ALRESCHA (Alpha Piscium). While only third brightest in the
ancient and sprawling zodiacal constellation of Pisces, the Fishes, Alrescha has a central place, at
the southeastern point of two lines of stars that represent the
ribbon that connects the two celestial fish, and thus was chosen by
Bayer to be the Alpha star. At mid-fourth magnitude (3.94), it is
not third by much, and is exceeded only by brighter fourth
magnitude Eta and
Gamma. One of the few named stars in the
constellation, "Alrescha" (which appropriately comes from the
Arabic word for "the cord") may refer to the original fishy
Babylonian constellation. Alternatively, it may refer an Arabic
constellation and to a string of stars that runs through both
Pisces and Andromeda that have more
to do with a rope that raises pail from a well. Whatever the
name's origin, the star (a delight in a small telescope) is a close
double, and consists of pair of class A stars that take 720
years to orbit each other. Though both stars are white, subtle
contrast effects can make the stars seem colored, one observer
even reporting pale green and blue.
Now separated by 1.3 seconds of arc (a
bit of a challenge to separate), they will make their closest
approach about 2060. The brighter eastern component, Alrescha-A
(by itself magnitude 4.3 and class A0), is the hotter, its
temperature about 9500 Kelvin. The fifth magnitude companion,
Alrescha-B (5.23, class A3), is some 900 Kelvin degrees cooler.
Both are ordinary hydrogen-fusing dwarfs. From their distance of
139 light years, we find respective luminosities of 31 and 12 times
that of the Sun, implying masses of 2.3 and
1.8 solar (the orbit is not well-enough determined to provide
accurate masses). The two stars average about 120 astronomical
units (three times the distance between Pluto and the Sun) apart,
the distance varying from 50 to 190 AU over the 720-year period.
Each star has been reported as a spectroscopic double, rather like
those that make Mizar. Alrescha-B's
orbital period may be about a week, though nothing else is known of
the system. These results may be spurious, however, and almost
certainly are for Alrescha-A. Neither star rotates especially
quickly (about 70 kilometers per second at the equator), and as a
result each is chemically peculiar (again like Mizar). Alrescha-A
is an "Ap" (class A peculiar) star with a magnetic field a thousand
or so times that of Earth that is wobbling with a 1.5 day period as
a result of the star's rotation. Alrescha-A shows enhancements of
silicon, strontium, and chromium produced by separation of atoms in
the relatively quiet atmosphere coupled with the action of stellar
magnetism. Concentration of elements into magnetic regions coupled
with rotation then make the star's spectrum variable. Alrescha-B,
not to be outdone, is a metallic-line star, such stars typically
having enhancements of copper, zinc, strontium, zirconium, and
barium.