Object 54: MWC 137

Podcast release date: 23 August 2021

Right ascension: 06:18:45.5


Epoch: ICRS

Constellation: Orion

Corresponding Earth location: Just south of the Irrawaddy River delta in the Indian Ocean

MWC 137 is a type of star in the constellation Orion called a B[e] star (with a large B and a small e in square brackets). I will continue saying B[e] throughout this podcast and assume that you can hear me articulating both the capitalization and punctuation of this phrase. This was originally identified as one of 416 stars with peculiar spectra in a 1933 survey by Paul W. Merrill and Cora G. Burwell with the Mount Wilson Observatory [1]. The stars that they found were given designations starting with MWC, which stands for Mount Wilson Catalog, and this is how MWC 137 got its name.

So, before I discuss MWC 137 in detail, let me take a moment to describe B[e] stars. These stars are generally defined as stars with specific types of spectra characterized by emission at very specific wavelengths from multiple neutral and ionized elements in the surrounding interstellar gas. The gas produces this type of spectra because the B[e] stars are generally hot and produce a lot of light but particularly ultraviolet light that not only heats up the surrounding gas but also ionizes some of the elements within the gas. To make things confusing, multiple different types of stars match this definition of what a B[e] star is. This includes protostars that are still forming out of interstellar gas and dust, blue supergiants, planetary nebulae around relatively ordinary hot blue stars, and binary star systems where the two stars are sufficiently close that one star is able to strip the outer gas layers from the other star, as well as a lot of stuff labelled with the technical term "we have no clue" [2].

So back to MWC 137. Everyone has agreed with the fact that this star has the right type of spectrum to classify it as a B[e] star. However, astronomers have had a lengthy debate about exactly what type of B[e] star MWC 137 is because it was difficult to determine the exact distance to the star. The distance measurements ranged from about 3300 light years (1 kpc) to 42000 light years (13 kpc) [2]. This meant that MWC 137 could be very close and relatively faint or very far and really bright. In recent years, however, astronomers have settled on a distance of about 16800 light years (5.15 kpc) [3,4]. They were able to measure this more accurate distance in part because newer telescopes and instruments, particularly the Gaia spacecraft, make it easier to measure the distances to more distant stars and in part because MWC 137 has been shown to be associated with a cluster of stars within an nebula of ionized gas called S 266 (or Sharpless 266), and all of those stars are at distances that are close to 16800 light years [2].

This relatively large distance points to the star being a B[e] blue supergiant with an estimated mass of 37 times the mass of the Sun [5]. For review, the blue supergiants that we see today were initially blue stars with smaller diameters, although they were still wider and more massive than the Sun. These blue stars would have initially fused hydrogen into helium in their cores like the Sun, but when they ran out of hydrogen in their cores, the fusion of helium into carbon would have started in the stars' cores, with hydrogen fusing into helium in shells around that. At the same time this happens, the stars would have expanded to become the blue supergiants that we see today. When the supergiants' cores run out of helium for fusion, they could start fusing carbon for energy, and when they run out of that, they could fuse even heavier elements in their cores. As fusion processes change within these stars over time, they can potentially expand more and evolve into cooler red supergiants. Also, just to make things confusing, sometimes red supergiants contract and evolve back into blue supergiants [5]. These stars run out of material to fuse for energy in their cores after a few million years, which is relatively quick by astronomy standards, and when this happens, they explode as supernovae. The age of MWC 137 specifically is estimated to be about 4.7 million years [6]. To put this age into context, the star is much younger than the dinosaurs but older than the last ice age.

One of the unusual things about MWC 137 is that jets of gas are emerging from above and below its poles [2,6,7]. These jets extend to a distance of 5.5 light years (1.7 pc) [2], which is a little longer than the distance from our Sun to some of the nearest stars. Jets of gas like this often form when interstellar gas in a disk around a central object is falling inwards towards that object. Normally, the central object is either a protostar that is forming out of a surrounding disk of gas or a supermassive black hole millions of times more massive than the Sun that is absorbing material from a disk of gas falling into it. In both of these situations, as the gas falls inwards, it gets hot and increases in pressure, and some of it blows away out of the poles of the disk rather than falling into the central object itself. The problem with MWC 137 is that it's a supergiant star, and supergiants don't normally have gas disks, so the appearance of jets of gas above the poles of MWC 137 doesn't make sense.

The people studying this star have come up with half a dozen hypotheses for the origin of the jets. First, it could be possible that MWC 137 is a binary star system with a blue supergiant that we can easily see and a second star is too faint to detect with modern telescopes but, for some reason or another, has an infalling gas disk where some of the infalling gas gets forms jets from the poles of the disk [2]. If the companion star is something like a red dwarf that is orbiting too close to the blue supergiant, its possible that the gravitational interactions between the stars have distorted the dwarf's outer gas layers into a disk-like structure that can create conditions for hot gas in the disk structure to produce jets. If the companion star is something like a neutron star or black hole orbiting close to the blue supergiant, then the companion star could be stripping layers of gas off of the blue supergiant to form a disk of hot gas with jets. If the companion star is a protostar, then, regardless of where it is located in relation to the blue supergiant, it would by definition still be a star forming out of a gas disk where some of the excessively hot gas from the disk would be blown away in jets.

Another hypothesis is that MWC 137 is one large star that formed from the merger of two smaller stars and that it has a disk of gas left over from that merger that is falling into the star and some of the overly hot infalling gas forms the jets [2]. Yet another possibility is that MWC 137, which is only 4.7 million years old, somehow retained part of the gas disk that it formed out of and that the jets are related to that disk [2]. Finally, it's possible that the jets are from a protostar in a completely different star system that is right next to MWC 137 and MWC 137 itself is a relatively ordinary B[e] supergiant (as ordinary as an extremely large blue star that ionizes the surrounding interstellar medium can be) [2]. You yourself can probably make up your own theory at home using nothing more than a protractor, a screwdriver, a spare disk of interstellar hydrogen gas, an old USB drive with at least 5 GB of disk space, and a roll of duct tape. In any case, more observations are needed to disentangle exactly what is going on.


[1] Merrill, Paul W. and Burwell, Cora G., Catalogue and Bibliography of Stars of Classes B and A whose Spectra have Bright Hydrogen Lines, 1933, Astrophysical Journal, 78, 87

[2] Mehner, A. et al., VLT/MUSE discovers a jet from the evolved B[e] star MWC 137, 2016, Astronomy & Astrophysics, 585, A81

[3] Gaia Collaboration et al., The Gaia mission, 2016, Astronomy & Astrophysics, 595, A1

[4] Gaia Collaboration et al., Gaia Early Data Release 3: Summary of the contents and survey properties, 2020, arXiv e-prints, arXiv:2012.01533

[5] Meynet, Georges et al., Red Supergiants, Luminous Blue Variables and Wolf-Rayet stars: the single massive star perspective, 2011, Bulletin de la Societe Royale des Sciences de Liege, 80, 266

[6] Kraus, Michaela et al., Resolving the circumstellar environment of the Galactic B[e] supergiant star MWC 137. II. Nebular kinematics and stellar variability, 2021, arXiv e-prints, arXiv:2106.15981

[7] Kraus, Michaela et al., Resolving the Circumstellar Environment of the Galactic B[e] Supergiant Star MWC 137 from Large to Small Scales, 2017, Astronomical Journal, 154, 186


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

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