Messier 9: Globular Cluster in Ophiuchus

Messier 9 is one of the smaller and fainter globular clusters in Ophiuchus. It does not have the impact of M10 or M12, but it is easy to locate next to a bright star and makes a good stop on a summer tour of the Ophiuchus globulars.

What it is

M9 is a globular cluster, a dense, round swarm of old stars in the halo of our galaxy. It lies about 27,100 light-years away, and its 12′ apparent size corresponds to roughly 95 light-years across. What makes M9 interesting is its location: it is one of the globular clusters closest to the center of the Milky Way. Its light passes through thick clouds of interstellar dust on the way to us, which dims it and makes it look redder than it really is. The dust is uneven, so the cluster can look slightly lopsided, with darker patches nearby. Its neighbors in the same constellation include M10, M12, M14 and M19.

When to see it

M9 is a summer object. From about 40°N it is highest in the south around 11 p.m. daylight time in late June, and earlier in the evening during July and August. It never gets very high from mid-northern latitudes: it reaches only about 31° above the horizon, so a clear southern horizon helps. From the Southern Hemisphere it passes nearly overhead and is much easier.

How to find it

Find Eta Ophiuchi (Sabik), a 2.4-magnitude star in the southern part of Ophiuchus, about 14° northeast of Antares. M9 lies about 3.5° southeast of Eta Ophiuchi: about 2° east and 3° south. In a finder or binoculars, put Eta at one edge of the field and nudge southeast until a small round haze appears.

What you will see

In binoculars M9 is a small, faint fuzzy spot, best seen from a dark site. A 70–100 mm telescope shows a round glow with a brighter center but no individual stars. A 200 mm or larger telescope at about 150× begins to resolve the outer edge into a sprinkling of faint stars, while the core stays hazy. From suburban skies it is noticeably harder than M10 or M12, partly because it sits low. No filter helps.

History

Charles Messier discovered M9 in late May 1764 and described it as a round nebula without stars. William Herschel resolved it into stars in the 1780s. Modern studies have focused on the dust in front of it, which is one reason the cluster appears fainter than its true brightness.