Sagitta (“arrow”) is one of the original 48 constellations mentioned by Greek astronomer Ptolemy in his 2nd century treatise known as the Almagest. It is the night sky’s third smallest constellation, taking up an area of just 80 square degrees of the night sky, with its brightest star, the red giant Gamma Sagittae, shining with an apparent magnitude of 3.47. It also contains a few deep sky objects, including the Necklace Nebula and the loose globular cluster Messier 71.
Sagitta lies in the northern celestial hemisphere, with its nearest neighbors being the constellations of Aquila, Delphinus, Hercules, and Vulpecula. It can be seen by observers located between +90° and -70° of latitude, meaning it is visible from everywhere except the Antarctic circle, although it is best seen during the month of August.
Hercules Family of Constellations
Sagitta belongs to the Hercules Family of Constellations, which contains the largest number of individual constellations at 19. This includes Aquila, Ara, Centaurus, Corona Australis, Corvus, Crater, Crux, Cygnus, Hercules, Hydra, Lupus, Lyra, Ophiuchus, Scutum, Sextans, Serpens, Triangulum Australe and Vulpecula.
– Gamma Sagittae, the brightest star in Sagitta, is a red giant located 274 light years from our solar system that shines with an apparent magnitude of 3.47. This 750 million year old star has 55 times out sun’s radius, 2.5 times its mass, and 640 times its luminosity.
– Delta Sagittae, the constellation’s second brightest star, is a binary star situated around 448 light years away of magnitude 3.68. The system consists of a red giant and a blue-white dwarf star companion that have an orbital period of around 10 years.
– Alpha Sagittae (Sham), the third brightest star in Sagitta, is a yellow giant (G1 II) located 620 light years distant with an apparent magnitude of 4.39. The star has 20 times the Sun’s radius, 4 times its mass, and around 340 times its luminosity. The name Sham derives from the Arabic word for “arrow”.
Other stars of interest in Sagitta includes the blue supergiant 9 Sagittae; the yellow stars Beta Sagittae, 15 Sagittae and HD 231701; the orange giant Eta Sagittae; the Cepheid variable S Sagittae; the eclipsing variable U Sagittae; and the multiple star systems of Zeta Sagittae, Epsilon Sagittae and Theta Sagittae.
Notable Deep Sky Objects
– Messier 71 (NGC 6838) is one of the least concentrated of all types of star clusters, with its 20,000 stars extending across 27 light years of space, and possibly a few stars stretching as far as 90 light years. M71 is located around 13,000 light years away, with its member stars orbiting our Milky Way Galaxy’s core as a satellite. This 9.5 billion years old globular cluster has a luminosity 13,2000 that of our sun, and shines with an apparent magnitude of 6.1. M71 appears as a fuzzy patch when viewed through binoculars, while an 8-inch telescope will help resolve many of its 11th and 12th magnitude stars.
– Necklace Nebula (PN G054.2-03.4) is a planetary nebula measuring 2 light years across that is situated 15,000 light-years from the Sun. This aptly named nebula consists of a binary pair of stars in which the larger bloated component has engulfed and consumed its smaller companion. The pair nonetheless continue to revolve around each other at a furious rate of one full orbit per day, and the gaseous envelope which ensued has subsequently travelled into space along the star’s equator, producing a beautiful ring.
Other objects of interest in Sagitta includes the planetary nebulae IC 4997 and NGC 6886; and the nebula M1-67.
The constellation resembles an arrow, and consequently a number of ancient civilizations associated it with the weapon, including the Persians, Hebrews, Greeks, Romans, and Arabs.
According to ancient Greek mythology, the Titan Prometheus stole fire from Mount Olympus before gifting it to humankind. As punishment for his actions, Zeus chained him to a mountain rock and sent an eagle to eat the immortal’s liver, only for the process to be repeated daily as his liver regenerated. Sagitta then came to represent the arrow that Hercules used to kill the eagle, while the eagle itself is represented by the nearby constellation of Aquila.
Other Greek legends, however, associate Sagitta with the arrow that Hercules shot at the man-eating Stymphalian birds, while others see is as either an arrow belonging to Eros the god of love, or alternatively the arrow Apollo used to kill the Cyclops who originally gifted the weapons of thunder and lightning to Zeus.