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SAGITTARIUS

The Archer

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Sagittarius - Celestial Atlas by Alexander Jamieson - 1822






Sagittarius is one of the oldest and largest constellations in the sky. It also has the distinction of being the constellation that sits at the center of our Milky Way galaxy. This means that when you look at Sagittarius, you are looking down through the entire length of the saucer shaped disk of our galaxy, and seeing literally billions and billions of stars. So many stars, that the center of the galaxy is completely hidden behind them. The constellation is filled with astronomical treasures, including no less than 15 Messier objects. There is so much to say about Sagittarius that Robert Burnham Jr. devotes over 100 pages to the constellation in his famous Celestial Handbook.

Sagittarius is a centaur, the mythological creature that is half man, half horse, and there are in fact two centaurs in the sky, one on each side of the Milky Way, facing each other. Opposite Sagittarius, straddling the southern cross (Crux), is the constellation Centaurus, which represents the centaur Chiron, the wise and benevolent tutor of Apollo. Sagittarius, on the other hand, is represented as mischievous and malevolent. Both centaurs, however, appear to have the killer instinct. As Sagittarius prepares to send an arrow through the heart of Scorpius, the scorpion, Centaurus is in the process of skewering Lupus, the wolf, with a spear. If you happen to be below about 20 degrees north latitude, you can go outside in the summer and see both centaurs at once, facing each other across the great hazy divide of the Milky Way.

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Many modern star charts reduce the grand vision of the centaur archer to the simple outline of a teapot. I have no idea why.

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The traditional image of a centaur with a bow and arrow, as the ancients saw it, illustrated below by the 17th century artwork of Johannis Hevelius on the left, and Johann Bode on the right.

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The Stars of Sagittarius

RUKBAT ALRAMI (Alpha Sagittarii) - One of the rare instances where the Alpha star is not the brightest star in the constellation. And this one isn't even close. With a magnitude of 3.97, there are no less than 14 stars in the constellation that are brighter. When Johann Bayer allotted the stars their Greek alphabetic designations back in 1602, he normally named them in order of brightness. But when he came to Sagittarius, he seemed to throw the rule book out the window. No-one knows why. The name of the star is Arabic for knee of the archer. It is a B8 blue dwarf, 170 light years away.

ARKAB (Beta Sagitarii) - Arabic for achilles tendon, Arkab is an optical double, meaning it can be easily seen as two distinct stars with the naked eye. Arkab Prior (Beta 1) leads the way across the sky. It has a magnitude of 3.96 and is 378 light years from Earth. Arkab Posterior (Beta 2) follows close behind, with a magnitude of 4.27 and is 137 light years from Earth.

EL NASL (Gamma Sagitarii) - Arabic for the point, this star marks the point of the arrow, aimed not only at the heart of the scorpion, but also directly towards the black hole at the center of our galaxy. It is a K0 red giant, magnitude 2.97, 125 light years away.

KAUS MEDIA (Delta Sagittarii) - The middle of the bow. A K2 red giant, it has a magnitude of 2.71, and lies 85 light years away.

KAUS AUSTRALIS (Epsilon Sagittarii) - The southern bow. At a magnitude of 1.81, this is the brightest star in the constellation. It is a B9 blue/white star, 125 light years away.

ASCELLA (Zeta Sagitarii) - Latin for arm of the centaur, it is an A2 main sequence blue/white star, 140 light years away.

KAUS BOREALIS (Lambda Sagittarii) - The northern bow. It is a K2 yellow giant, with a magnitude of 2.8, 70 light years from Earth.

NUNKI (Sigma Sagittarii) - With a magnitude of 2.1 this is the second brightest star in Sagittarius. Its name is Babylonian, roughly translated as proclamation of the sea, possibly referring to the "water" constellations that closely follow it across the sky: Capricornus, (the sea goat), Piscis Australis, (the southern fish), Aquarius, (the water carrier), Pisces, (the fish), and Cetus, (the sea monster).





Messier Objects in Sagittarius

M8
The Lagoon Nebula

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M8 - The Lagoon Nebula - European Southern Observatory, Chile - April, 2010



M17
The Swan/Omega/Horseshoe Nebula

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M17 - The Swan/Omega/Horseshoe Nebula - European Southern Observatory, Chile - June, 2011



M20
The Trifid Nebula

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M20 - The Trifid Nebula - European Southern Observatory, Chile - August, 2009



M55
globular cluster

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M55 - Globular Cluster - European Southern Observatory, Chile - December, 2009


Exoplanets

So far seventeen stars in Sagittarius have been found to support planetary systems. Unfortunately all these star are well beyond the limits of naked eye visibility, and all the planets so far discovered are massive gas giants unlikely to support life.


Winter: Orion   Canis Major   Canis Minor   Monoceros   Lepus   Eridanus   Taurus   Auriga   Camelopardalis   Lynx   Gemini   Cancer  
Spring: Hydra   Sextans   Crater   Corvus   Leo   Leo Minor   Ursa Major   Ursa Minor   Canes Venatici   Coma Berenices   Virgo   Bootes  
Summer: Draco   Corona Borealis   Hercules   Ophiuchus   Serpens   Libra   Scorpius   Sagittarius   Scutum   Aquila   Sagitta   Vulpecula   Lyra   Cygnus  
Autumn: Andromeda   Perseus   Pegasus   Cassiopeia   Cephus   Cetus   Lacerta   Delphinus   Equuleus   Capricornus   Aquarius   Pisces   Aries   Triangulum  
Southern Skies: Centaurus   Crux   Lupus   Corona Australis   Piscis Australis   Sculptor   Tucana   Fornax   Dorado   Columba   Puppis   Carina   Vela  

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