Constellation Sagittarius- The Archer

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A person lying on the ground, looking at the stars

If the stars should appear one night
in a thousand years,
how would man believe and adore,
and preserve for many generations
the rememberance of the city of God
-Ralph Waldo Emerson

Short form:Sgr
Area:867 sq deg
Co-ordinates1: 19h,-25°

On a deserted beach long ago a young man looked up at more stars than he could possibly count. He was looking at the centre of our galaxy, the Milky Way, which lies in the constellation Sagittarius.

Douglas Adams started the first book in the Hitchiker series with the words

Far out in the uncharted backwaters of the unfashionable end of the Western arm of the Galaxy lies a small unregarded yellow sun.–  Douglas Adams (Intro) The Hitchhiker's Guide to the Galaxy.
He was of course referring to our own sun, and while we may quibble with the designations uncharted or unfashionable, no one is arguing about the words 'far out'. Those who read science-fiction might note that the opening quote from Emerson was also used to begin a story by Isaac Asimov called Nightfall. That story was about a civilisation living in the middle of a globular cluster much closer to the heart of the Galaxy. That civilisation also did not have street lights, and so, on that world, when it got dark it got really dark; and one was faced with unimaginable numbers of stars. Those numbers really exist, but, on Earth, for several reasons, a person living in some place like London, or Tokyo, or Los Angeles may never see them. Even those with a clear dark sky cannot see things like the Pistol Star without using an infrared telescope, because of all the dust and dark matter between our arm of the Galaxy and the central core.

The constellation Sagittarius appears in the summer sky east of Scorpius between 20° and 40° south of the Celestial Equator. Many star charts will show an asterism called
'The Teapot' here.
Scientific study of the sky in Sagittarius indicates that a resident
black hole
lurks at the heart of the Milky Way here with a mass which has been calculated at greater than 2.6 million times that of our sun2.


Saggitarius depicts a centaur holding a bow and arrow. Many sources say Ptolemy patterned both this constellation and the more southerly Centaurus after the same figure in mythology: the centaur named Chiron3. Most centaurs were a surly lot, but not Chiron. Educated by the Greek gods he was wise and kind. Amongst his students were Achilles,
Jason ,
Hercules, and Asclepius4 the latter two having their own constellations. According to some versions the Archer has his arrows poised to shoot the scorpion lest he decide to sting anyone.


In the table below you will find the first column contains letters such as alpha or beta. The scientific star names are from the lower case Greek alphabet. Where two stars share the same Greek letter they have been given a suffix such as xi-2. Sagittarius is one of those constellations where alpha is not the designation for the brightest star (epsilon is actually the brightest). Some stars have proper names as well, and where no such names were found HR designations have been used.

Star Table

StarDesignationName or
catalogue number
Brightness (m)Distance
(light years)
α Alpha Rukbat+3.97170B8
β1 Beta-1 Arkab Prior+4.01378B9
β2 Beta-2Arkab Posterior+4.29139F2
γ1 Gamma-1 6742+2.691,500F4
γ2 Gamma-2 Alnasl+2.6996k0
δ Delta Kaus Meridionalis+2.70306K3
ε Epsilon Kaus Australis+1.85145B9
ζ Zeta Ascella+2.6089A2
η Eta 6832+3.11149M2
θ1 Theta-1 7623+4.37617B2
θ2 Theta-2 7623+4.37157A4
ι Iota 7581+4.13189K0
κ1 Kappa-1 7779+5.59244A0
κ2 Kappa-2 7787+5.64371A5
λ Lambda Kaus Borealis+5.6477K1
μ Mu Polis+2.813,000B1
ν1 Nu-1 Ain Al Rami+4.831,850K1
ν2 Nu-2 7120+4.9270K1
ξ1 Xi-1 7150+5.12,350K0
ξ2 Xi-2 Nergal+5.08372G8
ο Omicron 7217+3.77139K0
π Pi Albaldah+2.89440F2
ρ Rho 7340+3.93122F0
σ Sigma Nunki+2.02224B2
τ Tau 7581+3.32120K1
υ Upsilon 6832+4.611,672B2 (A1)
φ Phi 7039+3.17231B8
χ Chi 7362+5.43; 5.03220A5
ψ Psi 7039+4.85330K0 (A8)
ω Omega Terebellum+4.777.6G5

The Pistol Star

Until the advent of orbiting telescopes this star was unknown. It has been calculated that if the space between that solar system and ours was clear it would be a fourth magnitude star and appear in the chart above. However because of the intervening matter it is only visible in Infra-Red or X-ray.

Located in our galaxy's central bulge, originally this star was one of the most massive in existance ( 200 times the size of our sun). It is still a giant and in the course of its life has thrown out an emission nebula over four light years in diameter.

Nebulae   Star Clusters and Galaxies

The NGC catalogue was compiled by John Louis Emil Dreyer (the director of the Armagh Observatory from 1882 to 1916). The M numbers are from the Catalogue of Charles Messier. These Messier Objects were compiled during the 1700s and predate the NGC Catalogue.


CatalogueNameTypeBrightness (m)Distance
(light years)
M8Lagoon nebula N+6.05200
M17Omega nebula E+7.0greater than 5,000
M18 Star clusterO+7.54,000
M20Trifid nebula E/R+9.0less than 9,000
M21Open clusterO+7.54,250
M22 Star clusterG+5.110,400
M23Open clusterO+5.52,000
M24Star cloud----10,000 to 16,000
M25Open clusterO+4.62,000
M28Star clusterG+6.915,000
M54 Star clusterG+7.687,000
M55 Star clusterG+6.317,300
M69 Star clusterG+7.929,700
M70 Star clusterG+7.629,300
M75 Star clusterG +8.567,500
NGC6520Star clusterG+8.06,300
NGC6522Star clusterG+8.626,000


E=Emission, R=Reflection, P=Planetary, G=Globular, O=Open, ElG=Elliptical galaxy, IrG=Irregular galaxy

A Stellar Mystery

The stars in M24 are not gravitationally bound together.
They just happen to lie in the same direction. Astronomers sometimes bring up the idea that the sky should be lit up everywhere- but it is not. So why is the sky dark, if we have so large a number of stars out there? Some say that interstellar dust is the reason, but their arguments are hardly convincing. Others blame
dark matter , that unknown substance that makes up a sizable percentage of the mass of the universe. Whichever answer is correct, it is visibly apparent that M24 has less of it than most places; and so we see many stars ranging from 10 to 16 thousand light years away clearly which would otherwise be somewhat dimmed.

Extrasolar Planets

Over 2,000 years ago the Greek philosophers Aristotle and Epicurus argued about whether there were other Earth-like planets orbiting distant suns. Today scientists are coming up with ways to actually detect such planets, and the techniques are being fine-tuned so planets throughout our galaxy can be discovered and studied.

There have been 10 extrasolar planets found in the constellation Sagittarius, two of which orbit the star HD169830. The size of the extrasolar planets use as reference the mass of Jupiter, our Solar System's largest planet. This is known by astronomers as the 'Jovian scale'.

Extrasolar Planets Table

Star name or
catalogue number
catalogue number
year discoveredPlanet size
(Jovian scale)
Orbital distance
Orbital Period
HD169830HD169830b20002.880.81225 earth days
HD169830HD169830c20034.043.65.75 earth yrs.
HD179949HD179949b20000.9160.043.092 earth days
HD190647HD190647b20071.92.072.84 earth years
OGLE TR-10OGLE TR10b20040.630.413.1 earth days
OGLE 2005-BLG-169LOGLE 2005-BLG-169Lb20050.0412.79 earth yrs.
OGLE-2003-_BLG 235LOGLE 235-MOA 53b20042.65.14.69 earth days
OGLE-2005-BLG-390LOGLE-2005-BLG-390Lb20050.01882.69.6 earth yrs.
SWEEPS-04 9SWEEPS-04b20063.80.054.2 earth days
SWEEPS-11SWEEPS-11b20069.70.031.8 earth days

1Current IAU guidelines use a plus sign (+) for northern constellations and a minus sign (-) for southern ones.2Galactic Centre Exposed.3 or Cheiron4, Ophiuchus 'The Serpent Bearer'. 5Spectral Classification. 6The term Nebulosa was coined by Giovanni Battista Hodiena in the 1600s. This cloud of gas does not fit the other categories listed being a combination of Emission and Dark Nebulae.7 Astronomical Units8 This is about 5.5 Earth Masses.9Sagittarius Window Eclipsing Extrasolar Planet Search

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