Occupying 867 square degrees and containing the most stars with known planets (16), Sagittarius is the largest constellation in the Southern Hemisphere and the 15th largest constellation overall. The constellation has many bright stars and is very visible with the naked eye.
As is the case with all of the Zodiac constellations, Sagittarius was recorded in the 2nd century by the Greek astronomer Ptolemy, and it is Latin for archer.
Sagittarius is at the center of the Milky Way Galaxy, and the galaxy is at its densest point as it makes its way through Sagittarius.
In addition to Capricornus and Scorpius, the constellation borders Aquila, Scutum, Serpens Cauda, Ophiuchus, Corona Australis, Telescopium, Indus and Microscopium.
- Right ascension: 19.11 hours
- Declination: -25.8 degrees.
- Visible: between 55 degrees and -90 degree
- Best viewed: August around 9 p.m.
Notable stars and objects
The constellation’s brightest stars — delta, epsilon, zeta, phi, lambda, gamma-2, sigma and tau Sagittarii — form a star pattern, or asterism, called the Teapot.
Also referred to as Kaus Media (middle bow), Kaus Australis (southern bow) and Kaus Borealis (northern bow). delta, epsilon and lambda Sagittarii come together to form the archer’s bow.
The brightest star in Sagittarius and the 36th brightest star in the sky, epsilon Sagittarii is 125 light-years distant and 375 times brighter than the sun. While typical naming conventions have the alpha star as the brightest star in a constellation, German celestial cartographer Johann Bayer did not follow his own naming rule when he got to Sagittarius in 1602, therefore naming the brightest star epsilon Sagittarii.
Forming the top of the Teapot, lambda Sagittarii is 77 light-years from the sun and an orange giant.
The arrowhead is marked at the tip by gamma Sagittarii, an orange giant that is about 95 light-years from Earth.
With a magnitude of 2.1, Sigma Sagittarii this is the second brightest star in Sagittarius.
Making up the armpit, zeta Sagittarii, also known as Ascella, is the third brightest star in the constellation. It is a double star about 90 light-years distant.
Sagittarius is home to the bright blue hypergiant Pistol Star, one of the brightest stars discovered in the Milky Way. While it is bright, it is barely visible to the naked eye due to a great deal of interstellar dust that surrounds it. It is part of a dense region full of massive young stars known as Quintuplet Cluster near the center of the galaxy.
Sagittarius is also home to some interesting objects. Most notable is a bright radio source called Sagittarius A* (pronounced "Sagittarius A-star). Scientists think that area of space may hold a black hole.
That region of space also contains several nebulae, including the Lagoon Nebula (M8 or NGC 6523), a large interstellar cloud about 50 by 110 light-years in dimension. The Omega Nebula (M17 or NGC 6618) and the Trifid Nebula (M20 or NGC 6514) are star nurseries birthing dozens of new stars.
The Sagittarius Dwarf Elliptical Galaxy was the first globular cluster ever discovered outside the Milky Way. [Related: Hubble Photographs Dozens of Colliding Galaxies]
Myth of Sagittarius
The Greeks identify Sagittarius as a centaur, horse-human hybrid with the body of a man atop the four legs of a horse. The creature is aiming an arrow toward its neighbor Scorpio. The lore has Sagittarius the Archer shooting Scorpio the Scorpion, which had been sent to kill Orion the Hunter.
Sagittarius is sometimes confused with another centaur, Chiron of the Centaurus constellation.
The Babylonians associated Sagittarius with the god Pabilsaĝ, who had two heads (one human, one panther), wings and was sometimes called "the wild bull with multicolored legs."
In astrology, which is not a science, Sagittarius is the ninth sign in the Zodiac and represents those born between Nov. 22 and Dec. 21.
— Kim Ann Zimmermann, SPACE.com Contributor