Constellations: Sagittarius 'the Archer' Content from the guide to life, the universe and everything

Constellations: Sagittarius 'the Archer'

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The shield of the Science, Mathematics and Engineering faculty of the h2g2 University.Constellations: Overview | Andromeda | Antlia | Apus | Aquarius | Aquila | Ara | Aries | Auriga | Boötes | Caelum
Camelopardalis | Cancer | Canes Venatici | Canis Major | Canis Minor | Capricornus | Carina | Cassiopeia | Centaurus
Cepheus | Cetus | Chamæleon | Circinus | Columba | Coma Berenices | Corona Australis | Corona Borealis | Corvus
Crater | Crux | Cygnus | Delphinus | Dorado | Draco | Equuleus | Eridanus | Fornax | Gemini | Grus | Hercules | Horologium
Hydra | Hydrus | Indus | Lacerta | Leo | Leo Minor | Lepus | Libra | Lupus | Lynx | Lyra | Mensa | Microscopium | Monoceros
Musca | Norma | Octans | Ophiuchus | Orion | Pavo | Pegasus | Perseus | Phoenix | Pictor | Pisces | Piscis Austrinus
Puppis | Pyxis | Reticulum | Sagitta | Sagittarius | Scorpius | Sculptor | Scutum | Serpens | Sextans | Taurus
Telescopium | Triangulum | Triangulum Australe | Tucana | Ursa Major | Ursa Minor | Vela | Virgo | Volans | Vulpecula
If the stars should appear one night in a thousand years, how would man believe and adore, and preserve for many generations the remembrance of the city of God?
- Ralph Waldo Emerson
Short form:Sgr
Area:867 sq deg
Co-ordinates1:Right Ascension 19h, Declination −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 Hitchhiker 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. The 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 when it got dark it got really dark there were an unimaginable numbers of stars. Those numbers really exist on Earth, but a person living in a densely-populated city such as London, 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 infra-red 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 here called 'The Teapot'. Scientific study of the sky in Sagittarius indicates that a resident black hole lurks at the heart of the Milky Way, with a mass calculated at greater than 2.6 million times that of our sun.


Sagittarius 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 Chiron2. Most centaurs were a surly lot, but not Chiron. Educated by the Greek gods, he was wise and kind. Among his students were Achilles, Jason, Hercules, and Asclepius3, the latter two having their own constellations. According to some versions the Archer has his arrows poised to shoot the scorpion in case he decides 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 gamma-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)
Spectral Classification
εEpsilonKaus Australis+1.85145B9
σ Sigma Nunki+2.02224B2
δDeltaKaus Meridionalis+2.70306K3
μ Mu Polis+2.813,000B1
π Pi Albaldah+2.89440F2
φ Phi 7039+3.17231B8
τ Tau 7581+3.32120K1
ο Omicron 7217+3.77139K0
ρ Rho 7340+3.93122F0
β1Beta-1Arkab Prior+4.01378B9
ι Iota 7581+4.13189K0
β2Beta-2Arkab Posterior+4.29139F2
υ Upsilon 6832+4.611,672B2 (A1)
ω Omega Terebellum+4.777.6G5
ν1 Nu-1 Ain Al Rami+4.831,850K1
ψ Psi 7039+4.85330K0 (A8)
ν2 Nu-2 7120+4.9270K1
ξ2 Xi-2 Nergal+5.08372G8
ξ1 Xi-1 7150+5.12,350K0
χ Chi 7362+5.43; 5.03220A5
κ1 Kappa-1 7779+5.59244A0
κ2 Kappa-2 7787+5.64371A5
λ Lambda Kaus Borealis+5.6477K1

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 existence (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 (Director of the Armagh Observatory, 1882 - 1916). The M numbers are from The Messier Catalogue, by Charles Messier. These Messier Objects were compiled during the 1700s and predate the NGC Catalogue.


CatalogueNameTypeBrightness (m)Distance
(light years)
M6Butterfly clusterG+5.31,600
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
M24 Star 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
NGC 6520Star clusterG+8.06,300
NGC 6522Star clusterG+8.626,000
NGC 6537The Red Spider Planetary NebulaP+135,000

Legend: N=Nebulosa4, 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 there are such a large 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 10,000 - 16,000 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 so fine-tuned that planets throughout our galaxy can be discovered and studied.

There have been many extrasolar planets found in the constellation Sagittarius, two of which orbit the star HD 169830. 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 mass
(Jovian scale)
Orbital distance
(Astronomical Units)
Orbital Period/
Earth days
HD 169830HD 169830 b20002.880.81225.6
HD 169830HD 169830 c20034.043.62,102
HD 179949HD 179949 b20000.950.0453.092
HD 190647HD 190647 b20071.92.071,038
OGLE TR-10OGLE TR-10 b20040.630.413.1
OGLE-05-169LOGLE-05-169L b20050.042.83,300
OGLE-TR-56OGLE-TR-56 b20021.290.02251.21
OGLE235-MOA53OGLE235-MOA53 b20042.65.11,569
SWEEPS-045SWEEPS-04 b20063.80.054.2
SWEEPS-11SWEEPS-11 b20069.70.031.8
HD 187085HD 187085 b20060.752.05986
MOA-2007-BLG-400-LMOA-2007-BLG-400-L b20080.90.85N/A
OGLE-06-109LOGLE-06-109L b20080.712.31,825
OGLE-06-109LOGLE-06-109L c20080.274.65,100
MOA-2007-BLG-192-LMOA-2007-BLG-192-L b620080.010.66241
HD 171238HD 171238 b20092.62.541,523
HD 181720HD 181720 b20090.371.78956
HD 164604HD 164604 b20102.71.13606.4
HD 180902HD 180902 b20101.61.39479
HD 181342HD 181342 b20103.31.78663
MOA-2009-BLG-387-LMOA-2009-BLG-387-L b20112.61.81,970
WASP-67WASP-67 b20110.420.054.61
WASP-68WASP-68 b20110.950.065.08
WASP-110WASP-110 b20140.50.053.78
HD 169142HD 169142 b20143022.78,285
WASP-123WASP-123 b20150.90.042.98
HD 165155HD 165155 b20162.91.13432

HD 165155 b orbits within that system's habitable zone.

1Current IAU guidelines use a plus sign (+) for northern constellations and a minus sign (−) for southern ones.2Or, alternatively, Cheiron.3Ophiuchus, 'the Serpent Bearer'.4The term 'nebulosa' was coined by Giovanni Battista Hodiena in the 1600s. This cloud of gas does not fit the other categories listed as it is a combination of emission and dark nebulae.5Sagittarius Window Eclipsing Extrasolar Planet Search.6This is sometimes referred to as MOA-192 b. It is a terrestrial (rocky) planet, orbiting a brown dwarf (failed star). The planet is estimated to be 3.3 Earth mass.

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