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.