Alpha Mensae
The dimmest lucida in the sky, a G-type star with a red dwarf companion.
Alpha Mensae, also known as Hoerikwaggo, is the most luminous star in the constellation Mensa. Despite holding that title, it is the faintest lucida—the brightest star in any given constellation—in the entire night sky, and it ranks as the second-dimmest star to carry the "alpha" designation, trailing only Alpha Octantis. Its declination means it is best observed from the higher latitudes of the southern hemisphere, though it can also be glimpsed low on the horizon just north of the Equator when it reaches its highest point in the sky. The star lies 33 light-years from the Solar System. It is a G-type main-sequence star that has a red dwarf companion, making it a binary system. The Bayer designation for this star is α Mensae, Latinised as Alpha Mensae and abbreviated α Men or Alpha Men. It lacked a traditional proper name until the IAU Working Group on Star Names approved the name Hoerikwaggo for Alpha Mensae A on 12 December 2024, which now appears in the IAU Catalog of Star Names. The name comes from the Khoekhoe term Huriǂoaxa, rendered in Afrikaans as Hoerikwaggo, and refers to Table Mountain in South Africa—the mountain that the constellation Mensa represents. Classified as G7 V, Alpha Mensae is a G-type main-sequence star that generates energy by fusing hydrogen into helium in its core. It is slightly cooler than the Sun and similar in size, with 96.4% of the Sun's mass, 96% of its radius, and 81% of its luminosity. Its effective temperature is 5,569 K, and it has a metallicity—the proportion of elements heavier than hydrogen and helium—that is 129% of the Sun's. Estimated to be 6.2 billion years old, the star rotates slowly, with a projected rotational velocity of 0.6 km/s. It has a high proper motion across the sky and made its closest approach to the Sun roughly 250,000 years ago, coming within about 10 light-years (3.2 parsecs). Its red dwarf companion is separated by 3.05 arcseconds, equivalent to a projected distance of about 30 AU. The companion has only 16.9% of the Sun's mass and is fully convective. A candidate infrared excess was once detected around Alpha Mensae, suggesting a circumstellar disk extending beyond 147 AU with dust temperatures below 22 K. However, observations from the Herschel Space Observatory did not confirm this excess, leaving the finding uncertain.
- Bayer designation
- α Mensae
- Magnitude
- 5.09
- Distance
- 33 light-years
- Spectral type
- G7 V
- Mass
- 96.4% of the Sun
- Radius
- 96% of the Sun
- Luminosity
- 81% of the Sun
- Effective temperature
- 5,569 K
- Metallicity
- 129% of the Sun
- Age
- 6.2 billion years
Lore & Background
Alpha Mensae has the Bayer designation α Mensae, Latinised as Alpha Mensae, and was traditionally without a proper name. On 12 December 2024, the IAU Working Group on Star Names approved the name Hoerikwaggo for Alpha Mensae A, derived from the Khoekhoe name Huriǂoaxa for Table Mountain in South Africa, which the constellation Mensa represents. The star is best visible from higher latitudes of the southern hemisphere but can also be seen low in the sky just north of the Equator near its culmination. This G-type main sequence star has a stellar classification of G7 V, generating energy by fusing hydrogen into helium at its core. It is of similar size but slightly cooler than the Sun, with 96.4% of the mass, 96% of the radius, and 81% of the Sun's luminosity. The effective temperature of the stellar atmosphere is 5,569 K, and it has a slightly higher (129%) proportion of elements other than hydrogen and helium—what astronomers call the star's metallicity—compared to the Sun. The estimated age of this star is 6.2 billion years, and is rotating at a relatively leisurely projected rotational velocity of 0.6 km/s. Located 33 light-years distant from the Sun, Alpha Mensae has a relatively high proper motion across the sky. It has already made its closest approach to the Sun, coming within about 3.2 pc around 250,000 years ago. It has a red dwarf companion star at an angular separation of 3.05 arcseconds; equivalent to a projected separation of roughly 30 AU. With a mass just 16.9% that of the Sun, the companion is fully convective.
Reader's Guide
Alpha Mensae is notable as the dimmest lucida in the sky, with a magnitude of 5.09, and the second-faintest star labeled 'alpha' after Alpha Octantis. Its binary nature, with a red dwarf companion at a separation of 3.05 arcseconds, adds to its interest as a nearby system 33 light-years away. The star's high proper motion and past close approach to the Sun about 250,000 years ago provide context for its motion through the galaxy. A candidate planet was detected in 2023 via Doppler spectroscopy with a period close to one Earth year, though it may be an instrumental false positive and requires further confirmation. A candidate infrared excess suggesting a circumstellar disk beyond 147 AU was not confirmed by the Herschel Space Observatory, leaving the finding in doubt. The star's naming after Table Mountain via the Khoekhoe name Hoerikwaggo connects it to South African heritage and the constellation Mensa's representation of the mountain.
Did You Know?
- Alpha Mensae is the dimmest lucida in the sky at magnitude 5.09.
- The star's proper name Hoerikwaggo was approved by the IAU on 12 December 2024.
- A candidate planet with a period near one Earth year was found in 2023 but may be a false positive.
Frequently Asked Questions
What is Alpha Mensae?
Alpha Mensae, nicknamed Hoerikwaggo, is a G7-type main-sequence star roughly 33 light-years from Earth that serves as the brightest star in the small southern constellation Mensa. It belongs to a binary system that includes a red dwarf companion.
Why is Alpha Mensae called the faintest lucida in the sky?
At an apparent magnitude of 5.09, it is the dimmest star that holds the 'brightest in its constellation' title anywhere visible to the unaided eye. Among all stars carrying the Bayer letter alpha, only Alpha Octantis is fainter, making Alpha Mensae the second-dimmest alpha designation overall.
What kind of companion orbits Alpha Mensae?
The primary G-type star is paired with a red dwarf, forming a binary system. This makes it a textbook example of a near-solar star accompanied by a much smaller, cooler partner.
Where and when is the best time to observe Alpha Mensae?
Its southern declination means it is most accessible from higher latitudes in the southern hemisphere. Observers just north of the Equator can still spot it, but only when it crests low on the horizon at its nightly transit.
How closely does Alpha Mensae resemble the Sun?
The primary star is strikingly Sun-like, with a mass of about 96 percent of the Sun's and a radius roughly 96 percent as large. Its G7 spectral class places it just slightly cooler and less massive than our own G2V star.
More in Binary and Multiple Stars, Part 3 1-24
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