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SAGITTARIUS

The Archer

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Sagittarius - Celestial Atlas by Alexander Jamieson - 1822





Sagittarius, the archer, is one of the oldest and largest constellations in the sky. It is an image of the mythical centaur - half man,half horse - bow drawn and aimed at the heart of the scorpion. There are 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, an association often wrongly attributed to Sagittarius. The true mythology behind Sagittarius appears to be a blending of ancient Babylonian and Greek traditions. The original Babylonian image was of the two headed centaur god, Nergal, while the Greeks connected the constellation with a Satyr (half man, half goat) by the name of Crotus, a master archer who lived with the muses.

Both centaurs 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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Some modern star charts show Sagittarius as a teapot. I have no idea why.

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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, an optical double, two stars in the same line of sight that appear close together, but are actually vast distances apart. Arkab Prior (Beta 1 Sagittarii) leads the way across the sky. It has a magnitude of 3.96 and is 378 light years from Earth. Arkab Posterior (Beta 2 Sagittarii) follows close behind, with a magnitude of 4.27 and is 137 light years from Earth.

Al Nasl or Nash (Gamma Sagitarii) - Arabic for the point, this star marks the point of the arrow, aimed toward the black hole at the center of our galaxy, and the heart of the scorpion. 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.

Albaldah (Pi Sagittarii) - the city (thought to refer to Mecca). This star is named not for the archer, but for the ancient Arabic lunar station it was associated with: the 21st manzil, Al Baldah. It is an F2 yellow/white giant, 510 light years away. Even at that great distance it has a relatively bright magnitude of 2.89, making it about a thousand times brighter than our Sun.

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).


Exoplanets

So far twenty-five stars in Sagittarius have been found to support planetary systems. Unfortunately all these star are very far away and beyond naked eye visibility. All the planets so far detected are massive gas giants unlikely to support life.





Deep Sky Objects

Sagittarius has the distinction of hosting the exact center of our Milky Way galaxy, located at coordinates RA: 17h 45m 40.04s, DE: -29 00; 28.1". So when you look at Sagittarius, you are looking down through the densest part of our galaxy, and seeing literally billions upon billions of stars. So many stars that they all blur together, and the center of the galaxy is completely hidden behind them. As you might expect, this makes Sagittarius a very busy place, overflowing with astronomical treasures, including no less than 15 Messier objects, more than any other constellation. There is so much going on inside Sagittarius that Robert Burnham Jr. devotes over 100 pages to the constellation in his famous Celestial Handbook.

M8 (NGC 6523), known also as the Lagoon Nebula, is a truly immense cloud of gas and dust - 100 light years across - where stars are being born. It is classified as an emission nebula, because the energy of the new stars has ionized the cloud to the point where it emits its own light. With a magnitude of 6.0 it is one of only a handful of nebulae that can be seen with the naked eye under ideal conditions. M8 is about 5,000 light years away.

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M8 (NGC 6523) - The Lagoon Nebula - European Southern Observatory, Chile - April, 2010


M17 (NGC 6618), known also as the The Omega Nebula, The Swan Nebula, and the Horseshoe Nebula, is about 5,500 light years away, and about 15 light years across. Like M8, it is an emission nebula with a magnitude of 6.0.

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M17 (NGC 6618) - The Omega/Swan/Horseshoe Nebula - European Southern Observatory, Chile - June, 2011


M20 (NGC 6514), known also as the The Trifid Nebula, is another emission nebula about 5,000 light years away, where stars are being made. It is about 25 light years across with a magnitude of 6.3.

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M20 (NGC 6514) - The Trifid Nebula - European Southern Observatory, Chile - August, 2009


The globular cluster M54 (NGC 6715), is inside the Sagittarius Dwarf Elliptical Galaxy (SagDEG), a satellite of the Milky Way galaxy, at a distance of 87,400 light years. With a magnitude of 7.6 it is easily found in a small scope, but due to its great distance it takes the resolving power of Hubble to see individual stars. M54 is estimated to be about 300 light years across, one of the largest known globular clusters.

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M54 (NGC 6715) - Globular Cluster - Hubble Space Telescope - November, 2011


Globular cluster M55 (NGC 6809), is very much smaller and closer than M54. It is only 45 light years across, and only 17,600 light years away. It has a magnitude of 7.4.

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M55 (NGC 6809) - Globular Cluster - European Southern Observatory, Chile - December, 2009


Globular cluster M69 (NGC 6637), is about 61 light years across and 29,700 light years away, with a magnitude of 8.3. Unlike most globular clusters that live out on the fringes of the galaxy, M69 and its close neighbour M70 are both quite close to the galactic core, within about 6,200 light years.

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M69 (NGC 6637) - Globular Cluster - Hubble Space Telescope - October, 2012


Globular cluster M70 (NGC 6681), is only 1,800 light years from M69, and dangerously close to the black hole at the centre of our galaxy. It is about 68 light years in diameter, with a magnitude of 9.1.

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M70 (NGC 6681) - Globular Cluster - Hubble Space Telescope - April, 2012


The Sagittarius Dwarf Irregular Galaxy (SagDIG), is about 3.4 million light years away, not to be confused with the Sagittarius Dwarf Elliptical Galaxy (SagDEG), which is a satellite of the Milky Way galaxy and very much smaller and closer. With a magnitude of only 15.5, SagDIG is visible only in large telescopes.

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Sagittarius Dwarf Irregular Galaxy - Hubble Space Telescope - November, 2004


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