W ORI (W Orionis). The brighter stars carry proper names or are designated by Greek letters (Deneb in Cygnus being Alpha Cygni) or by Flamsteed numbers (Deneb also 50 Cygni). 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, 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) to be 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. Better, it was the first of the breed to be known, discovered by Angelo Secchi of the Vatican Observatory in 1868.

W Ori The carbon star W Orionis, the first of its kind to be found, shines like a little jewel against the dark sky around seven degrees west of Orion's belt. With a luminosity that might be as high as 20,000 Suns, the star, bigger than the orbit of Earth, does not have long to live before becoming a dense, cooling white dwarf.

The solar gases contain more oxygen than carbon. In an advanced evolutionary state, carbon stars reverse the ratio by dredging to the surface the carbon that they have made from helium in their nuclear-burning cores. The carbon forms molecules that block blue light, making carbon stars among the reddest of the sky, the color enhanced by dust grains forming in the stellar winds. At a large and very uncertain distance of 1200 light years (give or take a whopping 500 or so), W Ori, a class C6 or C5 bright giant, is quite luminous. Exactly how bright is arguable since the temperature (needed to assess the amount of infrared radiation) is also not secure, estimates ranging from a very cool 2600 to as high as 3200 Kelvin. At the lower end, the star would radiate 29,000 solar luminosities and would have a very large radius of 3.8 Astronomical Units (AU), nearly four times the distance between Earth and Sun, three-fourths the size of Jupiter's orbit, just over 800 solar radii. Direct measure of angular diameter, however, yields a radius of "only" 1.8 AU, about 400 solar radii. That and the higher temperature give a luminosity of 14,000 Suns. "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 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 very uncertain mass might be five times 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 generate a carbin-rich planetary nebula, its core becoming a white dwarf. W Ori's 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 before 1997 and revised byJim Kaler 8/12/16. Return to STARS.