1 PER (1 Persei). Perseus, The hero who killed Cetus, the Sea Monster, and thereby won Andromeda's heart, is a prominent, spread-out cluster that sprawls between our heroine and Auriga. One might think that the number 1 star (1 Persei) would be Perseus's luminary, visually outshining the vast number of stars that lie within the northern Milky Way. One would be wrong. Barely sixth magnitude (5.52), 1 Per is visible without optical aid to only the best eyes and under the best of conditions. Yet among its neighbors, it's a rare and luminous class B (B1.5) dwarf that rather stands out among the local field stars, compliments of its bluish color, the result of a temperature that falls somewhere between 17,000 and 22,100 Kelvin, and most significantly, that it's a weak eclipsing binary star in which two nearly identical members orbit with a period of 25.935...days, an eclipse depth of 0.20 magnitudes, and a high (but controversial) eccentricity of 0.39. (Eclipsers are always a joy to find as so much information can be gotten from them.) One of the members apparently fills its "equipotential surface," its surrounding teardrop-shaped tidal surface where the "effective gravity" is zero and mass can flow from one star to the other. Instead of a physical property, the "number 1" refers to position as the westernmost numbered star in the eighteenth-century Flamsteed Catalogue. Assuming simple identical components, and adopting a dimming of 0.29 magnitudes by interstellar dust, and a large distance of 1290 light years (give or take 170), we find individual luminosities of 2285 Suns for each star and effective radii somewhat more than solar. Theory suggests masses of about 8.2 Suns. More sophisticated analyses give 8.7 and 8.3 suns, above which stars are expected to end their lines as supernovae. The orbital period and Kepler's laws suggest a separation of about a solar diameter, as expected. The star, though spinning with an equatorial rotation speed of at least 198 kilometers per second, shows no sign of a circmumstellar disk that would make it into an emission-line star like Gamma Cassiopeiae. The greatest significance of 1 Persei seems more to lie in the course of its evolution. Since higher-mass stars evolve and expand first, the more massive will encroach upon the other and mass transfer will begin in earnest. The star seems to be in the first stage that will take it into the realm of eclipsers that look like Algol to the west, so perhaps "number 1" may mean something after all other than helping to define the border between Perseus and Andromeda.
Written by Jim Kaler, 06/15/18. Return to STARS.