40 Per (40 Persei). Many are the treasures of Perseus, the Hero who slew the Sea Monster in the rescue of Andromeda. Among them are collections of massive stars, many of which are fainter than those that have garnered proper or Greek letter names. One of them is fifth magnitude (4.97) 40 Persei, a hot class B (B0.5) hydrogen-fusing dwarf known best by its Flamsteed number. The star's other popular name, "o" Persei (lower case Roman "oh", the Roman letters used by Bayer after he ran out of Greek ones) probably should not be used, as the letter is prone to confusion with Greek "omicron." It's a special issue with 40 Per, as Omicron Persei (Atik), a brighter and somewhat similar star, lies just 1.7 degrees to the south-southeast of 40 Per. As a result, 40 Persei sometimes gets the Greek letter name, and is mistakenly called "Omicron Per," which it isn't.

It is, however, one magnificent star that is dimmed by almost a full magnitude by intervening interstellar dust. Were the pathway clear of the obscuring tiny dust grains (made largely of silicates and carbon), 40 Per would shine at magnitude 4.14 and be more a part of its parent constellation. The obscuration carries a side benefit, however, as (especially given the simplicity of a class B spectrum) the starlight provides a fine background against which to study the composition of and motions within interstellar gas, which always goes along with the dust (indeed, the mass of interstellar gas being some 100 times that found in the dust). With a high surface temperature of 28,700 Kelvin, 40 Per's radiation lies mostly in the ultraviolet. If it were all stuffed into the visual spectral domain, and if we could get rid of the intervening dust, we'd see 40 Per as first magnitude! Using the star's distance of 1055 light years (give or take 72), we find a great luminosity of 23,600 times that of the Sun, from which is calculated a radius of 6.2 solar. Measurements of projected equatorial rotation speeds are all over the place. Adopting 30 kilometers per second yields a rotation period under 10 days, relatively long for the class. The theory of stellar structure and evolution then tells of a mass 14 times that of the Sun. Confirmed as a dwarf, 40 Per is about halfway through its hydrogen-fusing lifetime. The star is clearly above the limit at which stars explode. After it completes its helium core, 40 Persei will go on to become a grand red supergiant that will "burn" its core helium through other products such as neon, magnesium, and sulfur to iron. The iron cannot fuse to anything, leading to total core collapse and a magnificent supernova. Forty Per is listed with two "companions." Its tenth magnitude optical "mate," 26 seconds of arc away, is moving much too fast and is clearly just a line of sight coincidence. A spectroscopically-detected neighbor does not seem to have been confirmed. The star is probably a loner. But not quite, as 40 Per is a member of the large, expanding, gravitationally unbound Perseus OB2 association of hot stars that includes a far more famed star, third magnitude Zeta Persei, and possibly (its membership disputed) nearby Omicron Persei as well, thus completing the circle.
Written by Jim Kaler 2/03/12. Return to STARS.