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.
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.