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.