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SEGIN

(Epsilon Cassiopeiae). At the eastern end of the "W" that makes Cassiopeia, and at the top of her "Chair," lies third magnitude (3.38) Epsilon Cassiopeiae, appropriately the fifth brightest star in the constellation. The star is full of contradictions. At one time called Ruchbah, the name was lost to Delta Cas; another, "Navi," which is "Ivan" spelled backwards to honor Ivan "Gus" Grissom of the Apollo program, is also applied to Cih; Segin, the most common name, is also of more modern, but obscure, origin. The star is a sparkling blue-white class B (B3) giant, but is sometimes listed as a peculiar B3 dwarf. The measured temperature of 15,400 Kelvin is rather on the cool side for a B3 star, and more indicates a star of class B5. At least the distance of 440 light years is fairly secure, which gives a total luminosity of 2500 times that of the Sun and a radius of 7 times solar. The angular diameter of 0.43 thousandths of a second of arc gives 6.2 solar, which given the errors on measurement, is fine agreement. The star's properties give it a mass 6 times that of the Sun, an age of 65 million years, and place it at the edge of giving up hydrogen fusion, if it has not done so already. But then the star again confounds. For a B star, Segin is a slow rotator, only 19 kilometers per second (minimum) at the equator, 10 times solar, but far less than most of its class. Consistently, it is also chemically peculiar, falling into a rare class of "helium-weak" stars that displays anomalously weak spectral absorptions of helium. These are a high-temperature extension of the "mercury- manganese" stars like Alpheratz and Alpha Tel, which bear wild peculiarities in their chemical compositions as a result of gravitational settling of some elements, radiative lofting of others. Slow rotation is required to keep the star's gases from being too stirred up and mixed. The weak helium is the source of the too-hot classification. The contradiction comes in the identification of Segin as a "shell star" (like Pleione). These have surrounding disks of ejected gas that are related to HIGH rotation. Yet more, Segin is identified as part of the controversial Cassiopeia-Taurus association of O and B stars. Most such associations are reasonably compact, and confined to one constellation. The Cas-Tau association, however, is nearby, spreads over an enormous stretch of sky, and is probably falling apart.