Ara (the Altar) is a small southern constellation with a history stretching back to ancient Greek astronomy. It was one of the 48 constellations described by Ptolemy in the 2nd century AD and represents an altar associated with the gods of Greek mythology.
Although Ara contains relatively few bright stars, its position along the Milky Way makes it particularly rich in star clusters and nebulae. Highlights include NGC 6397, one of the nearest globular clusters to Earth, the spectacular NGC 6188 star-forming region, and the unusual Stingray Nebula, which has undergone dramatic changes within a human lifetime.
Where Is Ara and When Can You See It?
Ara is the 63rd-largest constellation, covering approximately 237 square degrees of the southern sky. It lies immediately south of Scorpius, near the scorpion’s curved tail. Its neighboring constellations include Apus, Corona Australis, Norma, Pavo, Telescopium, and Triangulum Australe.
The best time to observe Ara is June through August, particularly July, when it is well placed in the evening sky. Ara is best sen from the Southern Hemisphere. Its northernmost stars can be seen from some tropical northern locations, but the constellation is not visible from Britain.
To find Ara, first locate Antares and the curved tail of Scorpius. Follow the scorpion’s tail southward toward the Milky Way, where Ara lies among dense star fields. Beta and Alpha Arae, its two brightest stars, provide useful starting points for identifying the constellation.
History and Mythology of Ara
Ara is one of the oldest recognized constellations, appearing among the 48 listed by the Greek astronomer Ptolemy in his Almagest. In Greek mythology, Ara was associated with the altar upon which the gods of Olympus swore allegiance before their great battle against the Titans.
Another tradition connects the altar with the centaur Chiron and the neighboring constellations of Centaurus and Lupus, which were sometimes depicted together in ancient star maps.
The constellation was traditionally imagined with its smoke rising toward the south, following the bright band of the Milky Way.
Unlike the southern constellations introduced by European navigators in the 16th century, Ara was already known to classical astronomers, although its low position in the sky made it difficult to observe from northern latitudes.
Notable Stars in Ara
- Beta Arae – The brightest star in Ara, an orange bright giant or supergiant approximately 600 light-years away, shining at magnitude 2.8. It is a massive, highly luminous star that has expanded enormously since leaving the main sequence.
- Alpha Arae – A hot blue-white star approximately 270 light-years away, with an apparent magnitude around 2.9. It rotates extremely rapidly and is surrounded by a disk of gas, making it an example of a Be star.
- Zeta Arae – An orange giant approximately 490 light-years away, shining at magnitude 3.1. It is one of the constellation’s most prominent naked-eye stars.
- Gamma Arae – A luminous blue-white star approximately 1,100 light-years away, with an apparent magnitude around 3.3.
- Delta Arae – A hot blue-white star approximately 200 light-years away, shining at magnitude 3.6.
- Mu Arae – A yellow, Sun-like star approximately 50 light-years away. It is particularly notable for its planetary system, which contains four confirmed exoplanets, including a relatively low-mass planet.
Notable Deep-Sky Objects in Ara
Ara contains no Messier objects, but its location along the Milky Way gives it a particularly rich collection of stellar clusters, glowing nebulae, and distant galaxies.
NGC 6188 and NGC 6193 — A Stellar Nursery
NGC 6188 is a large emission nebula approximately 4,000 light-years away, forming part of an extensive region of gas and dust where new stars have developed. Its glowing clouds are illuminated by hot, massive stars associated with the nearby open cluster NGC 6193.
The cluster contains several dozen bright young stars, including the exceptionally hot stellar systems HD 150135 and HD 150136. Their intense ultraviolet radiation causes surrounding hydrogen gas to glow, while powerful stellar winds help sculpt the nebula’s intricate structures.
NGC 6188 is particularly impressive in long-exposure astrophotography, where bright emission regions contrast with dark clouds of interstellar dust. NGC 6193 itself can be observed through binoculars and small telescopes from suitable southern locations.
NGC 6397 — One of the Nearest Globular Clusters
NGC 6397 is a globular cluster approximately 7,800 light-years away, making it one of the nearest such clusters to Earth. It contains hundreds of thousands of ancient stars packed together in a roughly spherical formation.
With an apparent magnitude around 5.7, NGC 6397 can be detected without optical aid under exceptionally dark skies, although binoculars or a small telescope provide a much better view.
The cluster has undergone core collapse, a process in which stars become increasingly concentrated toward its center through gravitational interactions.
NGC 6397 also contains blue stragglers, stars that appear unusually young and bright compared with the surrounding ancient stellar population.
NGC 6362 — An Ancient Globular Cluster
NGC 6362 is another globular cluster in Ara, approximately 25,000 light-years away. Its stars are billions of years old, and the cluster contains a mixture of evolved red giants and blue stragglers.
With an apparent magnitude around 8.3, NGC 6362 is accessible through moderate-sized amateur telescopes under dark southern skies.
Astronomers have studied its stellar populations to better understand how stars evolve within dense globular clusters.
Stingray Nebula — A Planetary Nebula That Is Fading
The Stingray Nebula (Hen 3-1357) is a small planetary nebula approximately 18,000 light-years away. It formed when an aging star expelled its outer atmosphere, leaving a hot central star whose radiation illuminated the surrounding gas.
What makes the Stingray Nebula extraordinary is the speed at which it has changed. Hubble Space Telescope images taken in 1996 and 2016 revealed that the nebula had faded dramatically over just two decades, with some of its previously bright structures becoming difficult to distinguish. Astronomers believe changes in the central star’s temperature and ultraviolet radiation are responsible for the rapid fading.
This makes the Stingray Nebula a rare opportunity to observe the evolution of a planetary nebula over a timescale of decades rather than thousands of years.
Other Deep-Sky Objects
Other notable objects in Ara include NGC 6326, a planetary nebula with an irregular structure, and NGC 6300, a barred spiral galaxy.
The constellation also contains NGC 6221 and NGC 6215, two galaxies whose structures have attracted interest because of their star formation and gravitational environments.