===Castor Ab===
'''Castor Ab''' is a main sequence dwarf star, however its spectral type is not exactly known. The star may be either a class M red dwarf or a class A blue-white dwarf, similar to Castor Aa.<ref name="harv2">http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1983ApJ...271..264G&data_type=PDF_HIGH&type=PRINTER&ext=.pdf</ref> The star is estimated to have a mass that ranges from 40 to 60 percent greater then than our own Sun. The star's radius and luminosity is not known. Castor Ab has been repeatedly observed having frequent broadband and [[x-ray]] flares, normally associated with red dwarf stars.<ref name="harv"/>
The combined visual luminosity of Castor Aab is more than 34 times that of the Sun. For an [[terrestrial planet|Earth-like]] world to comfortably maintain liquid [[water]] on its surface, the hypothetical planet would have to orbit more than 6.0 AU from the Castor Aab pair, comparable to an orbit distance of [[Jupiter]] from our Sun.
===Castor Bb===
Like Castor Ab, the second star of the Castor B binary is a main sequence dwarf of an unknown spectral type. The star may be a class M [[red dwarf]] or a class A blue-white dwarf, much like its primary companion. The question of the star's spectral type stems from the fact, like Castor Ab, there are frequent [[x-ray]] flares from the star. The mass of this star is also estimated to range from 40 to 60 percent more then than our Sun, although its visual luminosity and diameter may be less than the Sun.<ref name="harv2"/>
For an [[terrestrial planet|Earth-like]] planet that orbits the Castor Bab pair, the distance the star needs to be for liquid [[water]] to comfortably exist on its surface, is around 4.0 AU - about the same distance of the [[Asteroid Belt]] from our [[Sun]].