How to find the M10 and M12 Globular Clusters in Ophiuchus

This article will cover M10 and M12, two prominent globular clusters in Ophiuchus. The two globular clusters are not only bright, but also only three degrees apart, making them an excellent target on even a full moon night. The best time to view these clusters is during the summer months of May, June and July.

Most globular clusters contain a few hundred thousand stars, formed when the Universe was only a billion years old. To resolve individual stars within globular clusters, you will typically need a powerful telescope and long exposures, as globular clusters typically span just 100 light-years or less. However, most people want to see the clusters themselves and thankfully these globular clusters are still visible starting as fuzzy balls in binoculars, and increasing in detail as larger instruments are used.

The M10 cluster is the brighter of the pair at magnitude 6.4, with a diameter of 19.3 arcminutes it is two thirds the size of the full moon, however a very large telescope is required to see its full size as the outer regions of the cluster are very dim.

An eight-inch or six-inch telescope, however, shows only a very limited portion of the globular cluster, missing the faint halo of stars that surround the cluster at its extremities. While a three-inch telescope will show about 40% of the cluster and binoculars will just show the core.

M12 is also known as the Gumball cluster is the dimmer of the two at magnitude 7.68. In a small 3-inch telescope, the Gumball Globular cluster appears as a fuzzy ball of light, while an 8-inch telescope reveals the cluster's brightest stars.

Telescopes with larger apertures detect stars over the entire cluster, which has an area of approximately 10 arc minutes for the halo and a diameter of 3 arc minutes for the core. Making M12 roughly a third the size of the moon.

When M10 and M12 are compared, it is noticeable that M12 has a much smaller number of stars in its centre than M10.

This is unusual, as during the lifetime of a globular cluster, the higher mass stars move towards the middle of the cluster while the lower mass stars move toward the edges. Low mass stars, such as red dwarfs at the extremities, are prone to being stripped off the globular cluster and as the gumball cluster orbits through the disk of the milky way galaxy close to the dense galactic centre. This seems to have been what has happened. During its many passages through the galaxy, the milky way stripped its stars off, forming a stream of stars behind M12 that are, or will eventually populate the halo of the galaxy.

This is causing M12 to shrink over time, with estimates of a million stars lost over its lifetime so far. Usually, a globular cluster has a predicted lifetime of around twenty billion years. However, due to what is happening to it the globular cluster only has about four and a half billion years left before it is destroyed completely by repeated passages through the disk, as there is no or very little star creation in globular clusters.

So how do you find M10 and M12?

Click to enlarge M10 star chart

M10 is the brighter of the two clusters so it is easier to locate. Find the northern hemisphere constellation Ophiuchus, the serpent bearer, which is a large oval shape west of Arcturus. This star is a part of the Spring Triangle. Ophiuchus is east of Altair, which is a part of the Summer Triangle, and north of Antares. With an oval shape about 25 degrees long and 25 degrees wide, its stars are of second and third magnitude, so they should be visible with the naked eye even in moderate light pollution.

Click to enlarge the M10 and M12 star chart
From the east to the west, observe the line of stars that forms the south end of Ophiuchus. In between is second-magnitude zeta Ophiuchi. Use Zeta and Yed Prior Ophiuchus to form a triangle extending toward the centre of Ophiuchus with M10 at the far northern end. M10 is just 1 degree to the west of magnitude 4.8 30 Ophiuchi, which will be easily visible in binoculars or a finderscope.

Then M12 can be found slightly more than 3 degrees northwest of M10.

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