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.
Within 5 light years (ly) of the Earth, there are 4 stars known (just the Sun and the Alpha Centauri system). Within 10 ly, there are 15. Within 15 ly, there are 58 stars. The number goes up—rapidly! Undoubtedly, more stars will be discovered within 15 light years of the Sun.
And, cool is the rule when it comes to nearby stars. Of the 58 known stars within 15 ly of Earth, an amazing 37 (64%) are class M stars. The remaining 36% include one A star, one F star, three G stars, six K stars, one L infrared dwarf, five very cool T infrared dwarfs, one extremely cool Y infrared dwarf, and three white dwarfs.
The hottest (and bluest) star within 15 light years of the Sun is none other than Sirius (α Canis Majoris)—the brightest star in the night sky—just 8.65 light years distant. Sirius A is an A1V (main-sequence) star, twice as massive as our Sun, 71% wider, 25 times more luminous, and only 237 to 247 million years old—just a single orbit around the galactic center. Sirius rotates much faster than the Sun, too, spinning around once on its axis every 5.4 days. Think about all these things the next time you look up and see Sirius chasing Orion across the meridian these late-winter eves. And that Sirius has a white dwarf companion that orbits it once every 50 years, too.
All but two of the nearest 48 stars that are not white dwarfs or infrared dwarfs have a luminosity class of V, meaning they are dwarf or main-sequence stars. The first exception is Procyon (α CMi A). Its luminosity class of IV-V indicates it is bright for its temperature and spectral type (F5) and beginning to evolve into a subgiant star on its way towards becoming a giant star. The other exception is Kapteyn’s Star, a red subdwarf star of spectral type and luminosity class M2VI. A subdwarf star is underluminous for its temperature and spectral type. This is caused by low metallicity. The scarcity of elements other than hydrogen and helium in the star results in a more transparent stellar photosphere and thus a star that is a little smaller than it normally would be. Incidentally, the fact that we have three white dwarf stars within just 15 light years of us suggests that white dwarfs are copious throughout our galaxy.
You might be wondering how many planets have been discovered orbiting these 58 nearest stars. Beyond the eight planets orbiting our Sun we find another eighteen confirmed planets, plus at least three more unconfirmed planets. This is a rapidly advancing field and no doubt many more planets will be added to the list in the coming decade.
The masses of the confirmed planets include one 55% more massive than Jupiter, one a little more massive than Neptune, one a little less massive than Uranus, thirteen super-Earths (1.14 M⊕ up to 7.7 M⊕), and two less massive than Earth (0.75 M⊕ and 0.98 M⊕). Their orbital periods range from 2 up to 636 terrestrial days, and then one planet (the super-Jupiter) orbiting once every 6.9 years. Orbital eccentricities range from circular (0.00) to 0.55, with the super-Jupiter in a very elliptical orbit having an eccentricity of 0.702. The super-Jupiter is orbiting Epsilon Eridani (K2V, 10.48 ly), with all the rest of the confirmed exoplanets orbiting M-dwarf stars except for the four close-in planets orbiting Tau Ceti (G8.5V, 11.89 ly).