W BOO (W Bootis). Arcturus so dominates Bootes (the Herdsman) that we sometimes forget to pay attention to the rest the constellation's stars. Next down in brightness is second magnitude (2.39) Izar (Epsilon Bootis), which ranks as one of the finest telescopic double stars in the sky. Lying right next to Izar, 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-letter variable-star name), which bears much the same relation to Izar as Sigma Boo does to Rho. Neither of these naked-eye pairs is "real": they are just lines-of-sight coincidences. At a substantial 890 light years, W Boo is over four times farther than Izar. Izar then just becomes a signpost that allows appreciation of W Boo, a fine example of a class M (M3) red giant and subtle semi-regular variable. The distance, apparent magnitude, and a temperature of 3760 Kelvin (from which we find a considerable correction for infrared radiation) all lead to a total luminosity of 2990 times that of the Sun and a radius of 130 times solar, or 0.60 Astronomical Units (83 percent the size of the orbit of Venus), appropriate to red giant status. Theory then yields an approximate initial mass (the star has lost some through winds) of about 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 and oxygen, or brightening for the second time with a dead C-O core as it prepares to slough off its outer envelope and turn into a massive white dwarf of about 0.9 solar masses, one similar to Sirius B. Stars in the last of these three phases are known for their semi-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 checkered history. The total variation seems to be under a magnitude (4.7 to 5.4 is quoted). The period was originally given as 25 days, which seemed to change to 50 days. However, the stars against which W Boo was compared turned out to be variable as well. W Boo seemed, though, to be multi-periodic with 25 and 33 day periods. The most modern work gives only a few-hundredths of a magnitude variation with a principal period of 35.2 days with very subtle superpositions of 4.5 and 2.0 days, all due to physical pulsation and all too small to easily watch. Unlike Izar, W Boo has no known companion as it runs through the later stages of its short 100 million year (so far) life.
Written by Jim Kaler. Return to STARS.