CHI-2 ORI (Chi-2 Orionis). The main figure of Orion is so bright that much of the rest of the constellation is ignored. To the west is the vertical string of stars that make his cloak (which includes the six stars all called "Pi Ori"), to the north Meissa (Lambda Ori), which marks his head, and much farther to the north (lying just south of the ecliptic and much closer to Gemini and Taurus) Chi-1 and Chi-2, which mark the tip of his upraised club. While given the same Greek letter, and having about the same apparent brightness (Chi-1 fourth magnitude at 4.41 and Chi 2 just fifth at 4.63), the two could not be more different. Chi-1 (the more westerly of the two) is a solar type dwarf only 28 light years away, while Chi-2 is a magnificent hot (19,000 Kelvin) class B (B2) bright supergiant so far away that its distance is unmeasurable by parallax. Were it not for absorption by intervening interstellar dust, the star would be over a magnitude brighter (3.41). The distance can be estimated in two ways, one by its spectral class (which gives absolute brightness, and which is then compared with apparent brightness) and by its probable membership in the Gemini OB1 association of bright stars, the former giving 3400 light years, the latter a much-farther 4900. Astronomers generally adopt the latter, which makes all the parameters more consistent. If that far away, Chi-2 Ori has an incredible luminosity 410,000 times that of the Sun, which puts it into the fringe of the ultra-rare "hypergiants" (the set of which contains P Cygni and Eta Carinae). From the luminosity and temperature we derive a radius 59 times that of the Sun (0.28 Astronomical Units, some 70 percent the size of Mercury's orbit), a whopping mass that falls between 35 and 40 times that of the Sun, and an age of a mere five million years. (An uncertain minimum equatorial rotation speed between 36 and 72 kilometers per second gives a rotation period less than 82 days.) Such stars develop powerful winds, Chi-2 losing mass at a hundred-thousandth of a solar mass per year. Even the shorter distance of 3400 light years gives a luminosity 38,000 solar and a mass 14 solar, so there is no question that before too terribly long (on an astronomical time scale of course) that Chi-2 will blow up as a powerful supernova. Supernovae (at least this kind) are produced by the collapse of their nuclear- burning cores, which supergiants build to iron (which can "burn" no farther). Most of the cores collapse to neutron stars (balls of neutrons with radii in the tens of kilometers and densities of a million tons per cubic centimeter). If near the high end of the scale, which is most likely, Chi-2 crosses over into the realm in which the core may collapse into a black hole, a "star" so dense that even light cannot escape. Pity the poor (uncertain) companion, about which nothing is known but that seems to lie just 30 or so AU away. (Thanks to Reginald Quinto for suggesting this star.)
Written by Jim Kaler. Return to STARS.