Sagittarius the Archer

Sagittarius the Archer
In northern hemisphere summer, the ancient zodiac constellation Sagittarius stands low on the southern horizon. It's a special constellation, for when you see Sagittarius, you're looking into the heart of the Milky Way.

Sagittarius was a constellation in Babylonian astronomy well over three thousand years ago, and later adopted by the ancient Greeks. It may once have been associated with the god Nergal. However the Babylonian star catalog MUL.APIN identifies Nergal with the planet Mars, and Sagittarius with the god Pabilsag. You can see that he has two different kinds of head, a horse's body, wings, and a scorpion's tail. Nergal is also depicted as a mixture of human and animal, so it's not surprising that the Greeks also chose such a being for Sagittarius.

At first, the Greek Sagittarius was a satyr with two goat legs, and the head and torso of a human. He was drawing a bow, and represented Crotus the satyr who invented archery. However, even by the time of Eratosthenes (276-194 BCE) Sagittarius had become a centaur with a bow. Eratosthenes argued that this didn't make sense because not only was there already a centaur constellation, but also centaurs didn't use bows.

Observing Sagittarius
The constellation has been seen as a god, a satyr, and a centaur. John H. Rogers of the British Astronomical Association laments that “only in modern America has Sagittarius been ignominiously turned into a teapot – although to most Americans, a teapot is as exotic as a centaur.” I've never been able to see it as a centaur, but I quite like the teapot. Apparently, in a dark sky, the Milky Way looks like steam coming out of the spout.

Summer is the time when Sagittarius is visible in the northern hemisphere. Although it has no first magnitude stars, there are seven stars brighter than the third magnitude. (The lower the magnitude number, the brighter the star.) Observers with telescopes can also appreciate the wealth of deep sky objects, especially in the southern hemisphere. In winter in the southern hemisphere Sagittarius is directly overhead.

Alpha Sagittarii is traditionally known as Rukbat or Alrami, both meaning the archer's knee. You might expect Alpha Sagittarii to be the brightest star in Sagittarius, but it's only a fourth magnitude star. The brightest one is Epsilon Sagittarii, also known as Kaus Australis the southern part of the bow, a second-magnitude binary system. It's made up of a blue-white giant over 3.5 times the mass of the Sun, and a secondary star with a mass slightly smaller than that of the Sun.

Even though Sigma Sagittarii is 228 light years away, we still see it shining brightly in our sky. It's a long way off, but it's over three thousand times more luminous than the Sun. Oddly, the star's common name is Nunki, a Babylonian name whose meaning is uncertain. Although Nunki is an ancient name, the star acquired it rather recently, and there's no evidence the Babylonians named the star.

Over thirty of the stars of Sagittarius are known to have planets. New exoplanets are being discovered all the time, but as of November 2018, those in Sagittarius are mostly gas giants, though there are also two known super Earths. From the current data, it seems unlikely that any of them support life.

Deep Sky Objects
Sagittarius is a happy hunting ground for deep sky objects. It's full of nebulae and star clusters and other objects.

One of the best known Messier objects is the impressive Lagoon Nebula (M8), imaged here by Ignacio Diaz Bobillo. It's 5000 light years from Earth and some 140 light years across. You can tell from the red color that it's an emission nebula with hot young stars energizing the hydrogen gas and making it glow.

But there are also dark star-forming regions in M8. Dutch-American astronomer Bart Bok discovered a number of dark globules there, and suggested that they contained developing stars. Subsequent investigation confirmed his idea, so they're called Bok globules. There are two of these near the Small Sagittarius Star Cloud (M24), Barnard 92 and 93.

M22 is a splendid globular cluster. A globular cluster is a dense, nearly-spherical star cluster held tightly together by gravity. Globular clusters are much older than other stars in the Galaxy, probably older than the Milky Way itself.

The center of the Galaxy is marked by a strong radio source called Sagittarius A*, the asterisk signifying a radio source. It's associated with a supermassive black hole. We can't, of course, see a black hole. Because of the amount of gas and dust between us and the Galactic Center, we can't even see the area in which it's located. However its gravitational effect can be seen in the orbits of stars around the center. Their orbits can only be explained if they're affected a compact object with 4.6 million times the mass of the Sun.

(1) Rogers, JH, “Origins of the ancient constellations: The Mesopotamian traditions”
(2) UCLA Galactic Center Group, “Is there a black hole at the center of the Galaxy?”

You Should Also Read:
ABC of Astronomy - B Is for Bok Globule

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