The Dimmest Constellation

You are probably familiar with at least the names of the twelve constellations of the zodiac:


But are you familiar with the twelve constellations that have no stars brighter than 4th magnitude?

Coma Berenices
Corona Australis

All but two of these dim constellations are, at least in part, visible from southern Arizona; Chamaeleon and Mensa require a trip south to see.

The southern constellation Mensa, the Table Mountain (declination -70° to -85°) is a ghost of a constellation, exhibiting no star brighter than magnitude 5.1. That’s 17 times fainter than Polaris! In fact, that’s fainter than all the stars of the Little Dipper asterism! Mensa does have one claim to fame, however. The Large Magellanic Cloud, satellite galaxy of our Milky Way galaxy, straddles most of the border that Mensa shares with Dorado, the Swordfish.

Mensa is far and away the dimmest constellation. But Mensa is a small constellation, bested in size by 74 of the 88 constellations. So perhaps it is not too surprising that a small constellation is less likely to harbor a bright star. Another measure of faint, perhaps, is to determine which of these twelve constellations with no star brighter than 4th magnitude is largest. That might be more remarkable, because one is less likely to find no bright stars in a large area of sky than in a small area of sky. By this measure, Camelopardalis, the Giraffe, wins without a doubt. Camelopardalis is the 18th largest constellation, and yet contains no star brighter than magnitude 4.0. It is that empty region you might have not noticed midway between Capella and Polaris, best viewed at evening twilight’s end during the month of February each year.


The spectral type classification scheme for stars is, among other things, a temperature sequence. A helpful mnemonic for remembering the sequence is Oh, Be A Fine Girl (Guy) Kiss Me Like This, Yes! The O stars have the highest surface temperatures, up to 56,000 K (100,000° F), while the Y infrared dwarfs (brown dwarfs) have surface temperatures as cool as 250 K (-10° F).

Here are the brightest representatives of each of these spectra classes readily visible from the northern hemisphere. Apparent visual magnitude (V-band) is given unless otherwise noted.

Gentle Giant?

Pollux (Beta Geminorum) is the nearest giant star to Earth, between 33.7 and 33.9 light years away.  Its spectral type, K0III, indicates its photosphere is cooler than the Sun’s.  Relative to our Sun, Pollux is 8.8 times wider, 2.0 times more massive, and 43 times more luminous.  Many giant stars are larger than Pollux.

Beginning its life as an A-type main sequence star, but now evolved to a K-type giant, Pollux is only about 724 Myr old.

Pollux has the current distinction of being the brightest star in the night sky known to harbor at least one planet: a super-Jupiter 2.9 times as massive as Jupiter, and orbiting Pollux every 590 days at a distance of 1.7 AU.  The planet’s name is Thestias.

If you have trouble remembering which star is Castor and which star is Pollux in Gemini, here’s an easy way to remember: Castor sides with Capella, and Pollux sides with Procyon.  Another way to tell: Pollux is half a magnitude brighter than Castor.