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.