DELTA AND (Delta Andromedae). Not exactly a retiring star, at
third magnitude (3.27) Delta And is, as expected (and rarely
achieved), the fourth brightest star in the constellation of Andromeda. At first, it looks like an
ordinary class K (K3) giant, but one with a difference as a rather
odd double (really triple)
star. The main star, the one seen to the east of Alpheratz (Alpha And), shines with the
light of 73 Suns with a coolish (and very
accurately known) surface temperature of 4350 Kelvin. That and
temperature lead to a radius of 15.1 times that of the Sun, which
is in good accord with the radius of 14.0 times solar derived from
the measured angular diameter. And now we run into trouble. Two
measures of projected equatorial rotation speed give 6.5 and 1.0
kilometers per second, not very good agreement. The faster one
gives a rotation period of under 116 days, still pretty large.
Masses of such stars are hard to gauge, as different masses have
very similar properties. This one seems to be around 1.5 solar
with an age of 3 billion years. At that rate it was born as a
class F0 or hotter star, and is now (as best we can tell) either
brightening as a giant with a dead helium core, or fading a bit
with a core that has just fired up to fuse helium into carbon and
oxygen. Interestingly, it seems to have a debris disk still around
it, as is commonly found around the kinds of stars that include
Delta's hydrogen-fusing predecessor, which in turn might suggest a
planetary system. The telescope reveals two
visual companions. The motion of the outer 16th magnitude
component (Delta And C, 48 seconds of arc out) shows that it just
lies along the line of sight and has nothing to do with Delta And
proper. The inner one, however, 12th magnitude (12.4) Delta
Andromedae B, seems to be a real binary member about half a minute
of arc away, which translates to a real distance of at least 900
Astronomical Units. Nineteenth century astronomers viewed their
colors as "orange and dusky." From Delta-B's brightness, it must
be a class M3 or so dwarf that takes at least 20,000 years to orbit
our bright Delta And A. But then we look at Delta A itself with
the spectrograph, and find that it has an even closer companion
that takes some 21000 days, 58 years, to orbit, remarkably long for
a spectroscopic binary (given the
difficulty of such measure, enough so as to make one suspicious of
its reality). If it IS real, it would lie 19 AU from its bright K-
giant neighbor. Reality is affirmed, however, by a measure of
orbital eccentricity that would take the separation between 9 and
28 AU. Nothing else is known about it. Anyone on a planet
orbiting Delta-B would see the inner pair separated by a full
degree, with Delta And itself shining with the light of 15 full
Moons.
Written by Jim Kaler 12/12/08. Return to STARS.