W ORI (W Orionis). The brighter stars carry proper names, or are designated by Greek letters (Betelgeuse in Orion being Alpha Orionis) or by Flamsteed numbers (Betelgeuse also 58 Orionis). Fainter stars get catalogue numbers. Variables, however, fall into a special group. Unless they already have traditional names, they are given letters from the Roman alphabet, beginning with "R" for the first discovered, and then moving to Z, and then going through a complex pattern of double letters. W Orionis is thus the sixth variable (excluding those with other names, including Betelgeuse) discovered within the constellation of Orion. Reaching sixth magnitude (5.88) at its brightest, it also has the distinction of being one of the few naked eye (though just barely) "class C" carbon stars in the sky. The solar gases contain more oxygen than carbon. In an advanced evolutionary state, carbon stars reverse the ratio by dredging carbon that they have made in their nuclear-burning interiors to the surface. The carbon forms molecules that block blue light, making carbon stars among the reddest of the sky.
W Ori Shining with an obvious red light, the carbon star W Orionis, a semi-regular variable with a period around 200 days, lies near the center of the picture. Such stars are so cool and loaded with carbon compounds that the blue component of starlight is pretty much removed, leaving them a vivid red.
At a large distance of 700 light years, W Ori (a class C6 or C5 bright giant) is actually quite luminous. Exactly how bright is argued since the temperature (as is the case with many carbon stars) is not secure, estimates ranging from a very cool 2600 to 3200 Kelvin. At the lower end, the star would radiate 9400 solar luminosities (most in the infrared) and would have a very large radius of two Astronomical Units (AU), that is, double the distance between Earth and Sun. Direct measure of angular diameter, however, yields a radius of "only" one AU. That and the higher temperature give a luminosity of 4600 solar, which is probably closer to the truth. "W" is classified as a pulsating semi-regular variable in which the star changes its radius as well as its temperature and luminosity. Most such stars, however, have a more or less regular variation period, W Ori's measured between 186 and 212 days, during which time it goes from sixth to somewhat below naked eye visibility and back. (The variation in the infrared, where the star emits most of its energy, is much less). W's average brightness also changes over a much longer period of 2450 days (6.7 years) as result of either a secondary pulsation cycle or the effect of an otherwise undetected orbiting companion. The star, whose uncertain mass is somewhere around double that of the Sun, is slowly brightening with a dead carbon and oxygen core. Now losing mass at a rate of a tenth of a millionth of a solar mass per year, the star will before long remove its vast outer envelope as it prepares to become a white dwarf. Its internally manufactured carbon will add to the carbon content of interstellar space, where it will someday find its way into other stars and perhaps other earths.
Written by Jim Kaler. Return to STARS.