From the Observer’s Log: Four Great Globs

Four globular clusters visible in the May sky as imaged with a Mallincam Xtreme camera, 30 s exposure, through a VRC-6 telescope.

Four globular clusters visible in the May sky as imaged with a Mallincam Xtreme camera, 30 s exposure, through a VRC-6 telescope.

While it may take a little practise to see the subtle differences in each of these balls of ancient stars, and despite claims of many beginning stargazers, all globular clusters do not look the same. The images above show four of the brightest globular clusters visible in a telescope this time of year. Each has a distinctive appearance, pattern, and brightness gradient. All images were taken with the same exposure and filter and with the same 6″ RC telescope so you can get an idea of their comparative appearance.

Messier 3 – About halfway between the stars Arcturus and Cor Caroli lies the very fine globular cluster Messier 3. At magnitude 6.4, this cluster is visible in binoculars or a finderscope as a faint smudge. The cluster also serves as an excellent test of vision and sky clarity because its just on the limit of detection without optics. A 4” scope at 100x resolves the halo into grainy silver pinpricks, and an 8” scope nearly resolves the cluster to its core, where it bursts into a ball of of fine jewel-chip stars with arcs and streams flowing outward. Astronomers estimate M3 is younger than most globulars, with an age of some 8 billion years.

Messier 13 – By far the most famous globular in the northern sky, and possibly the prettiest in a small scope. M13 competes with M5 as the brightest globular cluster north of the celestial equator (it’s visible from most parts of the southern hemisphere too, as are all the globs described on this page). In dark sky the cluster is visible without optics. In a 4” scope at 100x or so, the cluster resolves into a halo of stars about 1/3 of a degree across, with a gradual brightening towards the core. An 8” scope resolves the cluster all the way to the core, revealing an amazing arrangement of tiny pin-prick white and yellow stars of 11th and 12th magnitude. Take a quick glance to the side with averted vision, and the hundreds more stars snap into view in an almost shocking manner.

Messier 92 – The poor stepsister to M13, this globular cluster in Hercules is a showpiece object in its own right. It’s a splendid cluster in a small scope. At moderate magnification in a 4”, it easily resolves into individual stars in the halo, and a larger scope reveals a core of fine diamond dust. The cluster appears somewhat oval compared to M13, and perhaps slightly more concentrated near the center. M92 is about the same distance away as M13, about 25,000 light years, so its slightly smaller apparent size is a consequence of its smaller diameter.

Messier 5 – In a large telescope this cluster has a tight sparkling core and innumerable tiny stars spraying throughout the halo in lines and arcs. A small telescope gives a lovely view as well, with fewer stars resolved but an “electric spark” seeming to emanate from the core of the cluster. Most agree the cluster is perceptibly non-circular, or at least non-symmetric, with the core appearing somewhat brighter to the north. At 13 billion years of age, M5 is one of the oldest globular clusters. And it’s one of the largest, at 130 light years across. The cluster is 24,000 light years away.

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