Sagittarius: The Coolest Constellation

Overview

Sagittarius is the ninth sign of the zodiac, with the sun passing through the constellation in December to January, so it is best visible to Earth in the summertime, from June through August along the southern horizon in the northern hemisphere. In the southern hemisphere it is more visible, passing almost directly overhead in many countries. It is called "the Archer" and forms sort of a tea pot shape, but to the ancient Greeks and Romans, Sagittarius is a centaur shooting an arrow into the heart of Scorpio. "Sagitta" is Latin for "arrow."

The story of Sagittarius is as follows: While Cronus, father of Zeus, was courting the mortal woman Philyra, he assumed the form of a stallion to avoid the fury of his jealous wife, Rhea. From this union was born Chiron, the most famous of centaurs and a skillful archer. Although centaurs, creatures who are part man and part horse, were generally savage and cruel, Chiron was wise, gentle and good. As the teacher of several famous heroes, he educated the boys about the mysteries of life. One fated day, Chiron was mistakenly slain by his student, Hercules. Realizing the wound was incurable, Chiron begged Zeus not to deprive him of immortality. Pitying his half-brother's predicament, Zeus granted the request and located Chiron in the sky as the constellation Sagittarius, where The Archer stands with bow and arrow for all time.

Sagittarius is one of the most interesting constellations because it contains the galaxy center of the Milky Way and thus an especially rich cluster of nebulas and star clusters, as well as a radio source at the center of the galaxy known as Sagittarius A. The prominent stars that make up the constellation's shape are in the table below.

Star	Right Ascension	Declination	Apparent Magnitude	Actual Magnitude	Spectral Class	Distance from Earth (pc)
Phi Sgr	18h 45m 39.4s	-26d 59m 26.8m	3.17	-1.08	B8.5III	70.8
Alnasi (Gamma Sgr)	19h 58m 45.4s	+19d 29m 31.7s	2.98	0.63	K0III	29.4
Kaus Media (Delta Sgr)	19h 47m 23.3s	+18d 32m 3.4s	2.72	-2.14	K3III	93.8
Kaus Australis(Epsilon Sgr)	19h 37m 17.4s	+16d 27m 46.1s	1.79	-1.44	B9.5III	44.5
Kaus Borealis (Lambda Sgr)	18h 27m 58.2s	-25d 25m 16.5s	2.82	0.95	K1IIIb	23.6
Nunki(Sigma Sgr)	18h 55m 15.9s	-26d 17m 47.7s	2.05	-2.14	B2.5V	68.7
Ascella (Zeta Sgr)	19h 48m 58.6s	+19d 8m 31.4s	2.60	0.42	A3IV	27.3
Tau Sgr	19m 6m 56.4s	-27d 40m 11.3	3.32	0.48	K1/K2III	36.8

Special Features

Star: Kaus Australis (Lambda Sgr)

Kaus Australis, also known as Epsilon Sgr (Sgr is the designation for stars in the Sagittarius constellation), is the brightest star in the constellation. It's name is part Arabic and part Latin; kaus is Arabic for "bow" and australis is Latin for "south." Thus, it is the southernmost part of the Archer's bow. It is a binary giant blue-white star, spectral class B9.5III, with an apparent magnitude of 1.79 and an absolute magnitude of -1.44. It is located at RA 19h 37m 17.4s and declination +16d 27m 46.1s. It is about 44.5 pc from Earth.

Galaxy: Sagittarius Dwarf Elliptical Galaxy

The Sagittarius Dwarf Elliptical Galaxy (SagDEG) is an elliptical loop shaped satellite galaxy of the Milky Way, discovered in 1994. When it was discovered, it was thought to be the closest galaxy to the ours, with a distance of a mere 50,000 light years from the center of the Milky Way, but now the Canis Major Dwarf Galaxy holds that honor. In the night sky, it can be found at right ascension 18h 55m 19.5s and declination -30d 32m 43s. It is about 10,000 light years in diameter and has an apparent magnitude of 4.5.

Nebula: Lagoon Nebula

The Lagoon Nebula (Messier 8, M8) is located in Sagittarius at right ascension 18h 3m 37s and declination -24d 23m 12s. It is classified as an emission nebula and is a hotbed for star formation. It is basically an oval with a slight tail to one side, and is one of only two star-forming nebulae visible to the naked eye in the northern hemisphere.

Other: Sagittarius A*

Sagittarius A* is one of the most interesting features of the entire galaxy. It is in fact the center of the Milky Way, located at right ascension 17h 45m 40s and declination -29d 0m 28s, an extremely compact and bright astronomical radio source, the strongest such source in the galaxy. This is believed to be due to the presence of a supermassive black hole in the center of the galaxy the size of more than 4 million suns. Kind of scary, huh? The radio source is most likely caused by the accretion disc of material that is attracted by and circles a black hole. A black hole the size that this one is suspected to be would have a huge amount of such material around it, which would then be compressed by the massive gravitational forces and emit radiation. Speculation about the center of the galaxy has been going on for years, but we still only have theories, since as yet the technology to see the event-horizon of a black hole does not exist.

Resources

Firmage, George.Sagittarius: A Guide. New York: Liveright, 1991. This book is a good overall guide to the constellation Sagittarius. It gets a little too technical for me, but contains some very useful charts and diagrams, as well as a good section on Sgr A*. I got a lot of my information from here, supplemented by internet sources.

Sagittarius. The Internet Encyclopedia of Science. Accessed December 1, 2009. http://www.daviddarling.info/encyclopedia/S/Sgr.html. This website offered a great table of most of the information needed to fill out my table. It also includes a list of prominent features of the constellation that was useful in figuring out which ones I wanted to report on. It was readable and well-reported.

Kaus Australis (Epsilon Sagitarii). The Internet Encyclopedia of Science. Acessed Dec 2, 2009. The Sagittarius Dwarf Elliptical Galaxy, SagDEG. Accessed Dec 1, 2009. http://seds.org/MESSIER/more/sagdeg.html. This site was a good resource for information about SagDEG. It was clear and fairly concise and provided the necessary information.

M8, the Lagoon Nebula. Night Sky Info. Accessed Dec 2, 2009. http://www.nightskyinfo.com/archive/lagoon_nebula/. This resource provided some beautiful images of the Lagoon Nebula and a bunch of information about it, including the coordinates and shape.

Brumfiel, Geoff."The Milky Way's Hidden Black Hole"Scientific American Accessed Dec 2, 2009. http://www.scientificamerican.com/article.cfm?id=the-milky-ways-hidden-bla. This article was immensely helpful. It provided a good background of Sgr A* and some specifics about the radio telescopes used to monitor it. I thouroughly enjoyed reading it.

Sagittarius: The Coolest Constellation by Bailey Graham is licensed under a Creative Commons Attribution-No Derivative Works 3.0 United States License