Object 119: Alpha Mensae

Podcast release date: 04 March 2024

Right ascension: 06:10:14.5


Epoch: ICRS

Constellation: Mensa

Corresponding Earth location: The interior of Antarctica.

Alpha Mensae is the brightest star system in the southern constellation Mensa. One of your first thoughts when you first hear the name of this constellation is that it might have something to do with the organization Mensa, and you would be kind of right. Both are named after the Latin word for table [1, 2]. I suspect that one of your next thoughts may be why did an organization of high-IQ individuals name themselves after a piece of furniture. However, this podcast is not about organizations of intellectuals but instead about astronomy.

So why did someone name a constellation after a table? Well, if you have listened to several of my past podcast episodes, you may recall my discussions about how, a few hundred years ago, weird astronomers liked to create new constellations out of relatively faint patches of stars in the sky, and no one was weirder than eighteenth century French astronomer Nicolas Louis de Lacaille. Lacaille created this constellation in the 1750s, but he originally did not name it a table but instead named it Table Mountain after the famous mountain outside of Cape Town in South Africa where he did a lot of his work [1]. The constellation consists of a bunch of scattered stars that aren't even above magnitude 5, which means that they are hardly visible without a telescope. Four of the stars form a trapezoid that could represent a table, so I guess it's better than most of Lacaille's other weird constellations, which are supposed to represent things like a microscope, an air pump, or a painter's easel but don't look remotely like those objects.

Anyhow, let's talk about Alpha Mensae, the brightest star system in the constellation Mensa at a magnitude of 5.1 [3]. This binary star system is located at a distance of 33.33 light years (10.22 pc) [4, 5], which is definitely shorter than the Kessel Run and which defintiely means that I get to use my Star Wars sound effects. The larger and brighter star in the system is a Sun-like star with nearly the same color, radius, and mass of the Sun [6]. The smaller star is a red dwarf with a mass of about 0.17 times the mass of the Sun [6]. The two stars, however, are separated by a distance of about 30 Astronomical Units (AU), which is equivalent to the distance from the Sun to Neptune, and the red dwarf orbits the Sun-like star about once every 162 years [6].

Although Alpha Mensae is not among the 50 closest star systems, it is still close enough for very detailed astronomical studies. One of the most information-rich references that I found was a paper on astroseismology measurements (or measurements based on the vibrations of the outer atmospheres of the stars in Alpha Mensae) published in 2021 by a group with Ashley Chontos as the lead author [6]. Her paper provided a lot of the detailed statistical information that I am using in this podcast episode. However, aside from the fact that we learn a lot of details about this star from Chontos's paper and other papers like it, we also get good quantified information on the stars in this system so we can infer the properties of more distant stars where it may be more difficult to make such measurements.

As the star system contains an Earth-like star and as the star system is very close to Earth, astronomers who like to search for exoplanets have repeatedly observed the system with the hope of detecting some sort of planetary object. No confirmed exoplanets have been found so far, but after over a decade of observing, astronomers have at least measured a slight amount of periodic Doppler shifting in the brighter star in the system that may be consistent with an otherwise unseen exoplanet in the system [7]. If this is a real exoplanet, it would have a mass of about 54 times the mass of the Earth, which is between the mass of Neptune and Saturn, and it would have an elongated orbit that takes the exoplanet around the Sun-like star once every 360 days [7].

In addition to this, astronomers may or may not have identified the presence of a debris disk, which is a disk of dust that forms from collisions between objects such as comets and asteroids orbiting a star. This disk was initially identified by the presence of extended infrared emission at 160 microns around the Sun-like star by the Herschel Space Observatory [8], which is very unusual, as most debris disks are identified at infrared wavelengths between 20 to 100 microns. However, a subsequent analysis was unable to demonstrate that the infrared emission from the star system was greater than what would be expected for the star itself [9], although, as best as I can tell from some of the cryptic comments describing the results, the analysis had difficulty with detecting excess infrared emission that only appears at really long infrared wavelengths. If this debris disk exists, the dust within the disk would have a temperature of about -250 degrees Celsius, which would be very cold by debris disk standards [7].

In any case, Alpha Mensae might be in one of the dumbest constellations in the Earth's sky, but it is an intriguing star system with a Sun-like star at its center. I look forward to the possible confirmation of either an exoplanet or a debris disk in this system sometime in the future.


[1] Ridpath, Ian, Star tales, 1988

[2] British Mensa Limited, About Mensa, 2024

[3] Høg, E. et al., The Tycho-2 catalogue of the 2.5 million brightest stars, 2000, Astronomy & Astrophysics, 355, L27

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

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

[6] Chontos, Ashley et al., TESS Asteroseismology of α Mensae: Benchmark Ages for a G7 Dwarf and Its M Dwarf Companion, 2021, Astrophysical Journal, 922, 229

[7] Laliotis, Katherine et al., Doppler Constraints on Planetary Companions to Nearby Sun-like Stars: An Archival Radial Velocity Survey of Southern Targets for Proposed NASA Direct Imaging Missions, 2023, Astronomical Journal, 165, 176

[8] Eiroa, C. et al., Herschel discovery of a new class of cold, faint debris discs, 2011, Astronomy & Astrophysics, 536, L4

[9] Sibthorpe, B. et al., Analysis of the Herschel DEBRIS Sun-like star sample, 2018, Monthly Notices of the Royal Astronomical Society, 475, 3046


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