Want to stay on top of all the space news? Follow @universetoday on Twitter
Object Name: Messier 55
Alternative Designations: M55, NGC 6809
Object Type: Class XI Globular Cluster
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)
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!
What You Are Looking At: 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. “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.” says M.J. Irwin and V. Trimble, “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.”
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. “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.” says Carlos Alvarez and Eric Sandquist, “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. “We present Stroemgren CCD photometry for the two galactic globular clusters M55 (NGC 6809) and M22 (NGC 6656).” says P. Richter (et al), “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. “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.” says N.A. Webb (et al), “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: M55 was originally discovered by Abbe Lacaille on June 16, 1752, when he was observing in South Africa. In his notes he writes: “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. Why? Because of low observing position from his location perhaps? But, nonetheless, Messier’s patience paid off when he observed and logged it on July 24, 1778: “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.”
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.”
Enjoy your own resolvability of this great globular cluster!
Top M55 image credit, Palomar Observatory courtesy of Caltech, M55 NASA Image, M55 courtesy of 2MASS, M55 courtesy of XMM-Newton and M55 color image courtesy Hillary Mathis, REU Program/NOAO/AURA/NSF.