Ara is a small constellation located in the southern sky. Its name means “the altar” in Latin. The constellation represents the altar used by Zeus and other Greek gods to swear a vow of allegiance before they went to war against Cronus and the Titans. In another Greek myth, Ara represents the altar of King Lycaon of Arcadia.
The constellation contains several notable deep sky objects: the Stingray Nebula, the open clusters Westerlund 1 (Ara Cluster) and NGC 6193, the globular clusters NGC 6397, NGC 6352 and NGC 6362, the barred spiral galaxy NGC 6300, the planetary nebula NGC 6326 and the pre-planetary Water Lily Nebula.
FACTS, LOCATION & MAP
Ara is one of the smaller constellations (63rd in size), with an area of 237 square degrees. It lies in the third quadrant of the southern hemisphere (SQ3) and can be seen at latitudes between +25° and -90°.
Ara has seven stars with known exoplanets: Mu Arae (four planets), Gliese 676 (binary system with four planets), HD 154857 (one confirmed and one suspected planet), Gliese 674, HD 152079, HD 154672 and HD 156411
Ara contains two stars brighter than magnitude 3.00 and three stars located within 10 parsecs (32.6 light years) of Earth. The brightest star in the constellation is Beta Arae. The nearest star, Gliese 674 (spectral class M3V), lies at a distance of 14.80 light years from Earth. There are no meteor showers linked to Ara. The constellation does not contain any Messier objects.
Ara contains two named stars. The names of stars that have been officially approved by the International Astronomical Union (IAU) are Cervantes and Inquill.
Ara belongs to the Hercules family of constellations, together with Aquila, Centaurus, Corona Australis, Corvus, Crater, Crux, Cygnus, Hercules, Hydra, Lupus, Lyra, Ophiuchus, Sagitta, Scutum, Serpens, Sextans, Triangulum Australe, and Vulpecula.
There are several myths associated with the constellation. In one of them, Ara represents the altar on which Zeus and other gods vowed to defeat the Titans and overthrow Cronus, who ruled the universe. Cronus was one of the 12 Titans who had deposed his father Uranus, the previous ruler.
When a prophecy said that the same fate would befall Cronus and he would be defeated by one of his own children, to prevent it from happening, he swallowed all his children – Hestia, Demeter, Hera, Hades, and Poseidon – all of them future gods and goddesses. When the youngest child, Zeus, was born, his mother Rhea hid him in Crete and gave Cronus a stone to swallow, telling him the stone was Zeus.
When Zeus grew up, he made Cronus vomit his brothers and sisters. Once freed, they swore to overthrow Cronus and the Titans. The war between the gods and the Titans lasted a decade and the gods won in the end. Zeus became the god of the sky, Poseidon became the god of the sea, and Hades the ruler of the underworld. Zeus placed the altar among the stars to commemorate the gods’ victory.
In another story, Ara represents the altar of Lycaon, the king of Arcadia who decided to test Zeus by serving him a meal of a dismembered child, and later tried to do away with the god while he slept. Zeus, enraged, transformed Lycaon into a wolf and struck down his 50 sons with lightning bolts. In one version of the tale, the sacrificed child was Arcas, the son of Zeus and Lycaon’s daughter Callisto.
MAJOR STARS IN ARA
β Arae (Beta Arae)
Beta Arae is an orange K-type bright giant (possibly supergiant), approximately 603 light years distant. With an apparent magnitude of 2.84, it is the brightest star in the Ara constellation.
α Arae (Alpha Arae)
Alpha Arae, the second brightest star in Ara, is a variable Be star, a B-type star with prominent emission lines of hydrogen in its spectrum. Its apparent magnitude varies between 2.76 and 2.90.
Alpha Arae is approximately 240 light years from Earth.
It rotates very quickly (estimated speed at the equator is 470 km/s) and, as a result of this, the star is surrounded by an equatorial disk of ejected material.
γ Arae (Gamma Arae)
Gamma Arae is a blue-white B-type supergiant, approximately 1140 light years distant. It has a visual companion 17.9 arc seconds away. The companion is a white A-type main sequence dwarf. Gamma Arae has an apparent magnitude of 3.5.
ζ Arae (Zeta Arae)
Zeta Arae is another orange K-type giant, about 574 light years distant. It has an apparent magnitude of 3.12.
μ Arae (Mu Arae)
Mu Arae (HD 160691) is a main sequence G-type star that has four known planets in its orbit. It is approximately 50 light years distant and has an apparent magnitude of 5.12. Three of the four planets in the star’s orbit have masses similar to that of Jupiter.
ε Arae (Epsilon Arae)
Epsilon Arae is a binary star system. Epsilon-1 Arae has an apparent magnitude of 4.06 and is about 300 light years distant. It is an orange K-type giant.
Epsilon-2 Arae is a binary star, 85.9 light years away, with an apparent magnitude of 5.27. The primary component is a yellow-white F-type main sequence dwarf. The closer companion star is only 0.6 arc seconds away and has an apparent magnitude of 8.6. The system has a third component, a 13th magnitude star found 25 arc seconds away from the primary star.
DEEP SKY OBJECTS IN ARA
NGC 6193 is a large open cluster that contains 27 stars, many of them binaries. The cluster lies eight degrees west and one degree north of Alpha Arae. Its estimated age is 3 million years.
The two hottest stars in the cluster are responsible for the illumination of NGC 6188, an emission nebula also located in Ara constellation.
NGC 6193 has a visual magnitude of 5.2, is 15′ in diameter, and lies approximately 4,300 light years from Earth.
NGC 6379 is a bright globular star cluster located three degrees to the northeast of Beta Arae. It contains about 400,000 stars. The cluster is approximately 7,200 light years distant, which makes it one of the nearest globular clusters to us. It was first discovered in the mid-18th century by the French astronomer Nicolas Lacaille.
The cluster is notable for the number of blue stragglers it contains. (Blue stragglers are main sequence stars that are bluer, two to three times more massive, and a few magnitudes above the stars at the main sequence turn-off point for the cluster.)
The Stingray Nebula (Hen 3-1357)
The Stingray Nebula is a planetary nebula approximately 18,000 light years from Earth. It is the youngest planetary nebula known. The central star is a white dwarf that has a companion 0.3 arc seconds away. The nebula has an apparent magnitude of 10.75.
Even though it is significantly smaller than most other planetary nebulae discovered so far, the Stingray Nebula is 130 times larger than our solar system.
The globular cluster NGC 6362 has an apparent magnitude of 8.3 and lies at a distance of about 24,800 light years from Earth. It is located near the border with the constellation Apus.
NGC 6362 was discovered by the British astronomer James Dunlop on June 30, 1826.
The cluster is estimated to be 13.57 billion years old and composed mostly of evolved red giant stars. It also contains a number of blue stragglers, stars that appear younger than they really are. These stars are bluer and more luminous than their neighbours either because they were formed in collisions of two old stars or because they have stolen mass from their companions.
NGC 6326 is an irregular planetary nebula in Ara. It has a visual magnitude of 12.2 and lies at an approximate distance of 11,000 light years from Earth. The nebula was discovered by the Scottish astronomer James Dunlop on August 26, 1826. It occupies an area 0.32′ in size.
NGC 6300 is a barred spiral galaxy classified as a Seyfert galaxy. It has a visual magnitude of 8.78 and lies at a distance of 50.9 million light years from Earth. The galaxy was discovered by James Dunlop on June 30, 1826. It is suspected to contain a black hole with a mass 300,000 times that of the Sun.
NGC 6352 is a globular cluster with an apparent magnitude of 7.8. It occupies an area 7.1′ in size and lies at a distance of 19,570 light years from Earth. Also known as Caldwell 93, the cluster is a relatively loose one. Individual stars can be resolved in 6-inch and larger telescopes.
Ara Cluster (Westerlund 1)
The Ara Cluster is a compact young open cluster located approximately 12,100 light years from Earth. It was named Westerlund 1 after Bengt Westerlund, the Swedish astronomer who discovered it in 1961.
Westerlund 1 is one of the most massive young clusters in our galaxy. It contains an unusually high number of rare high-mass stars, including the red supergiant Westerlund 1-26. Westerlund 1-26 is one of the largest stars known, with a radius approximately 1,530 times that of the Sun.
The Ara Cluster is also home to 6 yellow hypergiants, 3 other red supergiants, 24 Wolf-Rayet stars, a considerable number of OB supergiants, and a luminous blue variable.
Water Lily Nebula
The Water Lily Nebula, catalogued as IRAS 16594-4656, is a pre-planetary nebula found in Ara. It is in the process of evolving into a planetary nebula. The nebula was discovered and imaged by the Hubble Space Telescope.
Other deep sky objects in Ara include the intermediate spiral galaxy NGC 6328, the barred spiral galaxy NGC 6221, the young open cluster NGC 6250, the spiral galaxies NGC 6215 and NGC 6221, the open clusters IC 4651, NGC 6200, NGC 6204, and NGC 6208, and the globular cluster ESO 280-SC06.