is considered to be from 3500 to 5100 light years away. A dark band divides the nebula in two. While easily spotted with the eye, there is a wealth of detail that can only be brought out with at least a medium sized scope. The open cluster NGC 6530 is contained in the eastern part of the nebula. The young cluster (only several million years old) is nicely contrasted against the nebula. The Lagoon Nebula is five degrees west of lambda Sgr and one degree north. M17 (NGC 6618), the "Swan Nebula" or the "Omega Nebula", and occasionally known as the "Horseshoe Nebula". This nebula resembles the tail of a comet: a bright diffuse trail of light with a bit of a hook on it. It is about 5000 light years away. The Swan Nebula is five degrees north of mu Sgr, and one degree east. M18 (NGC 6613) is an open cluster of about twenty stars; a rather undistinguised member of the Messier group found one degree south of M 17. M20 (NGC 6514), the "Trifid Nebula", is another delight, but only with larger scopes, which will bring out the three dark lanes familiar on photographs. In the same field is M 21, an open cluster of about fifty stars. The Trifid Nebula is found 1.5 degrees north of the Lagoon Nebula. M21 (NGC 6531) is a rather unspectacular open cluster 0.7 degrees NW of M20. M22 (NGC 6656) is a fine globular cluster, a highly concentrated group of perhaps five hundred thousand stars in total, about 20,000 light years away. It lies two degrees NE of lambda Sgr. M23 (NGC 6494) is a pleasantly scattered open cluster of about 120 stars located four degrees northwest of mu Sgr and one degree north. M24 (no NGC) is a bright "star cloud", which contains the open cluster NGC 6603. M25 (no NGC) is a bright open cluster but without much interest. M28 (NGC 6626) is a bright condensed globular cluster, much less spectacular than M 22 but a fine object none the less. It is one degree NW of lambda Sgr. M54 (NGC 6715) is a globular cluster, difficult to resolve. M55 (NGC 6809) is another globular cluster, less concentrated than those previously mentioned. It is about 20,000 light years away, and lies between zeta Sgr and theta Sgr: seven degrees east of zeta and one degree south. M69 (NGC 6637) is a globular cluster of little merit. M70 (NGC 6637) is another globular cluster, two degrees east of M69. It too is of little interest. M75 (NGC 6637) is the faintest of globular clusters in this constellation. NGC 6822, "Barnard's Galaxy". Very faint; the larger the scope the better. This irregular dwarf galaxy is about 1.7 million light years away, making it one of the closest of its kind. It's in the same region as 54 Sgr, six degrees northeast of rho Sgr. Since Sagittarius sits at the very heart of the Milky Way, there are many more deep sky objects to study: planetary nebulae abound, as well as both bright and dark nebulae and of course star clusters, especially of the globular variety. For a more detailed appreciation of Sagittarius visit the Binocular Section. 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