Regor

REGOR (Gamma-2 Velorum). An amazingly complex star was boded by an equally complex name. In the nineteenth century, Argo, the Ship, was divided into Vela (the Sails), Puppis (the Stern), and Carina (the Keel) by Nicolas de Lacaille. He then applied Greek letters to Argo as if it were one constellation, so with the breakup of the Ship, they went with the pieces. Alpha (Canopus) and Beta (Miaplacidus) are therefore in Carina, while Gamma (our star Regor) is in Vela. The Arabs referred to several of Argo's stars, which skimmed their southern horizon, as "Suhail," including Canopus. Both Regor (Gamma Velorum) and Lambda Velorum are "Suhails," Regor "Suhail al-Muhlif," or "Suhail of the Oath." While Regor is still commonly called Suhail, that name most often goes to the Vela's Lambda star, so here we will use the modern name "Regor" for Gamma. (A small telescope shows a fourth magnitude companion not quite a minute of arc away. Since the companion is east of Regor proper, it is called Gamma-1, rendering Regor Gamma- 2.) Unfortunately, after all this, nobody seems to know what "Suhail" actually means! Regor, however, is "Roger" spelled backwards, and honors Roger Chaffee, one of the astronauts who died in the Apollo fire. Shining at bright second magnitude (1.78) with a hot blue-white light, the star itself is nothing less than spectacular. Called "the spectral gem of the southern skies," it is actually a pair of stars too close to be separated directly with the traditional telescope. The brighter of the two is a hot hydrogen-fusing dwarf of class O (traditionally O7.5 but perhaps 09.5), while the other is the visually brightest "Wolf-Rayet" star in the sky, the two in 78.5 day orbit about each other and (assuming a tilt of the orbit to the sky of 68 degrees, which could be in considerable error) separated by 1.2 Astronomical Units (the Unit the distance of Earth from Sun) or greater (the tilt of the orbit really not known). From the distant companion, itself a fairly impressive hot class B star at least 15,000 astronomical units away from Regor proper, the bright duo would seem to be a brilliant double star separated by about 15 seconds of arc or so. A fairly high eccentricity could take them as close as 0.8 AU. Wolf-Rayet stars, named after the astronomers who discovered them, are very rare and in an extremely advanced state of age. Enormously windy, they produce powerful emissions of radiation at particular colors. They have stripped off most of their mass, their outer hydrogen envelopes, and have exposed deep helium-rich layers heavily contaminated with the by-products of nuclear fusion. WR stars come in two flavors, nitrogen-rich and carbon-rich, Regor's belonging to the latter class. Relative to helium, carbon is typically enriched by a factor of 100, and is the product of the fusion of helium. Everything about Regor excites the imagination. Unfortunately, the distance is not all that well known. Direct parallax gives 840 light years, but light from background stars may have introduced error, so the system must be studied indirectly. Estimating from other stars, the O7 dwarf should have a luminosity of around 180,000 times that of the Sun, a temperature of 32,500 Kelvin, a radius of 13 solar, and a mass around 30 solar. The WR star is much hotter (57,000 to 70,000 Kelvin, most of the light coming out in the ultraviolet part of the spectrum) with a luminosity of 100,000 solar. Measures of the angular separation between the stars coupled with the known orbit then give a distance of 1200 light years, over 40 percent greater than previously adopted. More massive stars always evolve the faster, so while the Wolf-Rayet component is now less massive than the O star, it is much farther along in its evolution, and hence had once to be the more massive of the two. Both stars blow powerful mass-losing winds, the WR star at a rate of a hundred- thousandth of a solar mass per year (more than 100 million times that of the solar wind), while the mass-loss rate of the O star is some 25 times less than that of the WR star. The collision between the winds produces X-ray emission. The windy WR star probably started with somewhere around 40 solar masses and has now stripped itself down by an unknown amount, perhaps to under 10. Only a few million years old, the visually fainter Wolf-Rayet component is almost certainly in the last stages of preparing to blow up as a supernova. The O star will follow much the same path and go supernova as well.

Written by Jim Kaler 02/23/07. Return to STARS.