ZETA CRV (Zeta Corvi), along with HR 4691. In any reasonably dark sky, the small box-shaped constellation of Corvus, the Crow, stands out to the southwest of Virgo's Spica. The figure's brightest stars were appropriately given the first five letters of the Greek alphabet, though the brightest is bright third magnitude Gamma Corvi (Gienah). Tucked into the bottom of the box and not part of the classical outline is fifth magnitude (5.21) Zeta Corvi, a class B (B8) hydrogen fusing dwarf that makes its mark by being a "B emission" ("Be") star, one with hydrogen emissions caused by a circulating circumstellar disk. At first, Zeta looks like a naked-eye double with a sixth magnitude (6.0) "companion" (called HR 4691 to Zeta's HR 4696 in the "Bright Star Catalogue") just 7 minutes of arc to the west. Intriguingly, it might just be. Then maybe not.

Zeta Crv Zeta Crv At far left, the image of Zeta Corvi (center) is partially blended with that of HR 4691 seven minutes of arc away. Minkar (Epsilon Corvi) lies well up and to the right, while Alchiba (Alpha Crv) is near the right hand edge. At near left is a telescopic view of fifth magnitude Zeta and sixth magnitude 4691 showing them well separated. Are they related? (Near left picture from SIMBAD.)

Take them one at a time. From Zeta's temperature of 11,560 Kelvin (needed to allow for some ultraviolet light) and its well- determined distance of 415 light years (give or take just 12), we find a luminosity 190 times that of the Sun and a radius of 3.3 times solar. Like all "Be" stars, Zeta Crv is a fast rotator, going around with a projected speed of 236 kilometers per second, which gives it a rotation period of under 0.71 days (the relation between rotation and disk formation still not understood). If a small variation of a few hundredths of a magnitude with a period of 0.51 days is related to the actual rotation period, the true rotation speed is close to 330 kilometers per second with the rotation axis tilted by 45 degrees to the line of sight. Temperature, luminosity, and theory then tell of a star with a mass 3.4 times that of the Sun and that it is nearing the end of its 270 million year hydrogen fusing lifetime. Zeta's speed relative to the Sun of 68 kilometers per second, some four times "normal," suggests that the star had long ago been given some sort of gravitational kick. At a current separation of 20 seconds of arc lies a 13th magnitude "companion" that is moving so fast relative to Zeta proper that it is clearly a line of sight coincidence.

Then take a look at HR 4691. Of uncertain class, it was first listed as a G3 subgiant, but is possibly a common K0 giant. At a distance of 385 light years (give or take 20), it's only 30 light years closer to us than Zeta Crv. But given the uncertainties, it could actually be at the same distance. Moreover, Zeta and 4691 are moving through space in roughly the same directions at similar speeds, so perhaps they are really related. That aside for now, HR 4691 is itself a real double with an eighth magnitude (8.1, Zeta A then at 6.3) companion that is probably a mid class F dwarf just a few tenths of a second of arc away. With respective luminosities of say 50 and just under 10 times that of the Sun, HR 4691 A and B would appear to have masses perhaps around 2 and 1.5 Suns (?), which with a separation of at least 120 AU would imply an orbital period of more than 700 years. If HR 4691 is then really paired with Zeta, they would be separated by at least 50,000 AU and take more than 3.5 million years to make a full orbit. It's also quite possible that Zeta and HR 4691 are a coincidental pair. If at the stated distance, each would be roughly zero magnitude in the other's sky, pretend residents of Zeta still needing a telescope to make out the duplicitous nature of HR 4691.

Written by Jim Kaler 4/12/13. Return to STARS.