PLASKETT'S STAR (HR 2422 Monocerotis). Very few stars carry astronomers' names: fast moving Barnard's Star (a dim class M dwarf in Ophiuchus) comes quickly to mind as do a handful of others that include "Plaskett's Star," named after the Canadian astronomer J. S. Plaskett (1865-1941), who first studied its amazing properties in 1922 (and more formally known as HR 2422 in the "Bright Star Catalogue"). While not coming up to the "king," Eta Carinae, Plaskett's Star (just to the northeast of the famed Rosette Nebula in the modern constellation Monoceros) is still one of the most massive binaries (doubles) known, with two hugely massive blue-white class O (as best we can tell, O7.5 and O6) supergiants tightly orbiting each other with a period of only 14.40 days. The stars are so close together that it is very difficult to disentangle their properties from each other, though it HAS been done through observations of their spectra as they whirl around a common center of mass. While from the observed orbital velocities, the cooler is the less massive (43 Suns), it is visually the brighter and is called the "primary" (Plaskett A), the other (at 51 Suns) the "secondary" (Plaskett B). One of the biggest problems is the distance, as Plaskett's is too far away for any parallax measure, so far that its light is dimmed by 1.1 magnitudes through absorption of starlight by the interstellar dust of the Milky Way. Plaskett's has long been considered to be part of the Monoceros OB2 association of massive O and B stars, which (from spectroscopy) lies 4900 light years away. But membership in these systems is devilishly difficult to determine, and Plaskett's may be even farther, the spectroscopic evidence suggesting that it is as far as 6600 light years. If at the distance of Mon OB2, the determined parameters for temperature (T), luminosity (L), and radius (R) are for "A": 35,000 Kelvin, 295,000 Suns, 15 times solar; for "B": 40,000 Kelvin, 186,000 Suns, and 10 solar. All values, however, are more consistent for a distance of 6600 light years, at which L and R may be as high for "A" as 630,000 Suns and 21 solar radii, and for "B" as high as 372,000-870,000 Suns and 14-21 solar radii. The combined system mass is a whopping 97 Suns, which with the orbital period gives a separation of only half an Astronomical Unit, which in turn would fall between the sizes of the orbits of Mercury and Venus. That the stellar orbital period is much less than those of these planets is the consequence of the very high stellar masses. The stars are known for their colliding winds, that of the primary having whittled the star down to 43 solar masses from a birth mass of 54 solar. The primary (A) is rotating with a projected speed 75 kilometers per second, while the secondary is spinning much faster, the speed around 300 km/s (for a rotation period as short as two days), that and the surrounding mass loss rendering the system a unique sort of "shell star." There seems to be but one fate, that both stars will someday erupt in brilliant supernovae. The first one to go may well eject its mate at high speed, or, more interesting for future astronomers, the pair could stay together and end up as a binary neutron star, a double black hole, or some combination of the two. Keep your eye on the Unicorn! (Most of these data are from a paper by W. G. Bagnuolo Jr., D. R. Geis, and M. S. Wiggs that appeared in the Astrophysical Journal, vol. 385, p. 708, 1992. Thanks to Reginald Quinto for suggesting this star.)
Written by Jim Kaler. Return to STARS.