My first Magazine Sky & Telescope - 04.2019

Constellation History t THE GREATEST GLOBULAR Omega Centauri, the most mas- sive and luminous known globular star cluster in our galaxy, contains millions of stars. Unlike any normal cluster, its stars span several gen- erations, so it’s thought to be the core of a galaxy that was captured and stripped of its outer stars by our galaxy’s gravitational fi eld. The same is suspected of several other globular clusters, including 47 Tu- canae, Omega Centauri’s only rival in brightness and splendor. Sadly, 47 Tucanae lies so far south that it barely skims above the waves even from Hawaii‘i’s southernmost point. locations more familiar to most Sky & Telescope readers. Head and Shoulders Centaurus is a big constel- lation, stretching a long way north-south. So while the Centaur’s hooves are invisible from most of North America and all of Europe, its shoulders, marked by Theta (θ) and Iota (ι) Centauri, can be seen throughout the contiguous U.S. and well into Canada and northern Europe. Theta, also known as Menkent, is the brightest star in its sector of sky, and it makes a striking wide pair with Iota, though they’re nowhere near as tight nor as bright as the Alpha-Beta pair. You can compute approximately how far north a south- erly star is visible by subtracting its declination from 90°. For instance, Theta and Iota, at declination 36° south, barely This plate from Johannes Hevelius’s Firmamentum Sobiescianum (1687–1690) is scrupulously faithful to the descriptions in Claudius Ptolemy’s Almagest, except that the Centaur’s hind legs have been swung back to make room for the Southern Cross. Note that the chart is fl ipped as compared to what we see from the ground — the night sky is portrayed mirror-reversed, as though painted on the outside of the celestial sphere. According to ancient Greek tradition, centaurs in general were rude and crude — half human, half horse, and the worst half of both. But Chiron, half- brother to the chief god Zeus, was an exception — wise and pious. After his death he was placed into the sky where he offers Lupus, the Wolf, as a sacri- fi ce on Ara, the celestial Altar. Centaurus and Lupus were probably originally a single constellation; their bright stars are not easy to separate. 24 A PR I L 2 019 • SK Y & TELESCOPE . JOHANNES Moreover, Alpha Centauri is the sky’s third-brightest star, so it’s prominent even when it’s very low in the sky. And it forms a spectacular pair with Beta (β) Centauri, the 11th-brightest. Immediately west of the Alpha-Beta pair lies another ultrabright star formation, which was dubbed the South- ern Cross by 16th-century European navigators and eventually became the mod- ern constellation Crux. But in ancient times those stars, together with Alpha and Beta Centauri, marked the hooves of the Centaur. This ﬁ lled me with a great desire to see Centaurus as it appeared when the earliest surviving star catalog was compiled in Alexandria around AD 150 by the great Greek astronomer Claudius Ptolemy. How well does the constellation hang together with and without the Southern Cross? Did it make sense to separate those stars into another constellation? Fortunately we don’t have to guess, because we can counter the effects of precession by the simple expedient of traveling farther south. I fulﬁ lled my dream in April 2018 by ﬂ ying to Hawai‘i, where Centaurus appears almost exactly as it did from Alexandria in Ptolemy’s day. But before I discuss that view, let me describe the constellation’s appearance from

My first Magazine Sky & Telescope - 04.2019 | Page 26