Messier 9 (M9) – The NGC 6333 Globular Cluster

Due to Easter Weekend, we are celebrating Messier Monday a day later than usual this week. But as always, this latest installment is in honor of our dear friend and contributor Tammy Plotner.

In the 18th century, French astronomer Charles Messier kept noting the presence of fixed, diffuse objects in the night sky. At the time, he mistook them for comets, but eventually realized his mistake and began compiling them into a list.  Known as the Messier Catalog, this list was created to ensure that other astronomers did not make the mistake.

But more than that, it would go on to serve as a milestone in the history of astronomy and the study of Deep Sky Objects. One of these objects is Messier 9 (aka. M9 or NGC 6333), a globular cluster that is positioned in the southern part of the constellation of Ophiuchus. It is one of the nearer globular clusters to the center of the Milky Way Galaxy, with a computed distance of 5,500 light-years from the Galactic Center.


Messier 9 is one of the closest globular clusters to our Milky Way galactic nucleus. At about 25,800 light years from our Solar system, it keeps its stars collected in a loose sphere that encompasses approximately 90 light years of space. Speeding away from us at a speed of 224 kilometers per second, M9 is also passing through an interstellar dust cloud that is dimming some of the light we see. Even though the light is blocked, the combined light of this ball of distant suns shines 120,000 times brighter than Sol!

If you look closely, you’ll notice Messier 9 is also a little flattened, too. That’s caused by the gravity of our own galactic core pulling on it. Deep inside there are 13 variable stars there, too! According to its color diagram, M9 is a very metal poor star cluster, but has well-defined giant branches and blue horizontal branches. It is entirely possible that its red giant population is well fed because it passes through enriched proto-globular clusters’ clouds.

The constellation Ophiuchis. Credit:
The constellation Ophiuchis, with M9 visible towards the southern end. Credit:

History of Observation:

M9 is one of the original discoveries of Charles Messier, which he observed on June 3rd, 1764. As he recorded the discovery:

“In the night of May 28 to 29, 1764, I have determined the position of a new nebula which is situated in the right leg of Ophiuchus, between the stars Eta and Rho of that constellation; that nebula doesn’t contain any star; I have examined it with a Gregorian telescope which magnifies 104 times; it is round its light is faint, and its diameter is about 3 minutes of arc: its right ascension is 256d 20′ 36″, and its declination 18d 13′ 26″ south.”

It would be a few years before M9’s true nature became apparent when Sir William Herschel first resolved it:

“May 3, 1783. I viewed the nebula between Eta and Rho Ophiuchi, discovered by Mr. Messier, in 1764. With a 10-feet reflector, and a magnifying power of 250, I see several stars in it, and make no doubt a higher power, and more light, will resolve it all into stars. This seems to be a good nebula for the purpose of establishing the connection between nebulae and clusters of stars in general. On June 18, 1784, the same nebula viewed with a large Newtonian 20-feet reflector showed a very large and very bright cluster of excessively compressed stars. The stars are but just visible, and are of unequal magnitudes: the large stars are red; and the cluster is a miniature of that near Flamsteed’s 42d Comae Berenices.”

The Messier 9 globular cluster, located . Credit: NASA
The Messier 9 globular cluster, located 5500 light years from the Galactic Center. Credit: NASA

However, it was Admiral Smyth who describes it as we see it today in most backyard telescopes:

“A globular galaxy-cluster, on the Serpent-bearer’s left leg, with a coarse telescopic double star in the np quadrant. This fine object is composed of a myriad of minute stars, clustering into a blaze in the center, and wonderfully aggregated, with numerous outliers seen by glimpses. It was registered by Messier in 1764; and described by him as a nebula, “unaccompanied by any star.” The mean apparent place was carefully differentiated from Eta Ophiuchi. Sir William Herschel resolved it with his 20-foot reflector, in 1784; and he estimated its profundity as, at least, of the 344th order. He thinks it a miniature of No. 53 Messier; and it is one of those which forms a capital object, for proving space-penetrating power of a telescope. It lies 3 deg to the south-east of Eta, and rather more than a quarter of the way from Antares to Altair.”

Locating Messier 9:

Unfortunately, there is no easy way to locate M9. The constellation of Ophiuchus is sprawling and it will take some practice to identify Eta. Once you do, you’ll find this small globular cluster about 3 degrees (two finger widths) to the southeast. Look for a small triangular formation of stars and aim slightly above the southernmost.

While M9 is achievable in binoculars, keep in mind that low magnifying powers and aperture, such as 6X30 models, will mean the image you will see is tiny and you’ll have to keep your binoculars steady to pick it out of the field. Larger models will improve the situation, but you’ll need dark skies to capture this Messier without more power. In a small telescope, M9 will still have difficulty resolving – but will show hints of a concentrated core region. As you approach the 6″ and larger range, resolution begins and large aperture scopes can fully resolve this loosely constructed globular cluster fully.

And here are the quick facts on this globular cluster:

Object Name: Messier 9
Alternative Designations: M9, NGC 6333
Object Type: Class VIII Globular Cluster
Constellation: Ophiuchus
Right Ascension: 17 : 19.2 (h:m)
Declination: -18 : 31 (deg:m)
Distance: 25.8 (kly)
Visual Brightness: 7.7 (mag)
Apparent Dimension: 12.0 (arc min)

Enjoy your viewing!

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

2 Replies to “Messier 9 (M9) – The NGC 6333 Globular Cluster”

  1. Interesting that both M. Messier and Wm. Herschel describe its location as between Eta and Rho Ophiuchi when it’s clearly between Eta and Ksi Ophiuchi. Were the Bayer designations changed since that time?

    Typo? The intro describes the distance of M9 from the Galactic Centre as 5,500 ly, while the caption under the NASA image of M9 gives it as 55,000 ly.

    Despite my comments, I AM enjoying this series.

  2. In my view globular clusters did not form into a small galaxy, because they lack the center black hole, which would give it momentum to form into a spiral form…
    Comments appreciated 😉

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