FURUD (Zeta Canis Majoris). The proper name of this prominent
near-dead-on-third-magnitude (3.02) star derives from the Arabic
"al-furud," meaning "single" or "solitary ones," a plural applying
to any such star, but erroneously stuck on Bayer's Zeta CMa alone.
It certainly qualifies, as it is set rather well away from the main
figure of Canis Major, to the west and
more or less in between the Greater Dog's bottom triangle (with
bright Adhara) and the modern
constellation Columba, the Dove (making
Furud quite easy to find). And as a luminous, hot, blue class B
(B2.5) dwarf, it's well worth locating. The star's rather goodly
distance of 335 light years (known to an accuracy of about six
percent) and proximity to the Milky
Way causes it to be dimmed a bit by interstellar dust, by 0.16
magnitudes (about 16 percent). Making that correction, and another
for a large amount of ultraviolet light radiated by a surface
heated to 21,500 Kelvin, we find a high luminosity 4020 times that
of the Sun, which in turn leads to a radius
of 4.6 times solar. A minimum equatorial rotation velocity of just
25 kilometers per second (low for the class, suggesting that the
star may appear more or less pole-on) gives a rotation period of
under 9 days. Stellar structure theory leads to a mass very close
to 8 times that of the Sun, and shows the star to be roughly half
way through its hydrogen-fusing dwarf lifetime of only 32 million
years. While there is a rumor that the star is variable, there
seems to be no supporting evidence. In spite of its name, Furud is
not solitary, but has a lower-mass "spectroscopic" companion that shifts
Furud proper back and forth as the two orbit over a period of 675
days (1.85 years). Nothing is known about it. A guess of two
solar masses (consistent with the degree of shift) gives a
separation of 3.2 Astronomical Units (60 percent Jupiter's distance
from the Sun), a high eccentricity of 0.57 moving the stars between
5.1 and 1.4 AU apart. Furud is listed as having an eighth
magnitude companion at an angular separation of nearly three
minutes of arc, but the dimmer star's class and motion clearly show
the "companionship" to be merely a line-of-sight coincidence.
Furud is perilously close to the mass limit above which stars
explode as supernovae, but
odds are it will someday make a "
planetary nebula" and then a massive white dwarf instead, perhaps
a rare one composed of neon and oxygen rather than the usual
carbon-oxygen mix (the result of advanced nuclear processing). Zeta CMa
is an excellent marker for the "Antapex of the Sun's Way,"
the direction from which the Sun appears to be coming, which lies 3.5 degrees
due west of the star in the constellation Columba. The Apex
is in Hercules southwest of Vega.
(Thanks to Marco Trevisan for comments.)
Written by Jim Kaler 2/02/07. Return to STARS.