ZETA CMI (Zeta Canis Minoris). Even small constellations hold their charm,
curiosities, and treasures. Canis Minor, Orion's smaller Hunting Dog) glows with
zeroth magnitude (0.34) Procyon, the eighth brightest star of the sky, taking its
place at the northeastern apex of the Winter
Triangle (completed by Sirius at the
southern apex, and Betelgeuse to the
west). Here we find an odd and instructive coincidence. Third magnitude
(2.90) Gomeisa (Beta CMi) is a class B8 dwarf that lies some 4.5 degrees
to the northwest of Beta. Directly opposite Procyon, 4.5 degrees to
the southeast of the bright star, we find much fainter Zeta CMi (mag 5.14), an
almost identical class B8 "bright
giant" (but see below). The difference in apparent brightess is
almost all due to difference in distance, Zeta CMi falling 624 (give
or take 43) light years away, with Gomeisa at just 162 light years (plus
or minus 2) in the second Hipparcos reduction), almost four times as
far. The thin dust in the Milky Way toward the Anticenter of the Galaxy dims Zeta by a mere 0.06 of
a magnitude. A temperature of 11,680 Kelvin gives us the amount of ultraviolet light to add to the
visual, which results in a total luminosity of 478 Suns and thus a radius 5.4 solar radii. There
seems to be no excess infrared radiation, nor are there companions to be seen. The
application of theory gives a mass of 4.0 times that of the Sun and
shows the star to be at or near the end of its hydrogen fusing lifetime.
Not a giant at all, Zeta CMi is barely a subgiant (such classification
anomalies quite common among the B stars). What makes the two so
interesting is not their coincidental classification and placement
relative to Procyon, but their differences, seemingly produced mostly
by rotation. Zeta has a projected equatorial rotation speed of just
35 kilometers per second, giving it a rotation period of under 7.7 days.
Gomeisa on the other hand is really whirling around with a projected
velocity of 250 km/s that is much more appropriate to the class.
Probably as a result of its great rotation speed, Gomeisa has produced
a surrounding, radiating disk that turns it into a "B-emission") ("Be")
star similar to Zeta Tauri or Gamma Cas. Zeta, however, spins too slowly
to have created one. On the other hand Zeta CMi's slow rotation produces
a quiet atmosphere that allows gravitational settling of some chemical
elements and the lofting of others, making it into a "mercury-manganese
star" with huge surface overabundances of these and other elements such
as europium and strontium and underabundances of others such as
calcium. Gomeisa's faster spin keeps the outer atmosphere stirred up,
which helps the star maintain more or less normal chemical abundances.
We still do not know the exact mechanism for producing the "Be" disk.
Magnetic fields might be involved. Consistently, Zeta CMi seems to have
none of significance. If nothing else, the two seem to provide clues
to the astrophysical phenomena involved. Not massive enough to blow
up as a supernova, Zeta CMi
should slough its outer layers to produce a white dwarf with about 78
percent the mass of the Sun.
Written byJim Kaler 04/07/17. Return to STARS.