Messier 75 – the NGC 6864 Globular Cluster

Welcome back to Messier Monday! Today, we continue in our tribute to our dear friend, Tammy Plotner, by looking at the globular cluster known as Messier 75!

During the 18th century, famed French astronomer Charles Messier noticed the presence of several “nebulous objects”  while surveying the night sky. Originally mistaking these objects for comets, he began to catalog them so that others would not make the same mistake. Today, the resulting list (known as the Messier Catalog) includes over 100 objects and is one of the most influential catalogs of Deep Space Objects.

One of these objects is Messier 75 (aka. NGC 6864), a globular cluster roughly 67,500 light years from Earth near the southern constellation Sagittarius. This object is also about 14,700 light years away from the Galactic Center, and on the located on the other side relative to Earth. Because of its distance and location, this object is virtually impossible to see binoculars and difficult to resolve with small telescopes.


At a distance of around 67,500 light years from Earth, M75 is one of the most remote of of all globular clusters, positioned 47,600 light years beyond the Milky Way’s galactic center. Spanning 180 light years across, it blazes with the candlepower of 180,000 suns! What’s it doing out there? Who knows… Maybe generating new variable stars – or just crashing its red giants into each other. As Tim Adams (et al) said in a 2004 study:

“We investigate a means of explaining the apparent paucity of red giant stars within post-core-collapse globular clusters. We propose that collisions between the red giants and binary systems can lead to the destruction of some proportion of the red giant population, by either knocking out the core of the red giant or by forming a common envelope system which will lead to the dissipation of the red giant envelope. Treating the red giant as two point masses, one for the core and another for the envelope (with an appropriate force law to take account of the distribution of mass), and the components of the binary system also treated as point masses, we utilize a four-body code to calculate the time-scales on which the collisions will occur. We then perform a series of smooth particle hydrodynamics runs to examine the details of mass transfer within the system. In addition, we show that collisions between single stars and red giants lead to the formation of a common envelope system which will destroy the red giant star. We find that low-velocity collision between binary systems and red giants can lead to the destruction of up to 13 per cent of the red giant population. This could help to explain the color gradients observed in PCC globular clusters. We also find that there is the possibility that binary systems formed through both sorts of collision could eventually come into contact perhaps producing a population of cataclysmic variables.”

Hubble Legacy Archive image of the globular cluster Messier 75. Credit: STScI/NASA, ST-ECF/ESA, CADC/NRC/CSA.

But red stars mean old, doesn’t it? And if M75 is way out there – maybe it’s old, too. But just how old? According to Genevieve Parmentier and Eva K. Grebel of the Institute of Astronomy (in a 2005 study):

“We investigate what may be the origin of the presently observed spatial distribution of the mass of the Galactic Old Halo globular cluster system. We propose its radial mass density profile to be a relic of the distribution of the cold baryonic material in the protogalaxy. Assuming that this one arises from the profile of the whole protogalaxy minus the contribution of the dark matter (and a small contribution of the hot gas by which the protoglobular clouds were bound), we show that the mass distributions around the Galactic centre of this cold gas and of the Old Halo agree satisfactorily. In order to demonstrate our hypothesis even more conclusively, we simulate the evolution with time, up to an age of 15 Gyr, of a putative globular cluster system whose initial mass distribution in the Galactic halo follows the profile of the cold protogalactic gas. We show that beyond a galactocentric distance of order 2–3 kpc, the initial shape of such a mass density profile is preserved despite the complete destruction of some globular clusters and the partial evaporation of some others. This result is almost independent of the choice of the initial mass function for the globular clusters, which is still ill determined. The shape of these evolved cluster system mass density profiles also agrees with the presently observed profile of the Old Halo globular cluster system, thus strengthening our hypothesis. Our result might suggest that the flattening shown by the Old Halo mass density profile at short distances from the Galactic center is, at least partly, of primordial origin.”

History of Observation:

After its discovery on the night of August 27/28, 1780 by Pierre Mechain, this faint ball of stars was dutifully observed and cataloged by Charles Messier on October 5thand added to his catalog as object #75 on October 18th, 1780. As Messier noted at the time:

“Nebula without star, between Sagittarius and the head of Capricorn; seen by M. Mechain on August 27 and 28, 1780. M. Messier looked for it on the following October 5, and on October 18, compared it with the star 4 Capricorni, of sixth magnitude, according to Flamsteed: it seemed to M. Messier to be composed of nothing but very small stars, containing nebulosity: M. Mechain reported it as a nebula without stars. M. Messier saw it on October 5; but the Moon being above the horizon, and it was not until the 18th of the same month that he was able to judge about its form and determine its position.”

This star chart for M75 represents the view from mid-northern latitudes for the given month and time. Credits: Stellarium

By 1799, Sir William Herschel was on it – but not resolving it. “There is not the least appearance of its consisting of stars, but it resembles other clusters of this kind, when they are seen with low space-penetrating and magnifying powers,” he wrote.

It would take Herschel another 11 years before he was able to make out individual stars and pronounce in his private notes: “It is a globular cluster.” Twenty years later, his son John would say: “Not bright; small; round; pretty suddenly brighter toward the middle; 2′ diameter; mottled, but not resolved. An insignificant object.”

However, Admiral Smyth thought it just a little bit better. As he wrote upon viewing the object:

“A globular cluster in the space between the left arm of Sagittarius and the head of Capricorn, and 7 deg 1/2 degrees to the south-south-west of Beta Capricorni. It is a lucid white mass among some glimpse stars, with a large one in the nf verge field. It was discovered by Pierre Mechain in 1780, who considered it as a nebula without stars; but Messier viewed it as a mass of very small stars, which opinion, on an object which at best is rather faint, was bold. In 1784, it was resolved by William Herschel’s 20-foot Newtonian, and, on being gauged, was assigned a profundity of the 734th order. No wonder that this miniature of 3 Messier should be pale to gaze!”

The location of Messier 75 near the Sagittarius constellation. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Locating Messier 75:

Messier 75 is a difficult catch in binoculars because of its small size and low brightness, but with a simple trick, you can catch it under dark skies a little more easily than you might think. Rather than progress in right ascension, try declination! Using Theta Aquilae (the southernmost star in the Eagle’s wings) as your guide, identify the bright duo of Alpha Capricornii next. Draw a mental line between the two and think of this as one star hop.

Make another hop the same distance, keeping your finderscope or binoculars aligned due south of Theta and you’ll be there! Although it will be almost stellar in size in binoculars, M75 is very achievable under dark sky conditions and shows as a small, round contrast change in small telescopes. Mid-aperture scopes will pick out a graininess in texture and larger telescopes will begin resolution. Because it is a faint object, it does require darker skies and is not well-suited to light polluted areas or moonlit nights.

Enjoy your own observations of this distant ball of stars…

And here are the quick facts on this star cluster to help you get started:

Object Name: Messier 75
Alternative Designations: M75, NGC 6864
Object Type: Class I Globular Cluster
Constellation: Sagittarius
Right Ascension: 20 : 06.1 (h:m)
Declination: -21 : 55 (deg:m)
Distance: 67.5 (kly)
Visual Brightness: 8.5 (mag)
Apparent Dimension: 6.8 (arc min)

We have written many interesting articles about Messier Objects and globular clusters here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier ObjectsM1 – The Crab Nebula, Observing Spotlight – Whatever Happened to Messier 71?, 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.


Tammy Plotner

Tammy was a professional astronomy author, President Emeritus of Warren Rupp Observatory and retired Astronomical League Executive Secretary. She’s received a vast number of astronomy achievement and observing awards, including the Great Lakes Astronomy Achievement Award, RG Wright Service Award and the first woman astronomer to achieve Comet Hunter's Gold Status. (Tammy passed away in early 2015... she will be missed)

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