Sagitta, the Arrow, is one of the smallest constellations in the night sky, covering just 80 square degrees. Despite its modest size and relatively faint stars, it is surprisingly easy to recognize because its principal stars form a distinctive arrow shape.
Located within the Summer Triangle region, Sagitta is a useful constellation for stargazers exploring the northern summer sky. It also contains two particularly interesting deep-sky objects: the globular cluster Messier 71 and the beautiful Necklace Nebula, formed through the interaction of two closely orbiting stars.
Where and When to Find Sagitta
Sagitta is best observed from the Northern Hemisphere during July, August, and September, when it is visible high in the evening sky.
The constellation lies north of Aquila and south of Vulpecula, close to the bright star Altair. The easiest way to find it is to locate Altair, the southernmost star of the Summer Triangle. From Altair, look northward toward the middle of the triangle. Sagitta appears as a short line of relatively faint stars pointing eastward. Under dark skies, its arrow-shaped pattern can be recognized without optical aid, although binoculars make its stars easier to identify.
The Arrow-Shaped Star Pattern
Sagitta is unusual because its constellation outline closely resembles the object it represents.
Its four main stars form a simple arrow:
- Gamma Sagittae marks the arrow’s pointed tip.
- Delta Sagittae lies along the shaft.
- Alpha Sagittae (Sham) and Beta Sagittae mark the two sides of the feathered tail.
The entire pattern stretches across only a few degrees of sky, making it a convenient binocular target. Unlike the Summer Triangle, which is an asterism spanning three constellations, Sagitta is one of the 88 officially recognized constellations.
Mythology
Sagitta was among the 48 constellations described by the Greek astronomer Ptolemy in the second century AD.
One Greek myth identifies it as the arrow Hercules used to kill the eagle sent by Zeus to torment Prometheus. As punishment for stealing fire and giving it to humanity, Prometheus had been chained to a rock where an eagle repeatedly attacked him. Hercules eventually killed the eagle and freed Prometheus. The arrow was commemorated among the stars, while the eagle was associated with the neighboring constellation Aquila.
Other traditions link Sagitta to arrows belonging to Apollo or Eros, reflecting the constellation’s simple and recognizable shape.
Notable Stars
- Gamma Sagittae is the brightest star in Sagitta, shining at magnitude +3.5. It is a red giant approximately 275 light-years away and marks the arrow’s tip.
- Delta Sagittae is the constellation’s second-brightest star, shining at around magnitude +3.7. It is a binary system containing an evolved giant star and a hotter companion.
- Sham (Alpha Sagittae) is a yellow giant shining at magnitude +4.4. Despite its Alpha designation, it is not Sagitta’s brightest star. Its traditional name comes from the Arabic word for “arrow.”
- Beta Sagittae is another yellow giant, shining at approximately magnitude +4.4. Together with Sham, it forms the feathered end of the arrow.
Sagitta also contains several variable and multiple-star systems, including the pulsating variable S Sagittae and the eclipsing binary U Sagittae.
Notable Deep-Sky Objects
– Messier 71 (NGC 6838) is a relatively loose globular cluster located approximately 13,000 light-years away. For many years, astronomers debated whether M71 was an unusually dense open cluster or a loosely concentrated globular cluster. It is now classified as a globular cluster containing an ancient population of stars. M71 shines at around magnitude +8.2 and lies between Gamma and Delta Sagittae, making it straightforward to locate once the Arrow’s shape has been identified. Through binoculars, it appears as a faint, fuzzy patch. Small telescopes show a brighter central region, while larger instruments begin to resolve individual stars.
– The Necklace Nebula (PN G054.2−03.4) is a remarkable planetary nebula approximately 15,000 light-years away. Its distinctive appearance comes from a ring of glowing gas dotted with bright knots that resemble diamonds on a necklace.
The nebula formed when one aging star expanded and engulfed its smaller companion. Rather than being destroyed, the companion continued orbiting inside the giant star’s outer atmosphere, causing material to be expelled into space. The two surviving stellar cores remain extremely close together, completing an orbit in just over a day.
Photographs taken by the Hubble Space Telescope reveal the nebula’s delicate ring and bright gas clumps in extraordinary detail. The Necklace Nebula is primarily an astrophotography target rather than an easy object for visual observers.
Other deep-sky objects in Sagitta include the compact planetary nebulae NGC 6886 and IC 4997.