Mensa
What is Mensa?
Constellations· Southern summer · directly south of the Large Magellanic Cloud. Lacaille named it after Table Mountain in Cape Town, where he observed. Its brightest star is only magnitude 5.1, the faintest leading star of any constellation.
📍 Visible from where you are right now — computed in your browser
Stars of the figure
| Star | Magnitude |
|---|---|
| Alpha Mensae | 5.1 |
The 1 brightest of the 3 stars used in the figure (2 lines). Lower magnitude means brighter.
Deep-sky objects near this constellation
Constellation basics
| Season | Dec–Feb evenings; never visible from London or New York |
|---|---|
| Brightest star | α Mensae, mag 5.1 — faintest alpha star of any constellation |
| How to find | south of the Large Magellanic Cloud, toward the pole |
| Area rank | 75th of 88 |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
The quasar Chandra looked at first — an X-ray jet in Mensa
Right after the Chandra X-ray Observatory reached orbit in 1999, the first object it observed was the quasar PKS 0637-752 in the direction of Mensa. A quasar is the centre of a galaxy where a giant black hole swallows matter and shines extremely brightly; this one is about six billion light-years away. The observation was a test to focus the telescope, but the picture showed something unexpected.
A stream of matter, or jet, hundreds of thousands of light-years long was shining brightly in X-rays. The jet had been known only from radio waves, and there was no obvious reason for it to be so bright in X-rays. Scientists proposed a new explanation: fast electrons in the jet collide with the faint glow that fills the universe, the cosmic microwave background, and boost that light to X-ray energies. Dozens of other quasar jets have since been seen in X-rays.
The constellation in depth
Mensa is the only one of the 88 constellations named after a real feature of the Earth's landscape. Lacaille observed the southern sky from beneath Table Mountain in Cape Town in 1751–52 and made this constellation in its honour. Its original name was Mons Mensae, the Table Mountain.
A white cloud often settles on top of Table Mountain like a tablecloth. The constellation has something similar: the southern part of the Large Magellanic Cloud, a neighbouring galaxy that looks like a patch of mist, spills over its stars.
All of Mensa's stars are faint. Even Alpha Mensae, the brightest, is only magnitude 5.1 and hard to see from a city. It is a Sun-like star 33 light-years away with a small red companion. Pi Mensae has a planet about ten times Jupiter's mass and a 'super-Earth' larger than our planet; the super-Earth was one of the early finds of the TESS satellite.
At declinations of −70° to −85°, close to the south celestial pole, it cannot be seen from most of the northern hemisphere, while from the southern hemisphere it never sets. In the Sky view, set Cape Town on a January evening and the constellation appears just below the Large Magellanic Cloud.
Frequently asked
- When can I see Mensa?
- Mensa is best seen in Dec–Feb evenings; never visible from London or New York.
- How do I find Mensa?
- To find Mensa, look for south of the Large Magellanic Cloud, toward the pole.
- What is the brightest star in Mensa?
- The brightest star in Mensa is Alpha Mensae, at magnitude 5.1.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.