SAGITTARIUS, the ARCHER, pronounced saj-it-TAIR-ree-us

Chart showing the constellations.

Sagittarius is an ancient constellation depicting a centaur, half man, half beast, with a raised bow and arrow. It has been visualised this way, at least since times of the ancient Greeks. It probably originated with the Sumerian civilisation around the Euphrates, who saw the constellation as Nergal, their archer god of war.

Sagittarius is depicted with a threatening look, aiming his arrow at the heart of Scorpius the Scorpion. The bow is marked by the stars l (lambda), d (delta) and e (epsilon) Sagittarii. In modern times the constellation is often visualised as forming a teapot.

The centre of our Milky Way Galaxy lies in Sagittarius, so that the constellation is particularly rich in star clusters, planetary nebulae, and diffuse nebulae, both luminous and dark. Th actual centre is marked by a radio source known as Sagittarius A*.

To find Sagittarius look towards the north half way up the sky.

Chart showing Sagittarius as seen about 10.30 pm mid August and 8.30 pm mid September.

Sagittarius chart

Constellation Ara Constellation Norma Constellation Circinus Constellation Telescopium Corona Australis Constellation Microscopium Constellation Capricornus Constellation Aquarius Equuleus & Delphiunus Constellation Aquila Constellation Scutum Ophiuchus & Serpens Constellation Libra Constellation Lupus Constellation Scorpius

Some stars and interesting objects in the Constellation

α Sagittarii (Rukbat, knee, or Al Rami, the archer) is a magnitude 4.0 blue-white star 170 light years away. It is one of the many examples where the star labelled α is not the brightest in the constellation.

β Sgr consists of two unrelated stars seen separately by the unaided eye. β1 Sgr, Arkab Prior (front part of the leg) is a magnitude 3.9 blue-white star 378 light years away; it has a magnitude 7.2 companion visible in small telescopes. β2 Sgr, Arkab Posterior (rear of leg), is a magnitude 4.3 white star 139 light years away. These optical double stars appear nearby by chance.

γ Sgr (Al Nasi, point of the arrow) is a magnitude 3.0 yellow giant star 96.1 light years away.

δ Sgr (Kaus Meridionalis, middle of the bow), is a magnitude 2.7 orange giant star 306 light years away.

ε Sgr (Kaus Australis, southern part of the bow), is a magnitude 1.9 blue-white star 145 light years away. It is the brightest star of the constellation.

λ Sgr (Kaus Borealis, northern part of the bow) is a magnitude 2.8 orange giant 77.3 light years away.

σ Sgr (Nunki) is a magnitude 2.0 blue-white star 224 light years away.

M8 (NGC 6523), the Lagoon Nebula, was discovered by Messier in 1764. It needs a large field and shows very extensive luminosity with dark irregular lanes, followed by the bright open star cluster NGC 6530. It lies about 50,000 light years away.

M17 (NGC 6618), the Omega, Swan, or Horseshoe Nebula, is a beautiful emission nebula in the form of a figure "2" with a long base. It was discovered by Messier in 1764, who remarked on its extended form. It lies about 60,000 light years away.

M20 (NGC 6514), The Trifid Nebula, was discovered by Messier in 1764, but he could make little of it. It appears in photographs as an irregular mass on which are superposed three tortuous dark lanes meeting near the centre. The view through a telescope is less impressive.

M22 (NGC 6656) is one of the nearest globular clusters to us at about 10,000 light years. It has a broad centre and is a most beautiful object in a fine field. The globular is an easy object for binoculars and is faintly visible to the unaided eye in a dark sky.

M23 (NGC 6494) is a fine open star cluster with little central condensation. The stars form a pattern of lines and loops with some radial rays. Its distance is estimated as 2,200 light years.

M24 (NGC 6603) is an open cluster that resembles a loose globular cluster. The field is beautiful and filled with stars, but also includes a notable dark cloud, Barnard 92, making the immediate region quite indescribably rich and varied.

M25 (IC 4725) is an effective star group for small telescopes. This fine bright open cluster is somewhat gathered to a broad centre with three deep yellow stars in line, from which runs a remarkable widening curve of equal stars. Its distance is estimated as 1600 light years.

M55 (NGC 6809) was recorded by Lacaille in 1752 who compared it with the nucleus of a large comet. It is an open type of globular cluster, irregularly round, and rising broadly towards the centre, beautifully resolved into stars scattered in a haze of fainter ones. It is relatively nearby at about 16,000 light years.

NGC 6520 is an open cluster with an orange star in the centre round which are arcs of stars looking like a close spiral. It is set in a grand field, the background of which is luminous with the combined radiance of innumerable faint stars. On this is projected the remarkably dark nebula Barnard 86. This dark cloud has an orange on the north-west edge. Barnard found the dark nebula visually in 1883 and it is one of the best for telescopes, especially for small apertures. The cluster and dark cloud are estimated to be 5.500 light years away.

NGC 6522 is a small globular cluster discovered by William Herschel in 1784. It is a moderately condensed type with a star on the following edge. The field is sown profusely with stars and lies close to the direction of the galactic centre. Although somewhat obscured by absorbing matter, it was clear enough to allow Walter Baade to estimate the distance to the centre of the Galaxy, hence the field around the globular cluster is now known as Baade's Window. Distance is estimated as 20,000 light years.

NGC 6818 is a bright planetary nebula, relatively easy to find for these objects. No central star is visible. Distance is estimated as 5,000 light years.

M 75 (NGC 6864) was discovered by Merchain in 1780. It is an easy object for small telescopes and a good example of the strongly compressed type of globular cluster. It appears symmetrical with a strong central peak. Its distance is estimated as 60,000 light years.

Visibility

Sagittarius is a southern constellation, with its more southerly parts passing overhead in New Zealand. The brightest star, ε Sgr, passes overhead at North Cape. α Sgr passes over Levin and the two stars of β Sgr pass over Timaru and Waimate.

As a result of its southerly declination, the constellation is visible in the evening sky for several months of the year. The constellation is well placed for observing from later June until early November at 9 pm (10 pm after the start of NZDT in October).