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). Bayer
originally lettered the stars within huge Argo, and when the ship
was broken up, the Greek letters 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.