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.