VV ORI (VV Orionis). By far the most famed eclipsing double star in the sky is Algol (Beta Persei), against which all others pale. Even worse for an eclipser like VV Orionis is that it is tucked up beneath Orion's belt, where it is quite overwhelmed and ignored just to the west of Alnilam (VV belonging to the Belt subdivision of the Orion OB1 association). Even though fifth magnitude (5.34), it carries neither a Greek letter nor a Flamsteed number, just its Roman letter variable star name "VV" (in addition of course to catalogue numbers). While initially seeming to be no more than another hot class B (B1) star, some 1850 light years away (according to its parallax), the spectrum reveals an orbiting class B7 companion, both classed as hydrogen-fusing dwarfs. The orbit is tilted nearly into the line of sight such that every 1.48537769 days (1 day, 11 hours, 39 minutes), when a smaller and cooler B7 star goes in front of the larger and hotter B1 star, the combined brightness of the system drops by almost half a magnitude (5.3 to 5.7), the eclipse lasting for 2.5 hours. In between, when the smaller but dimmer star goes in back of the larger, is a "secondary" eclipse in which the brightness drops by about 0.2 magnitudes, which is more difficult
The light curve of VV Orionis begins with the deeper minimum of the secondary eclipse (at far left, Phase 0). The minimum of the secondary eclipse is in the middle (Phase 0.5); the curve again plunges at the primary eclipse at far right. Tidal effects that distort the stars from spherical shapes, and refelction effects, cause curve to be continuously variable even out of eclipse. (From an article by C. R. Chambliss and B. M. Davan in the Astronomical Journal, vol. 93, p. 950, 1987.)
to see without instrumentation. While the temperature of the smaller star comes in at about 15,500 Kelvin, that of the larger has been taken as anywhere from 21,000 to 25,000. Orbital solutions give a separation between the two components of a mere 0.063 Astronomical Units, only 15 or so percent of Mercury's distance from the Sun, accounting of course for the short period. Masses come in at 10.8 solar for the bigger star, which shines at a luminosity 9400 times that of the Sun, the values for the smaller respectively 4.5 and 330 solar. Radii of the pair are respectively about 5 and 2.5 times bigger than that of the Sun. These figures do not agree all that well with those derived directly from luminosity and temperature, which give up to 22,000 solar luminosities and 13 solar masses for the hot star. Quite likely, the distance determined from an uncertain parallax is just too large: 1200 light years is more like it. All the action is watched by a third component, a class smaller class A star that lies about an AU away and orbits with a period of 119 days. From there, the inner pair would appear no more than about 3.5 degrees apart. It may someday be witness to a supernova, as the main component of the triple is on the edge of the mass at which its core will eventually collapse in the creation of a neutron star.
Written by Jim Kaler. Return to STARS.