Object 33: Messier 56

Podcast release date: 02 November 2020

Right ascension: 19:16:35.6


Epoch: ICRS

Constellation: Lyra

Corresponding Earth location: About one-third of the distance from Bermuda to Miami

Messier 56 (M56), also known as NGC 6779, is actually the second Messier object to be featured in this podcast. The first object was the open cluster Messier 23, which I discussed in episode 20.

Like many of the other Messier objects, M56 was first identified by Charles Messier in the late eighteenth century [1]. Messier was much more interested in identifying comets, which, in eighteenth century telescopes, looked like fuzzy moving objects. He was irritated by finding fuzzy objects in the sky that didn't move, and he started his catalog to keep track of these things, which he treated as fake comets [1,2]. In fact, on the day in 1779 Charles Messier discovered M56, he also discovered a new comet [1]. Over time, the list of fake comets expanded to 109 objects. Many of these fake comets are among the most spectacular star clusters, nebulae, and galaxies that can be seen with an amateur telescope, and they are also scientifically much more important than any of the comets that Charles Messier really cared about. This just shows that Messier just hadn't set his priorities right.

M56 is one of multiple globular clusters in the Messier Catalogue. These are spheres of somewhere between about 100 thousand and 1 million stars that are found orbiting the Milky Way outside the plane of the galaxy in the galaxy's halo. M56 itself has a mass about 230 thousand times the mass of the Sun [3], which means that it probably has approximately that many stars. I found multiple distance measurements for the cluster ranging from about 30 to 36 thousand light years (9.4-11 kpc) [4,5,6,7,8]. Even though M56, like other globular clusters, spends most of its time in the halo, it may be currently passing through the plane of the galaxy [9]. It appears in the Earth's sky to have a diameter somewhere around 7 arcminutes in size or roughly one-quarter the diameter of the Moon [1,10], although the diameter seems to depend on who's measuring it and how they are measuring it.

The stars in globular clusters are old. The typical ages for globular clusters are between 10 and 13 billion years old, which is almost as old as the Universe. M56 is at the really old end of these old clusters of stars. You can think of it as older than old. It's age is estimated to be somewhere around 12.75 or 13 billion years old [11,12].

M56 is actually so old that it might be among the second set of stars to ever form in the universe, and this is indicated by the relatively low amount of elements heavier than hydrogen and helium in the atmospheres of the cluster's srars. When the universe formed from the Big Bang, virtually all of the atoms in the universe were hydrogen and helium. Any elements heavier than these were created within the first stars. Now, the stars in M56 are not pure hydrogen and helium; they seem to contain a few heavier elements, but the ratio of heavy elements to hydrogen is somewhere around 1% of the ratio for a star like our Sun [12]. The lack of heavy elements along with the really old ages of the stars indicates that M56 formed very, very early on in the universe.

Aside from the fact that M56 is old and that the stars are almost pure hydrogen and helium, two other scientific results stand out. First, a group of astronomers detected diffuse X-ray emission from the cluster, which seems rather unusual [9]. The origin of this X-ray emission is up for debate. As you may recall M56 generally spends most of its time in the halo but is currently passing through the disc of the galaxy, and one hypothesis that has been put forward to explain the X-ray emission is that diffuse interstellar gas inside the cluster is colliding with diffuse interstellar gas inside the disc of the Milky Way, thus causing the gas inside M56 to heat up and produce the observed X-ray emission [9]. I would expect this collision to cause M56 to lose all of its interstellar gas, although the stars should be unaffected. M56 will probably pass through the galaxy with just about all of its stars but without any gas. This phenomenon seems very rare, but if this is what is actually happening in M56, it could be something that also generally happens to all globular clusters and could explain why globular clusters seem to contain almost no interstellar gas.

The second and much more interesting scientific result regarding M56 involves something called the Gaia Sausage. This sounds like a meat-free sausage that you would be able to buy at your local organic supermarket, and you would almost be right. Instead, the Gaia Sausage is actually a collection of stars that were found by the Gaia spacescraft, which was designed to make ultra-precise distance and velocity measurements of nearby stars in the Milky Way. The stars don't actually look like a sausage in the sky, which is rather disappointing. Instead, the stars look like a sausage when they are put in a plot comparing their circular motion around the centre of the Milky Way to their motions towards or away from the centre of the galaxy. Most nearby stars in the Milky Way are moving in a circle and therefore, in the plot that I just described, would mostly be found around a single point. The Gaia Sausager stars are actually moving either into or out of the center of the galaxy and therefore fill out this oval, sausage-shaped region along one axis of the plot [13]. The stars in the Gaia Sausage probably came from a dwarf galaxy that fell into the Milky Way and got torn apart somewhere between 8 and 11 billion years ago [13]. Analyses of the orbit of M56 and the properties of its stars implies that it might have originally been part of this ancient dwarf galaxy that no longer exists [14]. In other words, M56 is a cluster of stars that was captured from another galaxy. It's not the only globular cluster that was potentially captured, either [14]. More work is going on regarding the Gaia Sausage, and hopefully astronomers will soon be marketing vegetarian sausages bearing this name.

So, I can say quite a few things about M56 as an object of scientific interest to professional astronomers, but it also attracts attention from amateur astronomers as well. Even though I have some experience with amateur telescopes, I have to admit that I do not recall ever seeing M56. When I have viewed this part of the sky with an amateur telescope, I normally look at M57, also known as the Ring Nebula, in the constellation Lyra, or I go look at the much closer and brighter globular cluster M13 in the constellation Hercules. I think I overlooked M56 completely. So, I tried looking up M56 in some amateur astronomy guides, and when those books discussed this part of the sky, they first suggested looking at M57, also known as the Ring Nebula, in the constellation Lyra and then taking a look at the much closer and brighter globular cluster M13 in the constellation Hercules. I think other amateur astronomers also overlooked M56 completely. Eventually, though, I did find some amateur astronomy books that did mention M56, and they said don't overlook it [1].

The best way to find M56 with a telescope is to look about halfway between Beta Cygni and Gamma Lyrae [1,10]. Beta Cygni is the bright star at the bottom of the cross in the constellation Cygnus, and Gamma Lyrae is the lower left corner of the parallelogram in the constellation Lyra. It has a magnitude of 8.1 [16], which is slightly more than two magnitudes lower than what can be seen with the unaided eye, but the cluster should be visible in a small telescope or even a finderscope as a compact fuzzy object [1,10]. In a 20 cm (8 inch) telescope, it should be possible to see more of a difference between the bright central core and the diffuse region around it [10].


[1] O'Meara, Stephen James, The Messier Objects, 1998

[2] Yeomans, Donald K., Comets: a chronomigical history of observation, science, myth, and folklore, 1991

[3] Boyles, J. et al., Young Radio Pulsars in Galactic Globular Clusters, 2011, Astrophysical Journal, 742, 51

[4] Pryor, C. and Meylan, G., Velocity Dispersions for Galactic Globular Clusters, 1993, in Structure and Dynamics of Globular Clusters, 50, 357

[5] Ivanov, Valentin D. et al., Extending the Red Giant Branch versus Metallicity Calibration toward Metal-poor Systems: Near-Infrared Photometry of the Galactic Globular Clusters M56 and M15, 2000, Astronomical Journal, 119, 2274

[6] Harris, William E., A New Catalog of Globular Clusters in the Milky Way, 2010, arXiv e-prints, arXiv:1012.3224

[7] Kharchenko, N. V. et al., Global survey of star clusters in the Milky Way. II. The catalogue of basic parameters, 2013, Astronomy & Astrophysics, 558, A53

[8] Piatti, Andrés E. and Carballo-Bello, Julio A., Extra-tidal structures around the Gaia Sausage candidate globular cluster NGC 6779 (M56), 2019, Monthly Notices of the Royal Astronomical Society, 485, 1029

[9] Hopwood, M. E. L. et al., A possible detection of diffuse extended X-ray emission in the environment of the globular cluster NGC 6779, 2000, Monthly Notices of the Royal Astronomical Society, 316, L5

[10] Eicher, David J., The Universe from Your Backyard, 1988

[11] Hatzidimitriou, D. et al., BVRI photometry of the galactic globular cluster NGC 6779, 2004, Monthly Notices of the Royal Astronomical Society, 348, 1157

[12] VandenBerg, Don A. et al., The Ages of 55 Globular Clusters as Determined Using an Improved Delta VHBTO Method along with Color-Magnitude Diagram Constraints, and Their Implications for Broader Issues, 2013, Astrophysical Journal, 775, 134

[13] Belokurov, V. et al., Co-formation of the disc and the stellar halo, 2018, Monthly Notices of the Royal Astronomical Society, 478, 611

[14] Myeong, G. C. et al., The Sausage Globular Clusters, 2018, Astrophysical Journal Letters, 863, L28

[15] Vanderbeke, Joachim et al., G2C2 - I. Homogeneous photometry for Galactic globular clusters in SDSS passbands, 2014, Monthly Notices of the Royal Astronomical Society, 437, 1725


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