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Messier 10 and Messier 12
June 12, 2020
Ophiuchus was one of the original 48 constellations listed by the second century Greek astronomer Ptolemy. At this time of year, it’s on its side, rising in the east and sweeping across the southern sky. Of course, the sun sets so late this time of year, Ophiuchus is fairly high in the sky by the time it’s dark.
A good time to see it is around 10:30. Look to the east for its brightest star, Rasalhague (RASS-ul-hayg). But don’t confuse it with Vega--the REALLY bright star to the east. Rasalhague is across the sky to the right of Vega. The second brightest star in Ophiuchus is Sabik (SAY-bik). You can find it lower to the southeast.
You can use those two stars to find two deep sky objects. First, find Sabik. Again, it’s a bright white star to the southeast. Above Sabik is Zeta Ophiuchi, the third brightest star in the constellation. If you were to draw a line between Zeta Ophiuchi and the bright star Rasalhague, the line would fall between globular clusters Messier 10 and Messier 12. To see them, you’ll need a telescope. Most cheaper telescopes should at least give you an idea.
Globular clusters are tightly-packed clusters of thousands of stars that form above the galactic plane. That’s how they’re able to clump together so closely--there’s less gravitational influence from the outside. But, they take billions of years to form. The two globulars inside Ophiuchus are over 11 billion years old. Just a couple billion years younger than the universe itself.
There are many deep sky objects around Ophiuchus, but M10 and M12 are the brightest. Given a decent telescope and extra time, you’ll find more.