The Altar, or Ara, is a small but significant constellation situated deep within the southern celestial hemisphere. It is one of the original 48 constellations cataloged in the 2nd century by the Greco-Roman astronomer Ptolemy. The name “Ara” is Latin for “the Altar.” This stellar figure is characterized by its location along the brilliant band of the Milky Way, giving it a rich density of deep-sky objects.
Celestial Identification and Location
The constellation Ara occupies a region of the sky in the third quadrant of the southern celestial hemisphere, ranking as the 63rd largest of the 88 modern constellations. Its borders encompass approximately 237 square degrees of the night sky. This extreme southerly placement means the entire constellation is visible only to observers south of the 22nd parallel north.
The best time for viewing Ara is during the months of June through August, when it culminates at midnight in mid-July. Portions of Ara are visible from low northern latitudes, such as those south of 25° N. However, it never rises above the horizon for observers north of 45° N.
Ara is surrounded by a collection of neighboring constellations. To its north, it borders the large Scorpius, while Corona Australis also touches its northern edge. The constellation is often found by looking south of Scorpius, near the celestial equator.
Its other neighbors are:
- Apus
- Pavo
- Telescopium
- Norma
- Triangulum Australe
The Mythological Origin
The celestial Altar’s origin is deeply rooted in ancient Greek mythology, primarily linked to the war between the Olympian gods and the Titans. Before the battle began, the younger gods, led by Zeus, swore an oath of allegiance upon an altar. This sacred altar was then placed in the heavens by Zeus as a permanent commemoration of their victory.
The constellation’s ancient Greek name was Thymiaterion, which translates to “censer” or “incense burner”. This name is related to the visual interpretation of the constellation, which is often depicted as a compact classical altar with its smoke rising southward. This rising smoke is symbolically represented by the brilliant, hazy band of the Milky Way, which passes directly through the area of the sky occupied by Ara.
In some less common interpretations, Ara is associated with the altar of the Centaur Chiron, who was renowned for his wisdom. This myth suggests the Altar was where Chiron performed a self-sacrifice to free the bound Prometheus, after which Zeus honored the act by placing the altar in the sky. Another depiction links Ara with the adjacent constellation Centaurus, showing the Centaur carrying Lupus, the Wolf, as an animal sacrifice to be offered upon the Altar. The constellation has maintained its identity as a sacred altar since its inclusion in Ptolemy’s influential 2nd-century catalog.
Notable Stars and Deep-Sky Objects
The constellation may lack stars of the first magnitude, but it contains several stellar objects, the brightest of which is the orange-hued supergiant Beta Arae. This star shines with an apparent magnitude of 2.8. Beta Arae is classified as a K3Ib-IIa star, located roughly 650 light-years from Earth.
Close in brightness is Alpha Arae, a hot, blue-white star of magnitude 2.8, which is a main-sequence star located about 240 light-years away. Another significant component is Gamma Arae, a blue supergiant with a magnitude of 3.31. The concentration of bright stars near the bottom of the constellation is often interpreted as the base or foot of the Altar.
Ara lies within the plane of the Milky Way. It hosts several notable globular clusters, including NGC 6397, which is one of the closest globular clusters to Earth at a distance of approximately 7,800 light-years. This dense cluster is home to over 400,000 stars.
The constellation is also home to the Stingray Nebula (Hen 3-1357), the youngest known planetary nebula. Ara contains massive stellar nurseries and clusters, such as the open cluster NGC 6193, which is embedded within a larger emission nebula. Westerlund 1, a super star cluster, is also located here, home to some of the most luminous and massive stars known in the galaxy.