Astronomy

Messier 72 – the NGC 6981 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 72.

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 72, a globular cluster about 54,570 light years away in the direction of the Aquarius constellation. Originally discovered by French astronomer Pierre Méchain a few years prior, Messier would go on to include this star cluster in his catalog. Located in close proximity to Messier 73, this globular cluster is one of the smaller and fainter Messier objects in the night sky.

Description:

Way out there in no man’s land 54,570 light years from Earth, this lonely globular cluster is one of the most remote star gatherings on the books. Although it’s not very compact, it has to be one of the most intrinsically luminous globular clusters of all – or we wouldn’t even be able to see it from this distance. But hold on… It’s coming at us at a speed of 255 kilometers per hour! Perhaps in a few hundred thousand years, this 106 light year diameter ball of stars will be much easier to observe.

So will it have changed much? Not hardly. Astronomers seem to believe it has been around more or less for at least 15 million years. As E. Brocato (et al) explained in a 2009 study:

“Galactic clusters have long been the stellar systems most suitable for testing the predictions of stellar evolution theories over the main evolutionary phases of both H- and He-burning structures. In more recent times, the color-magnitude diagrams (CMDs) of Galactic globular clusters (GCs) have become of critical importance in elucidating the early phases of galactic evolution, as well as providing severe constraints to the age of the universe. This has stimulated much effort on both the theoretical and observational fronts. During each night many standard stars were observed, in order to determine color equations, extinction, and zero points. Reductions for the primary and local standards followed exactly the same procedures, to mitigate the introduction of systematic errors. As part of a program studying the RR Lyrae variables in selected Galactic GC’s, the clusters NGC 6981 (M72) and NGC 1851 were also observed as part of the same program, and similar observing and reduction techniques were followed for each.”

Close up of Messier 72, taken by the Hubble Space Telescope. Credit: ESA/NASA/HST

So what really does happen when you start looking deep inside a globular cluster? You reach the core. And what do you find inside the core? Ask the Hubble Team! As A. Sollima (et al) indicated in a 2007 study:

“We used deep observations collected with Advanced Camera for Surveys (ACS) at Hubble Space Telescope (HST) to derive the fraction of binary systems in a sample of 13 low-density Galactic globular clusters. By analysing the colour distribution of main-sequence stars we derived the minimum fraction of binary systems required to reproduce the observed colour–magnitude diagram morphologies. We found that all the analysed globular clusters contain a minimum binary fraction larger than 6 per cent within the core radius. The estimated global fractions of binary systems range from 10 to 50 per cent depending on the cluster. A dependence of the relative fraction of binary systems on the cluster age has been detected, suggesting that the binary disruption process within the cluster core is active and can significantly reduce the binary content in time.”

But, is there anything hiding there we can’t see? Like perhaps mysterious dark matter? The answer is yes. According to the work of Lloyd Jones:

“Globular clusters are effectively approximated by using a King-Michie distribution. Sherbakov proposed that identifying the possible amount of dark matter present in globular clusters would improve understanding of their behavior. This thesis examines the properties of one-component and two-component models in order to maximize the percentage of dark matter. We find that we can obtain excellent density fits for some globular clusters (e.g., NGC 288 and NGC 6981) with up to 95% of the matter being dark.”

Messier 72 by the Hubble Space Telescope. Credit: ESA/NASA/HST

History of Observation:

And it was dark on the night of August 29-30th, 1780 when Pierre Mechain became the very first to lay eyes on this globular cluster and log it properly. His notes were then given the Charles Messier who added it to his catalog on October 4th, 1780 with the following description:

“Nebula seen by M. Mechain in the night of August 29-30, 1780, above the neck of Capricorn. M. Messier looked for it on the 4th and 5th October following: the light is as faint as for the preceding; near it is a small telescopic star: the position was determined from the star Nu Aquarii, of fifth magnitude.’ (diam 2′)”.

Beginning in 1783, Sir William Herschel would be the first to shatter Messier 72 into individual stars. Herschel kept his own notes on Messier objects private and non-competitive, and of his many observations, he writes:

“It is a cluster of stars of a round figure, but the very faint stars on the outside of globular clusters are generally a little dispersed so as to deviate from a perfectly circular form. The telescopes which have the greatest light shew this best.” and “It is very gradually extremely condensed in the centre, bit with much attention, even there, the stars may be distinguished.”

He would return again and again to this object: “There are many stars in the filed of view with it, but they are of several magnitudes totally different from the excessively small ones which compose the cluster.” For its resolvability in his telescope seemed to fascinate him. “It is not possible to form an idea of the number of stars that may be in such a cluster; but I think we cannot estimate them by hundreds.”

Sir William’s last observation of M72 came about on October 30, 1810. He writes; “Having been about 20 minutes at the telescope to prepare the eye properly for seeing critical objects, the 72nd of the Connois. came into the field. It is a very bright object.” Don’t you wish you were there with him? “A beautiful cluster of stars.”

The location of Messier 72 in the Aquarius constellation. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Locating Messier 72:

Messier 72 isn’t the easiest of the globular clusters to cut from the herd. It’s best found by identifying Epsilon Aquarii, then aiming your sites approximately two finger widths (3 degrees) south, then about a finger width (a degree and a half) east. If you have trouble distinguishing Epsilon, you can also try looking about a fistwidth (less than 10 degrees) east of Alpha Capricorni.

Small and faint, only larger binoculars under dark skies will be able to capture this Messier. Even small telescopes will have difficulty making out much more than a round contrast change that shows some graininess to the edges. Larger aperture telescopes will also have some difficulty resolving this particular globular. Dark skies are a must!

Object Name: Messier 72
Alternative Designations: M72, NGC 6981
Object Type: Class IX Globular Cluster
Constellation: Aquarius
Right Ascension: 20 : 53.5 (h:m)
Declination: -12 : 32 (deg:m)
Distance: 55.4 (kly)
Visual Brightness: 9.3 (mag)
Apparent Dimension: 6.6 (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.

Sources:

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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