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W BOO (W Bootis).Arcturus sodominatesBootes (the Herdsman) that wesometimes forget to pay attention to the rest the constellation'sstars. Next down in brightness is second magnitude (2.39)Izar (Epsilon Bootis), which ranks as one ofthe finest telescopic double stars in the sky. Lying right next toIzar, only 0.6 degrees away, as if it were a naked-eye companion,is fifth magnitude (4.81) W Bootis (known best by its Roman-lettervariable-star name), whichbears much the same relation to Izar asSigma Boo does toRho. Neither of these naked-eye pairs is"real": they are just lines-of-sight coincidences. At asubstantial 890 light years, W Boo is over four times farther thanIzar. Izar then just becomes a signpost that allows appreciationof W Boo, a fine example of a class M (M3) redgiant and subtle semi-regularvariable. The distance, apparent magnitude, and a temperature of3760 Kelvin (from which we find a considerable correction forinfrared radiation) all lead to a total luminosity of 2990 timesthat of theSun and a radius of 130 timessolar, or 0.60 Astronomical Units (83 percent the size of the orbitof Venus), appropriate to red giant status. Theory then yields anapproximate initial mass (the star has lost some through winds) ofabout five times solar. Starting life as a class B4 hydrogen-fusing dwarf, W Boo's current place in stellar life is uncertain. Like Rho Boo, it could be brightening with a dead helium core,dimming after firing up its core helium to fuse to carbon andoxygen, or brightening for thesecond time with a dead C-O coreas it prepares to slough off its outer envelope and turn into amassivewhite dwarf ofabout 0.9 solar masses, one similar toSiriusB. Stars in the last of these three phases are known for theirsemi-regular or regular (Mira-like)variability, so W boo is probably in its last throes as a giant. The study of the variability, however, has a bit of a checkeredhistory. The total variation seems to be under a magnitude (4.7 to5.4 is quoted). The period was originally given as 25 days, whichseemed to change to 50 days. However, the stars against which WBoo was compared turned out to be variable as well. W Boo seemed,though, to be multi-periodic with 25 and 33 day periods. The mostmodern work gives only a few-hundredths of a magnitude variationwith a principal period of 35.2 days with very subtlesuperpositions of 4.5 and 2.0 days, all due to physical pulsationand all too small to easily watch. Unlike Izar, W Boo has no knowncompanion as it runs through the later stages of its short 100million year (so far) life.
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