| Target | Type | R. A. | Dec. | Constellation | Magnitude | Size |
| M10 (NGC 6254) | Globular Cluster | 16h 57m 09s | -04° 06′ 01″ | Ophiuchus | +6.6 | 20.0 arcmin |
Overview
This is a bright cluster, on the edge of naked eye visible, in the constellation of Ophiuchus. It was discovered by Messier in 1764 who, like a few of his discovered clusters, thought it to be a “nebula without stars”.
William Herschel was the first person to see individual stars in the cluster but today, all of us with a small telescope can achieve the same feat.
Located away from the plane of the Milky Way, we see this as a rich, tight field of stars covering a large area of the sky, about 2/3rds the size of the full moon.
In reality, this cluster is over 80 light-years wide and is some 14,000 light-years distant. What we see through our telescopes is the brightest part of it, the core, which covers about half its total diameter.
This cluster is receding from our planet at a speed of 69 km per second.
Finder Charts
The first star chart (from SkySafari 6) shows the night sky in the middle of July at 10 pm. Note the position of M10 in the heart of Ophiuchus. It shines at the edge of naked eye visibility but is unlikely to be seen in any suburban setting. Instead, note the position of the star 30 Ophiuchi, which we’ll use to find M10.
Find the bright stars Rasalhague and Zeta Ophiuchi and imagine a line between them. To the southeast side of that line (below it, if you look in the middle of July), are four bright stars, circled in the star chart below. They all have magnitudes between 4.5 and 5.0, so are visible to the naked eye unless your sky is heavily light polluted.
Find the third one away from Rasalhague (or second away from Zeta Ophiuchi). This star is 30 Ophiuchi and we can just about see M10 in the same eyepiece view as it, if we use low magnification.
This final chart shows M10 inside the same 1.5° field of view as 30 Ophiuchi. When you see the star in your eyepiece, position it on the east side and you’ll have the cluster appear in the west.
In the last section, you’ll discover what you should expect to see of this cluster once you’ve got it in your eyepiece.
Individual Telescope Views
The following views will help you find M10 in different telescope types by presenting the images as your telescope will show them. The first image is with a black sky and white stars, the second picture is the same image but presented in inverse monochrome. The ring in each image is a 1° field of view and stars are shown to magnitude 9.0.
Each image can be clicked on for a full-screen version.
Upright View – This is what your eyes see unaided and through a reflex or red-dot finderscope
Upside-down view – This is what reflectors and magnifying finders show, and refractors / Cassegrains without a star diagonal
Mirrored View – Refractors and Cassegrain models with a star diagonal show this view
Observation
Under very dark skies, M10 can be seen with the naked eye. In reality, we need at least a pair of binoculars to see it.
A smaller telescope shows the bright, densely packed heart of this cluster but will struggle to resolve those stars. However, you should already be seeing resolved stars around its edges.
Larger telescopes reveal more of the individual stars in this beautiful cluster. Expect to see many tens of stars in this tight patch of sky if you have an eight-inch or larger telescope. Even so, the tight center may still appear like granular nebulosity.
PDFs for Printing
Each of the star maps above is reproduced as a pdf below. Each star map has a number in [square brackets] beneath it which corresponds to the file number below. If you want image [M10-1], for example, click the ‘download’ button next to it below and you’ll be able to open it as a printable pdf.