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.
Written by Jim Kaler. Return to STARS.