PHI-1 ORI (Phi-1 Orionis). With its seven bright stars (not to mention the Orion Nebula), the main figure of Orion presents such a magnificent sight that few venture upwards to the Hunter's Head, which is marked primarily by great Meissa (Lambda Orionis), which in turn is dominated by a rare class O star. Fewer might make note of the triangle made by Meissa with the two "Phi's" south of it, fourth magnitude (4.41) Phi-1 (the eastern of the two) and somewhat brighter (4.09) Phi-2 Ori. In spite of their common name, the two have nothing to do with each other, Phi-1 a luminous, hot class B (B0) "giant," perhaps subgiant (but see below), 1087 light years away (give or take 91), Phi-2 a rather ordinary (though cyanogen-weak) class K0 true giant nine times closer. We focus here on more interesting Phi-1. To be sure, the star's relative faintness comes from its large distance, but also from nearly half a magnitude of interstellar dust absorption. Were the path clear, Phi-1 would come out slightly brighter (magnitude 3.96) than Phi-2. Being in back of dusty clouds combined with its simple spectrum makes the star ideal for examining the nature of the intervening interstellar medium that pervades the Milky Way.

Distance and a temperature of 28,900 Kelvin (from which we can gauge the amount of ultraviolet radiation), a bit low for the class, lead to a great luminosity of 29,650 times that of the Sun, from which is found a radius of 6.9 times solar. An ill-determined projected equatorial rotation speed of 28 kilometers per second gives a rotation period of under 12.3 days, rather slow. The real significance comes with the calculation of mass as derived from theory. Carrying around 14 times the mass of the Sun, Phi-1 is well beyond the limit of 8-10 solar masses above which stars explode as core-collapse supernovae. Not a giant at all, with an age of 7.2 million years it's a hydrogen-fusing dwarf about 60 percent of the way through its 11.6 million year lifetime (massive stars living much shorter lives than those of lower mass, the Sun's analogous time coming in at 10 billion). After hydrogen exhaustion but before it explodes, the star will become a much brighter red supergiant like Betelgeuse. Though Phi-1 does not belong to its nearby namesake Phi-2, it does relate strongly to brighter Meissa. At Meissa's distance of 1100 light years (as calculated under the second Hipparcos reduction), the two are truly close to each other. Indeed, Phi-1 Orionis is considered to be part of the "Lambda Orionis association," otherwise known as the cluster Collinder 69. The whole affair is centered on a gigantic interstellar bubble 140 light years in diameter, which Phi-1 helps illuminate (Meissa doing most of the job). Closer in, Phi-1 is noted as having a spectroscopic companion with an 8.4 year period. If so (while it seems unconfirmed), if at low mass it would have an orbital radius of about 10 Astronomical Units. Though a bit on the faint side, perhaps to the point of un- noticeability, Phi-1 thus presents itself as part of greater Orion, of both the mythical constellation and more importantly of Orion's physically and temporally related stars. (Thanks to Bas Verhagen, who suggested this star.)
Written by Jim Kaler 2/10/12. Return to STARS.