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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 celestial 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 is named Rukbat Alrami, Arabic for knee of the archer. This is one of the not-so-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, or occasionally by priority of position, but when he came to Sagittarius, he seemed to throw the rule book out the window. Rukbat Alrami is a B8 blue/white main sequence star, 170 light years away.

Arkab

Beta Sagitarii is named Arkab, Arabic for achilles tendon. It is 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 is a B8 blue/white main sequence star with a magnitude of 3.96, 378 light years from Earth. Following close behind is Arkab Posterior (Beta 2 Sagittarii), a much different F0 yellow/white main sequence star, with a magnitude of 4.27, 137 light years away.

Nash

Gamma Sagitarii is Al Nasl or Nash, Arabic for the point (of the arrow), aimed toward the heart of the scorpion, as well as the black hole at the center of our galaxy. It is a K0 red giant, magnitude 2.97, 125 light years away.

Kaus Media

Next in Greek alphabetical order is Delta Sagittarii, with the name of Kaus Media - 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 follows logically as Kaus Australis, the southern bow. At a magnitude of 1.81, this is the brightest star in the constellation. It is an A0 blue/white giant, 125 light years away.

Ascella

Zeta Sagitarii has the name of Ascella Latin for arm of the centaur. It is an A2 white main sequence star with a magnitude of 2.60, 140 light years away.

Kaus Borealis

Next in line is Lambda Sagittarii, named Kaus Borealis the northern bow. It is a K2 yellow giant with a magnitude of 2.8, 70 light years away.

Albaldah

Pi Sagittarii is named Albaldah, 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.

Nunki

Finally we have Sigma Sagittarii, known as Nunki () an ancient Babylonian name, 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). With a magnitude of 2.05 this is the second brightest star in Sagittarius, 228 light years away.

Exoplanets

So far twenty-six stars in Sagittarius have been found to support planetary systems. Unfortunately all these star are very far away and beyond naked eye visibility. Almost all the planets so far detected are massive gas giants unlikely to support life. There are two super Earths in the mix, but they are thousands of light years away.





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 centre 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

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

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

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

M22

M22 (NGC 6656) is a large globular cluster about 100 light years across, and since it is only 10, 600 light years away, it is one of the brightest globulars in the sky, with a magnitude of 5.1.

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M22 (NGC 6656) - Hubble Space Telescope - June, 2001

M54

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

M55

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

M69

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

M70

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

SagDIG

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 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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