Between the stars Altair (Alpha [α] Aquilae) and Antares (Alpha Scorpii) and just west of the Milky Way’s stream lies a large asymmetrical hexagon of half a dozen faint stars that outline the constellation Ophiuchus.
The ancient Greeks saw Ophiuchus as embodying Asclepius, the god of medicine. According to legend, Asclepius once killed a snake with his staff. But a second snake brought the first back to life using mystical herbs. Witnessing this, Asclepius began using those same herbs on people to bring them back from the dead.
To commemorate his healing powers, Asclepius was placed in the sky as Ophiuchus the Serpent-bearer. He is depicted with the serpent’s head, Serpens Caput, and tail, Serpens Cauda, in either hand.
For backyard astronomers, Ophiuchus is best known as being home to 24 globular star clusters — more than any other constellation except Sagittarius. This month, let’s visit four of Ophiuchus’ globulars.
We’ll begin with the brightest two, M10 and M12. Both lie just south of the hexagon’s center. To find M10, first locate the stars that form the southern boundary of Ophiuchus, just north of the head of Scorpius. The line kinks a little to the south in the middle, to the star Zeta (ζ) Ophiuchi. Glance 6° — about one field of view — northeast of Zeta for 5th-magnitude 23 Ophiuchi. Look for the soft, circular glow of M10 2.5° northeast of 23 and 1° west of neighboring 5th-magnitude 30 Ophiuchi.
Once you locate M10, shift your attention toward the northwest edge of the field without changing your binoculars’ aim. Can you see another circular patch? That’s M12. Both are evident through most binoculars, with M10 shining at magnitude 6.4 and M12 reaching magnitude 7.7.
Messier discovered M10 and M12 just a day apart in late May 1764. In both cases, he referred to his new finds as “nebula[e] without stars.” That pretty much mirrors our view through binoculars. Each looks like a tiny cotton ball floating among the stars. But there is so much more there. Globular clusters are huge spherical agglomerations of 100,000 or more of the oldest stars known. The stars in M10 are estimated to be 11.4 billion years old, while those in M12 are a billion years older still.