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MENKHIB

(Xi Persei). The naming of stars was hardly systematic, some bright ones (like Cih) having no proper names at all, at least within our own culture. Perseus (the Hero) may be the champion not only of Andromeda but also of unnamed stars as well. The top two, Mirphak (Alpha) and Algol (Beta) have proper names, and then the list stops until we arrive at number 10, a fourth magnitude star called "Atik" and Bayer's "Omicron" star. Fainter yet is number 12 in brightness, mid-fourth magnitude Menkhib (Bayer's "Xi"). They both refer to a larger Arabic construction in which Atik is the "collarbone" of the Pleiades and Menkhib the "shoulder" (Caph in Cassiopeia representing the "hand." Why these dim stars of Perseus, rather than some of the brighter ones, were chosen is a mystery. In terms of real significance, however, the old Arabians chose well. Though Menkhib is not terribly bright to the eye, it is nevertheless quite the spectacular star. One of the very few naked eye stars that falls into the hottest of stellar categories (class O), it shines with a sparkling blue-white light. Indeed it is one of the hottest stars to be seen without the aid of a telescope, its surface temperature around 37,000 Kelvin, over six times hotter than the Sun. Its apparent faintness is caused by a combination of its uncertain distance of around 1600 light years and by the absorption of its light by interstellar dust in the Milky Way, which cuts its brilliance about in half. When these are taken into account, we see that to the eye the star would shine 13,500 times brighter than the Sun. And when we take into account the ultraviolet light from Menkhib's hot gases, the figure climbs to 330,000 times! To receive the same amount of energy as we do, an orbiting planet would have to be 15 times Pluto's from the Sun away. (Menkhib does have a companion: a much smaller star, about which nothing is known, in a 7-day orbit). Menkhib is slightly unstable, changing its brightness by about five percent, and is also blowing a fierce wind, causing it to lose about a millionth of a solar mass per year, ten million times the rate in the solar wind. It is one of the naked-eye sky's most massive stars, weighing in at birth at around 40 solar masses. Though the star's status is rather uncertain, some observers calling it a "giant," others a "supergiant," it has almost certainly shut down core hydrogen fusion, and may even be fusing helium, already having lost some 10 percent of its original mass. Still only a few million years old, its only recourse is to explode sometime in the next million years or so. Fortunately for us, stars like this are very rare, none nearby. Menkhib, like Naos in Puppis, is also one of the sky's few "runaway" stars. For reasons still uncertain, it is zipping at high speed from its birthplace in a group known as the "Perseus OB2 association (which oddly contains it sister star Atik), the acceleration caused either by a close encounter with another star or by the explosion of a now-dead and even more massive companion.