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.