George's Random Astronomical Object

Object 106: NGC 6712

Podcast release date: 04 September 2023

Right ascension: 18:53:04.3

Declination: -08:42:22

Epoch: J2000

Constellation: Scutum

Corresponding Earth location: The Andes in the state of Huanuco in Peru

The coordinates for this episode point to a location in another weird and obscure constellation that neither you nor I have ever heard of, which means that, once again, it's time for me to rant about stupid constellations. This time, the stupid constellation is Scutum, the shield, which is another collection of randomly arranged faint stars in the sky. This constellation was invented in 1684 by Polish astronomer Johannes Hevelius, and it was supposed to represent the shield of King John III Sobieski of Poland [1]. Although the Milky Way in this part of the sky is relatively bright, the constellation does not contain any notable bright stars, and, of course, the stars that are present do not look like a shield no matter how big your imagination is.

Anyway, the specific object featured in this episode is the globular cluster NGC 6712. If you ask an astronomer about globular clusters, they will tell you that they are spherical clusters of extremely old stars that orbit outside the plane of the Milky Way. If you don't ask an astronomer about globular clusters, they will still tell you that they are spherical clusters of extremely old stars that orbit outside the plane of the Milky Way. They will follow you around and tell you about things like color-luminosity diagrams and distance measurements using Cepheid variable stars until you find a way to escape.

Anyhow, NGC 6712 is a globular cluster located at a distance of about 22.8 thousand light years (7000 pc) [2] in roughly the direction the Milky Way's bulge. This makes it hard to study. One problem is the a large number of stars within the disk and bluge of our galaxy, including the much closer Scutum Stellar Cloud, are located between us and the cluster, making it tricky to identify which stars are part of the cluster. Another problem is that a large amount of interstellar dust within the plane of our galaxy sits between us and the cluster. Interstellar dust absorbs starlight, so NGC 6712 tends to look relatively faint as a result. Nonetheless, the cluster is still very interesting to astronomers for a couple of different reasons.

The first indication that NGC 6712 was rather unusual in terms of globular clusters came in a study published in 1999 by a group led by Guido de Marchi [3]. They found that the star cluster strangely seemed to lack the large numbers of small red dwarfs typically found in other globular clusters. After evaluating and rejecting various other explanations for why this was happening, de Marchi reached the conclusion that NGC 6712 probably had fewer red dwarf stars because itr was much more severely tidally disrupted than the average globular cluster, and the tidal disruptions appear to remove the smaller stars from the cluster first.

As I mentioned earlier, globular clusters usually orbit outisde the plane of the galaxy in our galaxy's relatively empty halo, which means that they don't interact with other stars within the plane of the galaxy very often, and this helps to explain how typical globular clusters have been able to remain relatively intact for billions of years. NGC 6712 also orbits outside the plane of our galaxy, but it orbits very close to the center of the galaxy.

The cluster's orbit takes it on an elliptical path that ranges from about 1500 to 15600 light years (450 to 4800 pc) [2] from the center of the Milky Way, or in other words, its orbit is several times smaller in diameter than the Earth's orbit. This means that the cluster is going to have a very short orbital period and is going to pass through the plane of the Milky Way relatively frequently. In fact, one study estimated that the cluster may have passed through the plane of the galaxy as recently as 4 million years ago [5]. For reference, it takes the Sun a couple hundred million years to orbit the Milky Way, so in that context, NGC 6712's passage through the plane of the Milky Way was very recent. Every time NGC 6712 passes through the plane of the galaxy, gravitational forces are going to strip away a few more of its stars, and eventually, the cluster will be completely obliterated by these gravitational interactions. As it is, this globular cluster looks like it currently has a mass 100 thousand times the mass of the Sun, which is small by globular cluster standards, but it may have originally been 100 times larger [6].

However, that is not the only interesting thing about NGC 6712. The cluster also contains a rather unusual X-ray source originally identified in 1976 [7] that has been called a couple of different names, including X1850-087 and 4U 1850-08. This object is an X-ray binary system, a star system containing the small core of a dead star (which could sometimes be a black hole but which in this case is a neutron star [8]) that is gravitationally stripping the outer layers of the other star in the binary system. The gas, as it falls into the neutron star, gets gravitationally compressed and really hot, which causes it to produce X-ray emission. What's extra weird about X1850-087 is that the star that is losing matter might be a white dwarf or another neutron star [8]. Usually, in an X-ray binary system, the star losing its outer gas layers is something like a red giant or red supergiant. It's really unusual to see a neutron star stripping the outer layers of a much more compact object like a white dwarf or neutron star. Additionally, the star that is losing it's outer gas layers appears to be smaller than 0.04 times the mass of the Sun [8]. It almost doesn't seem to exist anymore.

That X-ray binary star system, however, may not be the only unusual star system in the cluster. In 2021, a group using radio observational data identified a pulsar in a binary star system within the cluster [9]. This pulsar, which is named J1853-0842A, is again another neutron star, although this time a rapidly spinning one, and it is stripping the outer layers away from a companion star that seems to be smaller than 0.04 times the mass of the Sun [9].

OK, it sounds too much like the radio source J1853-0842A is the same object as the X-ray source X1850-087. Let me just check my references here... OK, they appear to be different objects. The radio emission from the pulsar was detected at a position that is about 14 arcseconds away from the position of the X-ray source listed in the SIMBAD database [9, 10]. Assuming those coordinates are correct, it seems like both binary systems happen to contain neutron stars that have nearly stripped away all of the material from their companion stars.

So, back to the system with the pulsar emitting radio waves, J1853-0842A. The two stars in the system orbit each other about every 3 hours and 34 minutes, which is actually quite rapid [9]. The two stars are also oriented in such a way that, as seen from Earth, they eclipse each other. The pulsar spins once every 2.1 milliseconds [9], which is quite rapid, and the reason why the pulsar spins this quickly is probably because the infalling material caused it to spin up. Radio astronomers call these types of pulsars "black widow pulsars", mainly because the pulsar is killing its companion, but also because it just sounds cool. As an additional note, it's rather unfortunate that X-ray astronomers did not come up with the name "black widow star systems. They're stuck with the relatively boring term "X-ray binary system".


[1] Ridpath, Ian, Star tales, 1988

[2] Vasiliev, Eugene and Baumgardt, Holger, Gaia EDR3 view on galactic globular clusters, 2021, Monthly Notices of the Royal Astronomical Society, 505, 5978

[3] de Marchi, Guido et al., A deep optical luminosity function of NGC 6712 with the VLT: Evidence for severe tidal disruption, 1999, Astronomy & Astrophysics, 343, L9

[4] Baumgardt, H. et al., Mean proper motions, space orbits, and velocity dispersion profiles of Galactic globular clusters derived from Gaia DR2 data, 2019, Monthly Notices of the Royal Astronomical Society, 482, 5138

[5] Cudworth, Kyle M., Astrometry and Photometry in the Globular Cluster NGC 6712, 1988, Astronomical Journal, 96, 105

[6] Deras, D. et al., NGC 6712: the variable star population of a tidally disrupted globular cluster, 2020, Monthly Notices of the Royal Astronomical Society, 493, 1996

[7] Seward, F. D. et al., X-ray sources in the Aquila-Serpens-Scutum region., 1976, Monthly Notices of the Royal Astronomical Society, 175, 39P

[8] Homer, L. et al., Periodic UV modulation of X1850-087: a double degenerate binary in the globular cluster NGC 6712?, 1996, Monthly Notices of the Royal Astronomical Society, 282, L37

[9] Zhang, Z. et al., Luminosity functions of LMXBs in different stellar environments, 2011, Astronomy & Astrophysics, 533, A33

Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

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© George Bendo 2023. See the acknowledgments page for additional information.

Last update: 4 September 2023