IOTA CNC (Iota Cancri). Almost due north of the Beehive Cluster (the Praesepe) and the four-star box
that makes the "Manger" of Cancer (the
Crab), lies Cancer's second brightest star, rather oddly given
Greek letter "Iota," showing that Bayer had more in mind than
brightness when he ordered the constellation's stars (the
brightness order going Beta, Iota, Delta,
Alpha). Iota Cancri's mid-fourth
magnitude (3.93) second-best is a bit of a cheat, however, since
the star is double, which makes it a mere 1/100 of a magnitude
(about 1%) brighter than Delta Cancri, the two stars looking
exactly the same brightness as seen with the human eye. To be
perfectly correct, the star should be known as "Iota-1," notably
dimmer Iota-2 lying a couple degrees to the northeast, but "Iota"
will serve here. Iota Cnc is a classic and pure example of a "wide
binary," consisting of a seventh magnitude (6.57) white class A
(A3) dwarf (called "Iota B") coupled with a fourth magnitude
yellow-white class G (G7.5) giant ("Iota A") that lie 30.6 seconds
of arc apart. Though the colors of the stars do not differ by all
that much, contrast effects caused early observers to declare them
"clear blue and pale orange," making them a fine sight in a small
telescope (lower power enhancing the contrast). Neither of the
stars is well-studied. Parallax measures of Iota Cnc B seem to
show it much closer than the 300 light year distance of Iota Cnc A,
suggesting that the two are merely a line of sight coincidence.
However, because of the proximity of brighter "A," the errors on
the measures for "B" are huge, rendering its distance measure
useless. That the stars are moving through space together and have
not changed their separation in a century show them to be a true
pair. Iota Cnc A shines with the light of 215 Suns from a 5000 Kelvin surface, while fainter
"B" glows at 16 Suns from an 8800 Kelvin surface. "A" has recently
been listed as a supergiant, but the calculated luminosity places
it squarely among the giants. "B" is an ordinary hydrogen-fusing
dwarf with a mass of 1.9 solar, while "A" is a 3.5 solar mass star
in transition, cooling at its surface with a dead helium core, the
pair about 260 million years old. Only about a million years ago,
"A" was a blue class B dwarf; "B" has about a billion years left to
it before following in "A's" footsteps. By then, however, "A" will
have become a vastly brighter giant and then a dim white dwarf.
The two are separated by at least 2800 Astronomical Units, and take
at least 65,000 years to orbit. From A, B would shine with the
light of our full Moon, while from B, A would appear 10 times
brighter. Odds are that gravitational encounters with other stars
will gradually separate them, leaving them as lonely singles.
Thanks to Bob Parvin who suggested this star, and to Matthew
Branham, who helped research it.