Messier 55 - The NGC 6809 Globular Star Cluster

By Tammy Plotner

The globular star cluster Messier 55 in the constellation of Sagittarius (The Archer) was obtained in infrared light with the VISTA survey telescope at ESO’s Paranal Observatory in Chile. This vast ball of ancient stars is located at a distance of about 17 000 light-years from Earth. - Image Credit: ESO/J. Emerson/VISTA

In the 18th century, while searching the night sky for comets, French astronomer Charles Messier kept noting the presence of fixed, diffuse objects in the night sky. In time, he would come to compile a list of approximately 100 of these objects, with the purpose of making sure that astronomers did not mistake them for comets. However, this list – known as the Messier Catalog – would go on to serve a more important function.

One of these objects is Messier 55, a globular star cluster located in the Sagittarius Constellation. Also known as the “Summer Rose Star”, this cluster is located 17,600 light-years from Earth and spans about 100 light-years in diameter. While it can be seen with binocular, resolving its individual stars can only be done with a small telescope and finderscope.


Located some 17,300 light years from planet Earth and spanning nearly 100 light years in diameter, this loose appearing ball of stellar points may not seem concentrated – but its home to tens of thousands stars. Does anyone really take the time to count them? You bet. M.J. Irwin and V. Trimble did just that during their 1984 study of Messier 55:

“We report star counts, as a function of position and apparent magnitude, in the rich, relatively open southern globular cluster NGC 6809 (M55). Three AAO 150arcsec plates were scanned by the Automatic Plate Measuring System (APM) at the Institute of Astronomy, Cambridge, and 20825 images were counted by its associated software. Previously known features of rich globular clusters which appear in the raw counts include a flattening of the luminosity function, increased central concentration of bright stars relative to faint ones (normally interpreted as mass segregation), and mild deviations in radial profile from King models. Crowding of the field, which causes the counting procedure to miss faint stars preferentially near the cluster center, contributes to all of these, and may be responsible for all of the apparent mass segregation, but not for all of the other two effects.”

Globular cluster Messier 55 (M55, or NGC 6809) in the constellation Sagittarius, as imaged by the ESO 3.6-metre telescope on La Silla. Release date: 3 December 2009. - Image Credit: ESO

But just want good does counting the stars do? Well, knowing how many stars are within a given area helps astronomers compute other things as well, like chemical abundances. Said Carlos Alvarez and Eric Sandquist in their 2004 study:

“We have compiled the asymptotic giant, horizontal, and upper red giant branch (AGB, HB, and RGB) stars in the globular cluster M55 (NGC 6809). Using the star counts and the R-parameter we compute the initial helium abundance. The ratio is unusually high for a globular cluster, being almost 2 away from the predicted values, and the highest recorded for a massive globular cluster. We argue that M55’s particular HB morphology and metallicity have produced long-lived HB stars that are not too blue to avoid producing AGB stars. This result hints that we are able to map evolutionary effects on the HB. Finally, although we find no evidence of variations in HB morphology with distance from the center of the cluster, the red HB stars are significantly less concentrated than the majority of HB stars, and the bluest HB stars are more centrally concentrated.”

Studying globular clusters photometrically also gives astronomers the advantage of comparing them to others, to see how each evolves. As P. Richter (et al) indicated in their 1999 study:

“We present Stroemgren CCD photometry for the two galactic globular clusters M55 (NGC 6809) and M22 (NGC 6656). The difference between M55 and M22 may resemble the difference in integral CN band strength between M31 globular clusters and the galactic system. The colour-magnitude diagram of M55 shows the presence of a population of 56 blue-straggler stars that are more centrally concentrated than the red giant-branch stars.”

And viewing globular clusters like Messier 55 in a different wavelength of light other than optical reveals even more stunning details – like the vision of the XMM-Newton. As N.A. Webb (et al) said in their 2006 study:

“Using the new generation of X-ray observatories, we are now beginning to identify populations of close binaries in globular clusters, previously elusive in the optical domain because of the high stellar density. These binaries are thought to be, at least in part, responsible for delaying the inevitable core collapse of globular clusters and their identification is therefore essential in understanding the evolution of globular clusters, as well as being valuable in the study of the binaries themselves. Here, we present observations made with XMM-Newton of globular clusters, in which we have identified neutron star low mass X-ray binaries and their descendants (millisecond pulsars), cataclysmic variables and other types of binaries. We discuss not only the characteristics of these binaries, but also their formation and evolution in globular clusters and their use in tracing the dynamical history of these clusters.”

History of Observation:

M55 was originally discovered by Abbe Lacaille on June 16th, 1752, when he was observing in South Africa. In his notes, he wrote: “It resembles an obscure nucleus of a big comet.” Of course, our own comet hunter, Charles Messier, would search for a good many years before he recovered it to add to his own catalog. By July 24th, 1778, he found the object and recorded it as follows in his notes:

“A nebula which is a whitish spot, of about 6′ extension, its light is even and does not appear to contain any star. Its position has been determined from zeta Sagittarii, with the use of an intermediate star of 7th magnitude. This nebula has been discovered by M. l’Abbe de LaCaille, see Mem. Acad. 1755, p. 194. M. Messier has looked for it in vain on July 29, 1764, as reported in his memoir.”

Messier 55 in Sagittarius. - Image Credit: Hewholooks/Wikimedia Commons

Johann Elert Bode, Dunlop and Caroline Herschel would follow, but it would be Sir William Herschel who would be first to glimpse the resolvability of this great globular cluster. In his private notes he writes:

“A rich cluster of very compressed stars, irregularly round, about 8 minutes long. By the observation of the small 20 feet telescope, which could reach stars 38.99 times as far as the eye, the profundity of this cluster cannot be much less than of the 467th order: I have taken it to be of the 400th order.”

Locating Messier 55:

M55 is by no means easy to find. One of the best ways to locate it is to begin at Theta 1 and Theta 2 Sagittarius, where you’ll find it approximately two finger widths northwest of this pair approximately four degrees. Both Thetas are on the dim side for the unaided eye – about magnitude 4 and 5 respectively, but you’ll recognize them when you find two stars separated by less than half a degree and oriented north/south.

For average binoculars, this will put M55 about a binocular field away to the northwest. For average image correct finderscopes, place the Thetas in the 8:00 position at the edge of the finderscope field and go to the eyepiece with the lowest possible magnification to locate it.

Although it has a high visual brightness, M55 has low surface brightness so it isn’t suitable to urban or light polluted skies. With dark sky conditions, binoculars will see it as a round hazy patch – like a diffuse comet, while small telescopes can begin to resolve individual stars. Larger aperture telescopes will pick out the fine grain of low magnitude stars quite easily!

Enjoy your own resolvability of this great globular cluster!

And as always, here are the quick facts on this Messier Object:

Object Name: Messier 55
Alternative Designations: M55, NGC 6809
Object Type: Class XI Globular Cluster
Constellation: Sagittarius
Right Ascension: 19 : 40.0 (h:m)
Declination: -30 : 58 (deg:m)
Distance: 17.3 (kly)
Visual Brightness: 6.3 (mag)
Apparent Dimension: 19.0 (arc min)

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.


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