Messier 19: Globular Cluster in Ophiuchus

Messier 19 sits in the southern tip of Ophiuchus, close to Antares and the bright heart of Scorpius. It is low for northern observers, but it is fairly bright, and its slightly squashed shape makes it different from the round globulars nearby.

What it is

M19 is a globular cluster, a dense ball of old stars held together by gravity. It lies about 27,200 light-years from us, and its 17′ apparent size corresponds to roughly 135 light-years across. It is also close to the center of the Milky Way, and the galaxy's gravity may have helped shape it. M19 is one of the most flattened (oblate) globular clusters known: its long axis is clearly longer than its short axis, and this shows even in a modest telescope. Dust along the line of sight dims it, so it would look brighter if it were in a clearer part of the sky. Other globulars in Ophiuchus include M9, M10, M12 and M14; the globular M4 lies next to Antares.

When to see it

M19 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. It is low from mid-northern latitudes, reaching only about 24° above the horizon at 40°N, so choose a clear night with a good southern horizon. From the Southern Hemisphere it passes almost overhead and is a much better sight.

How to find it

Start at Antares, the bright reddish star in Scorpius. M19 lies about 7.4° due east of Antares, at almost exactly the same declination. Another way is from Theta Ophiuchi, a 3rd-magnitude star east of Antares: M19 is about 4.5° west and slightly south of it. In binoculars, sweep east from Antares along the same height above the horizon until you find a small round glow.

What you will see

In binoculars M19 is a small fuzzy spot, slightly elongated if your sky is steady and dark. A 70–100 mm telescope shows an oval glow with a brighter center, clearly flattened at medium power. A 200 mm or larger telescope at high power begins to resolve the outer parts into faint stars, while the oval core stays bright. Its low altitude means haze and light pollution hurt it more than most globulars, so suburban views are limited. No filter helps.

History

Charles Messier discovered M19 in early June 1764 and described it as a round nebula without stars. William Herschel resolved it into stars in the 1780s. Later observers noticed its flattened shape, which modern measurements have confirmed.