Sagittarius (“archer”) is a zodiac constellation that contains an asterism of eight bright stars in its western region that stargazers refer to as “the teapot”, after the object it resembles. The brightest star in Sagittarius is Kaus Australis, a blue giant located 125 light-years distant, and shining 375 times brighter than our sun. Since the constellation lies in the direction of the Milky Way, it is relatively easy to find, but in far northern latitudes, some of the constellation’s stars may be lost in the haze close to the horizon.
The constellation represents a centaur holding a bow apparently aimed at the neighbouring constellation of Scorpius, or more precisely its heart represented by the red supergiant star Antares. In Greek mythology, Centaurs were half man and half horse, and one of the noblest and wisest of this hybrid race was Chiron, who tutored many Greek legendary heroes, including Heracles, Achilles and Jason. Unfortunately, Heracles accidentally wounded the immortal Chiron with a poison arrow dipped in the blood of the Hydra who was then in such pain that he pleaded with Zeus to put him out of his eternal agony. After his death, Chiron was placed among the stars on account of his noble life and deeds.
Shape: Archer’s Bow or Teapot
It takes a lot of imagination to see a centaur holding a bow, but with some effort, it is possible to make out the shape of a drawn bow armed with an arrow, the bow being represented by the stars Delta (Kaus Media), Lambda Sagittarii (Kaus Borealis), and Epsilon Sagittarii.
Location: Southern Constellation
Sagittarius is the 15th largest constellation, taking up a an area of 867 square degrees of the southern celestial sphere, and can be seen between +55° and -90° of latitude. It lies between Capricornus to the east and Scorpius to the west, with other neighboring constellations including Aquila, Corona Australis, Indus, Microscopium, Ophiuchus, Scutum, Serpens Cauda and Telescopium. Sagittarius is positioned along the Milky Way, with the easiest way to locate the constellation by following an imaginary line drawn through the Summer Triangle stars of Deneb (Cygnus) and Altair (Aquila), and prolonging it a similar distance until it terminates at the famous Teapot asterism in Sagittarius.
Best Seen: Autumn
From the northern hemisphere, Sagittarius is best seen during the months of August and September, which corresponds to spring in the southern hemisphere. However, from mid-northern latitudes, the constellation stays low in the south and never rises high in the night sky.
Notable Stars: Kaus Australis (1st magnitude)
– Kaus Australis (Epsilon Sagittarii), the constellation’s brightest star, is a binary system found 140 light years from Earth that shines with an apparent magnitude of 1.85. It primary component is a blue giant (B9.5 III) that is 7 times bigger than the Sun, with 3.5 times its mass, and 375 times its luminosity. Its name means “Southern Bow”, with the star also marking the bottom right star of the Teapot.
– Nunki (Sigma Sagittarii), the second brightest star in Sagittarius, is a blue dwarf (B2.5V) located 228 light years from our solar system with an apparent magnitude of 2.05. This hydrogen-fusing star is around twice the Sun’s size, with 7 times its mass, and 3,300 times its luminosity. It is also a very fast spinner, with a rotational velocity at its equator in excess of 200 km/s, which is around 100 times faster than our sun.
– Ascella (Zeta Sagittarii), the constellation’s third brightest star, is a binary system situated 89 light years away of magnitude 2.59. It consists of a blue giant and a blue subgiant that are around twice as massive as our sun, 30 times more luminous, and orbit each other once every 21 years at an average distance of 13.4 AU. Ascella means “armpit” in Latin.
– Kaus Media (Delta Sagittarii) is the primary star in a 2.72 magnitude multiple star system about 306 light years away. It is an orange giant that is about 62 times bigger than the Sun, 5 times more massive, and 1,180 brighter. Kaus Media means “Middle Bow”.
– Kaus Borealis (Lambda Sagittarii) is an orange giant (K1+IIIb) 77 light years distant of magnitude 2.82. It is 11 times bigger than the Sun, 3 times more massive, and 52 times more luminous. Kaus Borealis means “Northern Bow”.
– Pistol Star (V4647 Sgr) is a blue hypergiant located about 25,000 light years away in a dense region of space near the center of our galaxy known as Quintuplet Cluster. It has an apparent magnitude of +11.8, although it would appear significantly brighter if not for the intervening dust. In fact, this variable star is one of the brightest yet discovered in the Milky Way, and is generally believed to be up to 200 times more massive than the Sun, and between 4 and 10 million times more luminous, meaning it produces about as much energy in 20 seconds as the Sun does in a full year.
Other stars of interest in Sagittarius includes Alnasi (“Arrowhead”), Rukbat (“The Archer’s Knee”), Arkab Prior (“Hamstring”), and Arkab Posterior (“Hamstring”).
Notable Objects: Many Nebulae and Star Clusters
Sagittarius lies in the direction of the Milky Way’s centre, and being located in such a dense part of the night sky means it is packed with many interesting astronomical objects worth observing. This includes 15 Messier objects, which is more than any other of the 88 recognized constellations, namely:
– Nebulae: The 3 Messier classed nebulae in Sagittarius all have colorful names, such as the Lagoon Nebula (M8), Omega Nebula (M17), and the Trifid Nebula (M20).
– Star Clusters: The constellation’s 11 Messier star clusters includes 7 globular clusters (M22, M28, M54, M55, M69, M70, M75) and 4 open clusters (M18, M21, M23, M25). Incidentally, the Sagittarius Cluster (M22) at a distance of 10,600 light years is among the closest large star clusters to Earth.
– Star Cloud: The Small Sagittarius Star Cloud (M24) is a star cloud which forms a small section of the Sagittarius Arm of the Milky Way. It is around 300 light years across, 10,000 light years distant, and has an apparent visual magnitude of +4.6. It is also the densest concentration of stars visible using binoculars, with thousand seen in a single view.
The constellation is also home to numerous other non-Messier objects, such as the Sagittarius Dwarf Elliptical Galaxy, Sagittarius Dwarf Irregular Galaxy, and Barnard’s Galaxy (NGC 6822); several planetary nebula, including the Little Gem Nebula (NGC 6818), and Red Spider Nebula (NGC 6537); with other interesting objects including the famous radio source called Sagittarius A; and an enormous cloud of molecular dust and gas called Sagittarius B2, which is 150 light years across and located 390 light years from the Galactic centre.
Meteor Showers: None
Despite diligent searches by thousands of observers over many years, no meter showers that emanate from radiants in Sagittarius have ever been identified.
Planets: 25 Stars with Planets
Of the 25 stars with planets in the constellation, one has two planets, with the rest having one planet each. However, no star in the constellation has a planet in its habitable zone, although there are two potentially habitable moons.
Astrology: Nov 22 to Dec 21
The Sun currently passes in front of Sagittarius between December 18th and January 20th, which is off by around one month from the horoscope date of November 22st to December 21st. A distinguishing feature of Sagittarius is that the Sun shines in front of this constellation on the December 21st winter solstice.
Date of Birth: Nov 22 to Dec 21
Sign Ruler: Jupiter
Birth Stone: Amethyst
Color: Light Blue
Characteristics: Insightful, rational, brave, lively, optimistic
Compatibility: Virgo, Leo, and Aries