V766 CEN (V766 Centauri)= HR 5171. Not impressed by the names? You will be by the star. Bayer's successors in the southern hemisphere never got to this one, surely never even noticed it in southern Centaurus within the rich Milky Way between Alpha Centauri and the Southern Cross. Catalogued as barely seventh magnitude (6.51), it makes a good sixth by varying irregularly between 6.1 and 7.3. It's remarkable that the star actually got a listing in the Bright Star Catalogue as HR 5171 (actually 5171A, given a tenth magnitude companion 9 seconds of arc away). As a yellow hypergiant (a "YHG") more grand than a supergiant, V766 Cen falls among the rarest kinds of stars: only about ten are known in the Galaxy, including the iconic Rho Cassiopeia (HR 8752, also in Cas, seeming to belong as well). The spectral class is a bit problematic, the Bright Star Catalogue listing it as a K0 hypergiant, later astronomers making it as a G8 to K3 bright supergiant. V766 is dimmed by as much as 2.8 magnitudes (the actual amount not clear) by circumstellar and interstellar dust, which also reddens the starlight. Given a clear view, it might have been bright enough for a Greek letter.

Too far away for parallax, the star's distance is estimated from its Galactic location and other considerations to be nearly 12,000 light years. A temperature of around 4800 Kelvin adds a bit of infrared radiation to the total, which with distance gives a luminosity of 470,000 times that of the Sun and a radius 0f 980 solar or 4.6 Astronomical Units, nearly 90 percent the size of Jupiter's orbit. Interferometer measures suggest that HR 5171A may be even larger, 1315 times solar (6.1 AU, 12 percent BIGGER than Jupiter's orbit), which with distance and a temperature of 5000 Kelvin yields a luminosity of closer to a million Suns. Any way we look at it, the birth mass seems to be around 39 Suns, placing it into the realm of the most massive stars in the Galaxy. Given mass loss through strong winds, the mass must now be a lot less (stars always ending their lives with much less mass than they started with). A high abundance of sodium in the HR 5171A's atmosphere suggests that it is in the later stages of core fusion (beyond that of helium, the sodium cycled upward) and that the star was once a true red supergiant that has evolved beyond that state. Perhaps it is becoming a "Luminous Blue Variable" like P Cygni or Eta Carinae, or a Wolf-Rayet star (one with a stripped core and thick envelope from mass loss) like that in Gamma-2 Velorum.

The star is centered in a diffuse emission nebula (an ionized cloud like the Orion Nebula). The ultraviolet radiation required to excite the nebula comes not from the hypergiant (which is too cool), but from the companion. An impressive star on its own, HR5171B is a class B0 lesser supergiant with a temperature near 27,000 Kelvin, a luminosity of 160,000 Suns (providing it is at the same distance as "A," which is likely), and thus a birth mass of 25 Suns. Since the hypergiant was only recently a hot class O5 dwarf, it probably contributed mightily to the surrounding scene. If the two are gravitationally bound (which is questionable), they are at least 30,000 Astronomical Units apart and would take at least a million years to orbit each other. Given a hydrogen-fusing lifetime of maybe 5 million years, they have not done it many times. V766 is actually triple! Interferometry and an additional regular 0.2 magnitude variation tell of a very close companion that periodically gets in front of, and partially eclipses, the brighter star. With an orbital period of 3.57 years, the two are probably in actual contact at their surfaces, though it might more be contact of the smaller star with the inner part of the bigger one's thick wind, which has produced a nebula seen in the infrared that extends out to 6500 AU. HR 5171A seems destined to blow up as a supernova, and soon, which will do the close companion no good. The only real question is whether the core will collapse into a neutron star or a black hole. HR 5171B will then follow, both stars giving back most of their element-enriched matter (including a lot of iron) back into the interstellar medium as fodder for new, higher-metal, stars. (Thanks for data and discussion to O. Chesneau et al., Astronomy and Astrophysics, 563, A71, 2014.)

Written byJim Kaler 4/25/14. Return to STARS.