George's Random Astronomical Object

Object 80: WR 124 and M1-67

Podcast release date: 22 August 2022

Right ascension: 19:11:30.9

Declination: +16:51:38

Epoch: J2000

Constellation: Sagitta

Corresponding Earth location: About 110 km southwest of the southern tip of the Dominican Republic

The coordinates for this episode do not point to one object but instead two related objects. Both are located at a distance of 20900 light years (6.4 kpc) in the constellation Sagitta [1, 2]. Sagitta represents an arrow and has nothing to do with the constellation Sagittarius, which represents an archer and which is located a bit further south. If you talk to Sagittarius, he will tell you that he is not looking for a missing arrow.

Anyway, one of the two objects in this episode is a bright, blue star that was initially called Merrill's Star because its discovery was published by Paul W. Merrill in 1938 [3]. The scientific paper, which consists of one entire paragraph, explained that the observations were performed by William C. Miller at Mount Wilson Observatory in 1937 and that the star was identified as having a rather unusual spectrum by Cora G. Burwell [3], but because Merrill wrote the description of the discovery, he got all of the credit.

What makes Merrill's Star interesting is not that the fact that a couple of highly skilled people got jilted out of having a star named after themselves but that this star is a member of a class of stars called Wolf-Rayet stars [3]. This class of stars were named after Charles Wolf and George Rayet, who were willing and able to share credit at least with each other for their discovery. Because Merrill's Star is a Wolf-Rayet star, it has also been given the designation WR 124, where the WR indicates that it is a Wolf-Rayet star. Most people these days seem to use WR 124 instead of Merrill's Star as the name for this object, maybe because they like to indicate that it is a Wolf-Rayet star by using a name that begins with "WR" and maybe because other people are a little upset that Miller and Burwell didn't get any credit when the star was named. In any case, I am going to use the name WR 124 for the rest of the episode.

And now back to discussing science. The Wolf-Rayet class of stars, including WR 124, are stars typically about 10 to 25 times the mass of the Sun (but sometimes even larger) that would have initially started out in life by fusing hydrogen into helium in their cores [4]. Over time, the cores of all stars that fuse hydrogen into helium will eventually fill up with helium, and while this will take a total of 10 billion years for a star the size of the Sun to do, this could take just a few million years for a much more massive star. When that happens in a very large star, helium will start to fuse into carbon in the star's core, with hydrogen fusing into helium in a shell around that. When the core fills up carbon, the carbon will be fused to make heavier elements, with the fusion of helium into carbon in a shell around that and another shell of hydrogen fusing into helium around that. Eventually, the core of the really massive star will fill up with iron, which can't be fused to produce energy, and the star will first implode and then explode as a supernova. Before that, though, the star will reach a stage where the outer layers of the star get so hot that the outer hydrogen gas shell gets blown away at a very high speed, exposing the layer where helium is being fused into carbon. This is the stage where the star is classified as a Wolf-Rayet star.

One of the peculiar things about WR 124 specifically is that it lies at the center of the second object in this episode, which is a clumpy-looking but roughly spherical nebula called M1-67. This nebula should not be confused with the more famous Messier 1 or M1. The M in the name for M1-67 stands for Minkowski, or more specifically, Rudolph Minkowski, who discovered the nebula in 1946 [5]. Also, Rudolph Minkowski should not be confused with Hermann Minkowski, who did some of the mathematical work that laid the foundation for relativity [6].

Anyway, M1-67 is a thing that was initially thought to be a planetary nebula, which would be the cloud of gas produced by a Sun-like star after it dies and blows away its outer gas layers [7]. However, it didn't look quite like a normal planetary nebula in part because, if it was, it would be the only planetary nebula that did not have, in its center, a white dwarf about the size of the Sun but instead a Wolf-Rayet star multiple times the mass of the Sun. Calling it a planetary nebula just seemed wrong. Someone else classified it as an HII region [8], which is the type of region of ionized hydrogen gas that is typically found around newly formed hot blue stars, but that designation did not seem quite right, either. Eventually, people decided to just call it a "nebula" to avoid all of the semantics problems.

As you may guess, the M1-67 nebula was created by the star WR 124, but it's not as straightforward as you think. The inner part of the nebula is ionized gas created by the stellar winds from the central star, but that gas may have been ejected well before the star became a Wolf-Rayet star [9, 10]. However, the outer structure of the nebula has been created by where these winds are colliding with interstellar dust and gas; it's actually a kind of shock wave [9, 10].

The other thing that makes WR 124 and M1-67 rather peculiar is that both are moving together at a really high speed in a different direction from most of the other stars and other things in the disk of the Milky Way [11, 12]. This makes WR 124 what is called a runaway star. Most other stars travel in roughly circular orbits around our galaxy's center, and stars that are about the same distance from our galaxy's center will travel at roughly the same speed. WR 124 would have formed with a bunch of other stars in a nebula within the disk of our galaxy and would have started off moving at the same speed and direction as everything else. (Note that the nebula that WR 124 formed within is not the same as the one that currently surrounds the star now.)

Anyway, some sort of event flung WR 124 out of the nebula in which it formed at a really high speed. One possibility is that gravitational interactions between WR 124 and other stars that formed out of the same nebula led to WR 124 being ejected from the cluster [13]. The other possibility is that WR 124 was pushed out of the nebula in which it formed by a supernova explosion [14]. Both scenarios would have sent WR 124 moving in a seemingly random direction at an extreme speed. What I personally found rather interesting to learn while researching this is that WR 124 is not the only Wolf-Rayet star that is a runaway star. It's actually quite common for Wolf-Rayet stars to get ejected from where they formed at really high velocities [12].

In any case, WR 124 and the surrounding nebula M1-67 are a really weird and interesting pair of objects, and astronomers will continue to look at these objects just to learn as much as they can about Wolf-Rayet stars, how they shed their outer gas layers, and how they fly through our galaxy in different directions from everything else.

References

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

[2] Gaia Collaboration et al., Gaia Data Release 3: Summary of the content and survey properties, 2022, arXiv e-prints, arXiv:2208.00211

[3] Merrill, P. W., A Wolf-Rayet Star with High Velocity, 1938, Publications of the Astronomical Society of the Pacific, 50, 350

[4] Crowther, Paul A., Physical Properties of Wolf-Rayet Stars, 2007, Annual Reviews of Astronomy and Astrophysics, 45, 177

[5] Minkowski, R., New Emission Nebulae, 1946, Publications of the Astronomical Society of the Pacific, 58, 305

[6] O'Connor, J. J., and Robertson, E. F., Hermann Minkowski, 2015, MacTutor History of Mathematics Archive

[7] Bertola, F., A Planetary Nebula with WN Nucleus, 1964, Publications of the Astronomical Society of the Pacific, 76, 241

[8] Sharpless, Stewart, A Catalogue of H II Regions., 1959, Astrophysical Journal Supplement Series, 4, 257

[9] Vamvatira-Nakou, C. et al., Herschel observations of the nebula M1-67 around the Wolf-Rayet star WR 124, 2016, Astronomy & Astrophysics, 588, A92

[10] Sevigny, Marcel et al., New insights into the WR nebula M1-67 with SITELLE, 2021, Monthly Notices of the Royal Astronomical Society, 501, 5350

[11] Kharchenko, N. V. et al., Astrophysical supplements to the ASCC-2.5: Ia. Radial velocities of ~55000 stars and mean radial velocities of 516 Galactic open clusters and associations, 2007, Astronomische Nachrichten, 328, 889

[12] Marchenko, S. V. et al., Population I Wolf-Rayet Runaway Stars: The Case of WR124 and its Expanding Nebula M1-67, 2010, Astrophysical Journal Letters, 724, L90

[13] Evans, C. J. et al., A Massive Runaway Star from 30 Doradus, 2010, Astrophysical Journal Letters, 715, L74

[14] De Donder, E. et al., The number of O-type runaways, the number of O and Wolf-Rayet stars with a compact companion and the formation rate of double pulsars predicted by massive close binary evolution., 1997, Astronomy & Astrophysics, 318, 812

Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

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© George Bendo 2022. See the acknowledgments page for additional information.

Last update: 21 August 2022