ETA AND DELTA-1 CMI (Eta and Delta-1 Canis Minoris, a two-for-one
special). Canis Minor, Orion's smaller hunting dog, is well marked by bright
Procyon, but contains little else of
naked-eye interest. Closer examination, though, reveals a curious
coincidence, two remarkably similar (but gravitationally unrelated)
stars of rather rare class. Eta CMi (between Procyon and Gomeisa) and Delta-1 CMi (the brightest of
a trio of unrelated "Deltas" southwest of Procyon) look like twins:
both are fifth magnitude (5.25) class F (F0) giants in a tie for the
constellation's eighth brightest star. Yet even such pairs can
have significant differences. At a distance of 790 light years,
Delta-1 is 2.2 times farther than Eta and must therefore be 2.2
squared, or five, times more luminous. A measured temperature of
7450 Kelvin for Delta-1 is a bit high for its class (though adopted
here). It should be more like the 7000 Kelvin adopted for Eta.
The exact temperature, however, is in this range irrelevant for
calculation of luminosities, which for Delta-1 and Eta respectively
come in at 360 and 70 times that of the Sun,
in the expected ratio. Temperature and luminosity then yield
respective radii of 11 and 6 solar and rather different masses of
3.75 and 2.5 solar. Having begun life as class B stars
(respectively B6 and B9), both stars are now beginning to end their
lives as they prepare to expand and brighten further before they
lose their outer envelopes and become white dwarfs. Both are
also, for giants, rather rapid spinners (at least 54 kilometers per
second for Eta, 71 for Delta-1), giving respective rotation periods
less than 5 and 8 days. Delta-1 is metal-rich, with an iron-to-
hydrogen ratio 50 percent greater than that of the Sun, while that
of Eta is unknown. Eta CMi, however, distinguishes itself by
having a dim 11th magnitude (11.1) binary companion 4 seconds of
arc away that, from its absolute visual luminosity of but a third
that of the Sun, is most likely a class K1 dwarf much like Alpha Centauri B. Orbiting at a
distance of at least 440 Astronomical Units, Eta CMi B takes at
least 5000 years to make a full circuit around its much brighter
mate. From the companion, the bright star would appear to be but
a pinpoint that shines with the light of over 400 full Moons, while
Delta-1 would be three times brighter than it is viewed from Earth.