Object 97: Mu Colombae
Podcast release date: 01 May 2023
Right ascension: 05:45:59.9
Corresponding Earth location: Near where the Ninetyeast Ridge joins the Broken Ridge in the Indian Ocean
The coordinates for this episode point to the star Mu Columbae with a Greek letter Mu, although if you like cows, you can think of this as Moo Columbae. Anyway, this blue star is located in the constellation Columba, the dove, which was invented in the sixteenth century by Petrus Plancius, a Dutch astronomer and cartographer who devoted his life to making maps of things . The stars in Columbae form kind of a capital T shape where the right end of the T is bent, and Mu Columbae is located about three degrees north of where the two lines in the T cross. Mu Columbae has a visual magnitude of about 5 , so in case you want to find it, it's just barely visible to the naked eye.
While Mu Columbae may look faint compared to other stars as seen from Earth, it's actually a very bright blue star that just looks faint because its located very far away from Earth at a distance of 1920 light years (590 pc) [3, 4]. Astronomers would classify this as an O-type star . Mu Columbae still fuses hydrogen into helium in its center like the Sun, but it is 16 times more massive, 8 times larger in diameter, about 6 times hotter, and tens of thousands of times brighter . O-type stars like Mu Columbae produce so much ultraviolet radiation in particular that they ionize the interstellar gas around them.
However, what distinguishes Mu Columbae from many other bright blue stars is its proper motion. Proper motion is the term that astronomers use to describe how stars appear to move sideways in the sky over time relative to other stars in the background. This movement is related to the motions of the stars through the Milky Way relative to the Earth, which, just to remind you, is also moving through the Milky Way on its own orbit. In general, stars don't move enough that anyone will be able to notice the changes without a telescope during their lifetimes. Some of the stars with the highest proper motions on the sky will appear to move a few arcseconds a year (where 3600 arcseconds are equivalent to 1 degree) . This proper motion is equivalent to a star being able to move a distance equivalent to the diameter of the moon in a few centuries, although the fastest moving star, a faint red dwarf called Barnard's Star, can actually cover this distance in a little over 170 years .
By the way, I have actually avoided discussing proper motion in this podcast series up until now in part because it makes the coordinates that I read for some of the astronomical objects a little confusing. I am using coordinates from the International Celestial Reference System (ICRS) system that correspond to the year 2000. Some of the closer stars in my podcast series would have moved enough that the coordinates based on the year 2000 would be a few arcseconds off from where the stars are now.
Mu Columbae is not really moving all that fast in terms of proper motion; it moves 0.022 arcseconds a year [3, 4]. However, given that this star is relatively bright, it was possible for astronomers in the first half of the twentieth century to see the star change position in the photographic images of the sky that they used for their research. In 1954, Adriaan Blaauw and William Wilson Morgan published a paper that not only measured the proper motion of Mu Columbae but also determined that the star appeared to have originated from the Orion Nebula . As seen from Earth, Mu Columbae basically left the constellation Orion, crossed the constellation Lepus, and is now appoaching the center of the constellation Columba. That's equivalent to crossing one-seventh of the part of the sky visible above the horizon.
In terms of what's happened in space, Mu Columbae appears to have travelled roughly 860 light years from the Orion Nebula. What's also even weirder is that the two astronomers who discovered that Mu Columbae had originated from the Orion Nebula had also found that another very similar blue O-type star in the constellation Auriga named AE Aurigae was moving at nearly the same speed in the opposite direction from the Orion Nebula . This implied that something happened in the Orion Nebula to eject both of these stars in opposite directions.
My first impression was that this was a coincidence, but it turned out that the two stars were indeed linked. In 2004, Alessia Gualandris, Simon Zwart, and Peter Eggleton published a paper explaining that Mu Columbae and AE Aurigae had both been ejected from the Orion Nebula in a complex gravitational interaction with two more stars that currently comprise the binary star system Iota Orionis at the center of the nebula .
So, the two stars in Iota Orionis are named Iota Orionis A and Iota Orionis B. Iota Orionis A is a blue O-type star used to be in a binary star system with AE Aurigae, or if you want to anthropomorphize things, Iota Orionis A and AE Aurigae were in a relationship together . Iota Orionis B is classified a B-type star, which is slightly smaller and fainter than an O-type star but otherwise a very similar, and Iota Orionis B was in a binary system with or dating Mu Columbae . These two binary star systems would have formed in different locations within the Orion Nebula, which you can think of as like a crowded party. The systems then passed by each other and gravitationally interacted in such a way that Iota Orionis A and Iota Orionis B broke up with their partners and formed a new binary star system while Mu Columbae and AE Aurigae were ejected out of the party (or actually out of the nebula) into empty space, where they would never meet another star ever again . However, you don't need to feel sad for Mu Columbae or AE Aurigae. Mu Columbae has taken up painting, while AE Aurigae has become a gourmet cook and is now writing a novel. (OK, that's too much anthropomorphization, but you get the idea.)
In any case, Mu Columbae is one of the classic examples of a runaway star, which is the term astronomers use for stars ejected from clusters. It's not the first runaway star that was ever found, but it has taught us important lessons about how stars are ejected from the nebulae in which they are formed.
 Ridpath, Ian, Star tales, 1988
 Ducati, J. R., VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system., 2002, VizieR Online Data Catalog
 Gaia Collaboration et al., The Gaia mission, 2016, Astronomy & Astrophysics, 595, A1
 Gaia Collaboration et al., Gaia Early Data Release 3: Summary of the contents and survey properties, 2020, arXiv e-prints, arXiv:2012.01533
 Gualandris, Alessia et al., N-body simulations of stars escaping from the Orion nebula, 2004, Monthly Notices of the Royal Astronomical Society, 350, 615
 European Space Agency, High Proper Motion Stars in the Hipparcos Catalogue, 2023
 Blaauw, A. and Morgan, W. W., The Space Motions of AE Aurigae and mu Columbae with Respect to the Orion Nebula., 1954, Astrophysical Journal, 119, 625