Object 72: GX Monocerotis
Podcast release date: 02 May 2022
Right ascension: 06:52:47.0
Corresponding Earth location: The Gulf of Thailand
Monoceros is supposed to depict a unicorn (yes, a unicorn). It appears to have been created in 1612 by the Dutch astronomer Petrus Plancius, whose career seemed to be based on inventing weird constellations . Most of those constellations are now ingored, but Monoceros survived. Unfortunately, the constellation contains a bunch of relatively dim stars that are about magnitude 3.9 or fainter. For context, the stars in the Big Dipper have magnitudes between 1.8 and 3.3, while stars with magnitudes 5 or 6 are the faintest stars that can be seen without a telescope. This means that the constellation Monoceros looks like the equivalent of a random patch of faint stars on the sky.
In any case, the specific object that this episode's coordinates point to is the star GX Monocerotis. (I am going to call this GX Mon for short because "Monocerotis" sounds like a disease.) This star, located at a distance of 2120 light years (650 pc) [2, 3], is an evolved star called an asymptotic giant branch star. It sort of looks like a red giant star but it is actually a bit different. The "branch" part of the name "asymptotic giant branch star" refers to the fact that these stars lie along a curve or "branch" when placed in a plot where temperature is on the x-axis and luminosity is on the y-axis. The word "asymptotic" refers to how this curve seems to get close to but does not quite reach the line in this plot where "normal" red giant stars lie.
Asymptotic giant branch stars and "normal" red giant stars are in different phases of their lifespans. A star like the Sun is powered by the fusion of hydrogen into helium in its core, but when the core fill up with helium, it will evolve into a "normal" red giant star that is powered by the fusion of hydrogen into helium in a layer on top of an inert helium core. When the star does this, it gets larger and brighter but cooler and moves into the red giant branch in that plot of luminosity versus temperature. Eventually, though, the helium core will collapse, which will trigger the fusion of helium into carbon. The star will become hotter but also contract and become fainter, moving off the red giant branch. A star with a mass close to the mass of the Sun cannot fuse carbon to produce energy, so eventually, the core of the star will fill up with carbon, but the fusion of helium into carbon will continue in a layer around the core, with hydrogen still being fused into helium in a layer above all of that. At this stage, the star will cool down and become red in color but will also get bigger and brighter again, and it will move onto the asymptotic giant branch in the luminosity versus temperature plot.
So, GX Mon is one of many stars at the asymtotic giant branch stage in their lifespans. At this point, these stars begin to blow away their outer gas layers. This isn't quite the stage where a star will expel its outer gas layer entirely and become a planetary nebula, but it's very close. In fact, GX Mon, like other asymptotic giant branch stars, is surrounded by shells of gas that completely fill the star system. These shells of gas in the GX Mon system have a measured radius of 4000 Astronomical Units (AU) . For context, the distance from the Sun to the Earth is 1 AU, and the distance from the Sun to Neptune is 30 AU, so the gas shells surrounding GX Mon would completely envelop all of the planets in our Solar System as well as almost everything else, although the Oort Cloud would still be larger.
The gas shells themselves might be interesting, but what is extra peculiar about GX Mon is that these shells of gas contain spiral patterns . It's very hard for a star by itself to produce spiral structures like this. After all, stars are close to spherical, and even though they rotate, any gas they expel is going to more-or-less get blown outwards at the same rate in all directions, forming spherical shells. However, it's very easy for an asymptotic giant branch star to form gas shells with spiral structure if it is travelling in an elliptical orbit around another star while blowing away its outer gas layers, and that seems to be what is happening in the GX Mon star system.
The companion star has apparently not yet evolved into a red giant because no one has seen it yet. This makes it a bit difficult to actually say much of anything about the companion star other than that it is probably like the Sun and still fuses hydrogen into helium in its core. However, some analyses of the gas shell structures surrounding GX Mon indicate that the asymtotic giant branch star and its unseen companion seem to be on elliptical orbits that extend from about 7 to 61 AU, and the two stars orbit each other once every 138 years .
GX Mon is one of only a few asymptotic giant branch star where astronomers have seen outer shells of gas containing spiral structures. However, astronomers are still trying to get more images of gas shells around other asymptotic giant branch stars to see how many other spiral structures they can find. One thing that is certain is that this line of research is going to have implications for how planetary nebulae form and why they look the way they do, so astronomers will be spending much more time looking at GX Mon.
 Ian Ridpath, Star Tales, 2018
 Karovicova, I. et al., New insights into the dust formation of oxygen-rich AGB stars, 2013, Astronomy & Astrophysics, 560, A75
 Danilovich, T. et al., New observations and models of circumstellar CO line emission of AGB stars in the Herschel SUCCESS programme, 2015, Astronomy & Astrophysics, 581, A60
 Randall, S. K. et al., Discovery of a complex spiral-shell structure around the oxygen-rich AGB star GX Monocerotis, 2020, Astronomy & Astrophysics, 636, A123