Ara

Ara has no star bright or distinctive enough to have earned a proper name in any tradition, ancient or modern, an unusual gap for one of Ptolemy's original forty-eight. What the altar lacks in named stars, it makes up in what happens to be sitting inside its borders.

NGC 6397 lies within Ara, one of the closest globular clusters to Earth at roughly 7,800 light years away, a dense ball of several hundred thousand ancient stars packed into a volume only a few dozen light years across. Its closeness makes it one of only two globular clusters, along with Messier 4 in Scorpius, near enough for astronomers to study its individual stars in detail rather than only its overall glow, work that has helped establish how the oldest stars in the galaxy actually age.

That the cluster sits in a constellation with no bright named star of its own is a small irony worth noting: ancient observers charting the sky by eye had nothing here to single out, since a globular cluster at that distance is a faint smudge rather than a point of light, while modern telescopes found in the same patch of sky one of the most scientifically useful objects of its kind precisely because it is unusually near.

The altar's smoke, read by some ancient writers as the nearby Milky Way itself, was always the more visible feature of this stretch of sky than any single star in it.

Ara is a small southern constellation, one of Ptolemy's forty-eight, representing an altar. In Greek myth it is the altar on which the gods of Olympus swore their alliance before the war against the Titans, or the altar upon which the centaur bears its offering; the smoke of its sacrifice was sometimes identified with the Milky Way, which runs close by. It lies deep in the southern sky, below the tail of Scorpius, too far south to be seen from much of the northern hemisphere. Its stars are modest and set within a rich Milky Way field. In astronomical tradition the constellation is a pattern of stars rather than a zodiac sign. These are the features of Ara as the classical and modern traditions describe it, distinct from any astrological reading of the southern region of sky it occupies.

Ara sits beside Telescopium, introduced by Lacaille in the 1750s and named, appropriately for a region of sky otherwise carrying an altar's smoke and a centaur's sacrifice, for the instrument that made the systematic eighteenth-century survey of the far southern sky possible in the first place. Norma and Circinus, the level and the compasses, are two more of Lacaille's instrument names carved from the same general stretch of sky in the 1750s.