George's Random Astronomical Object

Object 67: Messier 71

Podcast release date: 21 February 2022

Right ascension: 19:53:46.5

Declination: +18:46:45

Epoch: J2000

Constellation: Sagitta

Corresponding Earth location: About 120 km north-northeast of Barbuda

Messier 71 or M71 is the third Messier object to be selected by chance for this podcast. At this rate, I should cover all of the Messier Catalogue sometime around episode 2434.

For people who don't know about the history of the Messier Catalogue, it consists of a set of star clusters, nebulae, and galaxies compiled by the eighteenth century astronomer Charles Messier [1]. Messier was interested in looking for comets, which look like faint fuzzy things that move slowly across the sky relative to the stars behind them, but he was annoyed by fuzzy things that did not seem to move and were therefore not comets. He therefore started a catalogue of these fake comets that are now known to be very beautiful amateur astronomy objects and/or objects that are much more important scientifically than any comet that Messier ever found. Some of these things were objects that Messier discovered himself, while others were things that he found out about in communications with other astronomers.

Messier 71 is one of these objects that was discovered by somebody else. It might have been discovered in 1746 by the Swiss astronomer Jean-Philippe Loys de Cheseaux, although his notes seem a little vague [2]. It was found again in 1775 by the German astronomer Johann Gottfried Koehler and then in 1780 by Pierre Mechain, who was another French comet hunter and an associate of Charles Messier [2].

In any case, M71 is a globular cluster that contains roughly 53000 stars (or, to be more accurate, it has a mass 53000 times the mass of the Sun) [3]. It's located at a distance of about 13050 light years (4000 pc) from Earth [4, 5]. Globular clusters are spheres of thousands of very old stars that typically orbit the Milky Way outside the disk of the galaxy in the galaxy's halo. However, M71 is a little abnormal in that it orbits the Milky Way within the plane of the galaxy, and it actually looks like it's more closely associated with either the bugle or the disk of our galaxy instead of the halo [6, 7, 8]. The stars in M71 also looks a little less densely packed than in a typical globular cluster [2]. At one point, it was debatable as to whether it was actually an open cluster [9], which is a smaller type of star cluster typically found in the plane of the Milky Way with stars that are younger and less tightly bound gravitationally than in a globular cluster.

One of the slightly unusual things about M71 is that its stars contain more elements heavier than hydrogen or helium than the average globular cluster [3, 9]. The reasons for this are a little unclear to me.

One reason could be because M71 is relatively young for a globular cluster. The key thing to keep in mind is that, when the universe initially formed, the only atoms in existence were mostly hydrogen and helium mixed in with an extremely rare lithium or beryllium atom. Heavier elements would have only formed in the stars that were created after the Big Bang, with those elements being ejected into space when the stars die. Over time, the heavier elements will accumulate in the gas that stars form out of, so younger stars will be expected to contain more heavier elements than older stars.

Most globular clusters are over 10 billion years old, and some would have formed less than 1 billion years after the Big Bang. In contrast, some measurements of the age of M71 place it around 8 billion years old [10]. Even though this still sounds extremely old by most people's standards, this age would mean that M71 contains more heavy elements because it formed at a time after the first few generations of stars had been able to produce lots of those heavy elements, whereas most other globular clusters formed at a time when the universe was younger and contained much more pristine hydrogen and helium gas. However, one relatively recent scientific analysis determined that the age of M71 could be around 12 billion years old [11], which would mean that it is the same age as most other globular clusters, so this age argument may not necessarily explain why M71 has more heavy elements.

If that is the case, my best guess is that the reason why M71 contains more heavy elements than the average globular cluster has to do with the fact that M71 is not associated with our galaxy's halo. Many other globular clusters look like they formed outside of our galaxy, possibly in someplace like a small dwarf galaxy, and then fell into our galaxy's halo. Dwarf galaxies generally contain few heavy elements, possibly in part because they contain fewer stars to produce these heavy elements to begin with and in part because, when the more massive stars die and form supernovae that eject the elements that they have formed into space, the gravity from the dwarf galaxies is too weak to stop the heavy elements from escaping the dwarf galaxies altogether. The stars in any clusters that formed in these dwarf galaxies would contain relatively few heavy elements as a result.

However, because M71 looks like it is associated with either the disk or the bulge of our galaxy and not the halo, it most likely formed within our galaxy. Even only 1 or 2 billion years after the Big Bang, which would have been the earliest that M71 would have formed, our galaxy would have contained a relatively large number of stars and therefore would have had a lot of stars that could produce heavy elements. Also, the Milky Way would have had the gravity needed to keep those heavy elements within our galaxy rather than letting them get blown out into intergalactic space. This means that the gas that the stars in M71 formed out of would have contained a lot of heavy elements than the gas that other globular clusters formed out of, thus leading to the differences that we see today between M71 and other globualr clusters [12, 13].

However, this is not what I personally found to be the most interesting thing about M71. Instead, what I think is most interesting is the fact that M71 seems to have lost about 90% of its stars as indicated by a lot of different observations and analyses [14]. First of all, as I mentioned before, M71 does not look very dense as a globular cluster, which kind of indicates that something's been happening to it [2]. Also, M71 is kind of on the small side for a globular cluster [3]. Additionally, M71 contains an awful lot of X-ray emitting objects for its size [14, 15]. These X-ray sources include pulsars, cataclysmic variable stars (which are binary star systems with a normal star and a white dwarf that is stripping gas away from the normal star), and chromospherically active binary star systems (where the stars are so close to each other that their magnetic fields interact and produce huge stellar flares). As a side note, if you want to learn more about these types of X-ray emitting star systems, I recommend listening to episodes 46 and 63 of my podcast. Anyway, the high number of X-ray sources implies that M71 used to contain many more stars than it does now.

The reason why M71 currently contains so few stars is because, as I mentioned before, it is orbiting the center of our galaxy through the disk of our galaxy [14]. The stars are not going to stick together for very long while this is happening. The visual that came to my mind was a small spherical flock of birds trying to fly through a much much larger flock of birds. The individual birds from the smaller flock probably aren't going to be able to stick together for very long, and they will probably end up joining up with the larger flock. This is kind of what is happening to M71. As it plows through the disk of our galaxy, gravitational interactions between the cluster and the stars from our galaxy's disk as well as the gravitational shocks from spiral arms in our galaxy are going to slowly strip stars out of M71 until virtually nothing remains of it.

This, however, will probably take billion of years, so you still have time to spot M71 in the sky beforehand. If you want to try to find it yourself, the cluster is located in the constellation Sagitta or the Arrow, which has nothing to do with the constellation Sagittarius or the Archer. Sagitta looks like a line with a V shape at one end, and M71 lines in the middle of this line of stars halfway between the third and fourth brightest stars in the constellation, Gamma and Delta Sagittae [2]. It's easily visible in even a pair of binoculars or a small telescope [2]. In a larger amateur telescope, like one with a 30 cm (12 inch) diameter, it should be possible to see hundreds of individual stars [2]. It's even a good target for amateur astrophotography. I really recommend checking it out.


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

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

[3] Dalgleish, H. et al., The WAGGS project-III. Discrepant mass-to-light ratios of Galactic globular clusters at high metallicity, 2020, Monthly Notices of the Royal Astronomical Society, 492, 3859

[4] Gaia Collaboration et al., Gaia Data Release 2. Kinematics of globular clusters and dwarf galaxies around the Milky Way, 2018, Astronomy & Astrophysics, 616, A12

[5] Baumgardt, H. and Vasiliev, E., Accurate distances to Galactic globular clusters through a combination of Gaia EDR3, HST, and literature data, 2021, Monthly Notices of the Royal Astronomical Society, 505, 5957

[6] Cudworth, Kyle M. and Hanson, Robert B., Space Velocities of 14 Globular Clusters, 1993, Astronomical Journal, 105, 168

[7] Geffert, M. and Maintz, G., First results of a photometric and astrometric study of the globular cluster M 71 (NGC 6838), 2000, Astronomy & Astrophysics Supplements, 144, 227

[8] Cadelano, M. et al., Proper Motions and Structural Parameters of the Galactic Globular Cluster M71, 2017, Astrophysical Journal, 836, 170

[9] Harris, William E., A Catalog of Parameters for Globular Clusters in the Milky Way, 1996, Astronomical Journal, 112, 1487

[10] Reid, Neill, HIPPARCOS Subdwarf Parallaxes: Metal-rich Clusters and the Thick Disk, 1998, Astronomical Journal, 115, 204

[11] Di Cecco, A. et al., On the Absolute Age of the Metal-rich Globular M71 (NGC 6838). I. Optical Photometry, 2015, Astronomical Journal, 150, 51

[12] Renaud, Florent et al., The origin of the Milky Way globular clusters, 2017, Monthly Notices of the Royal Astronomical Society, 465, 3622

[13] Massari, D. et al., Origin of the system of globular clusters in the Milky Way, 2019, Astronomy & Astrophysics, 630, L4

[14] Elsner, Ronald F. et al., Chandra X-Ray Observatory Observations of the Globular Cluster M71, 2008, Astrophysical Journal, 687, 1019

[15] Huang, R. H. H. et al., Study of Hubble Space Telescope counterparts to Chandra X-ray sources in the globular cluster M 71, 2010, Astronomy & Astrophysics, 513, A16

[16] Grelin, Antoine and Grelin, Dana, M71 - Globular Cluster in Sagitta, 2019, Galactic Hunter

Podcast and Website: George J. Bendo

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Last update: 25 April 2022