ZETA LYR (Zeta Lyrae). Lyra, the Harp or Lyre, is dominated by its luminary Vega, and so much by the "double-double" (Epsilon) and the strange eclipser Sheliak (Beta) that nobody pays much attention to another fascinating multiple, Zeta Lyrae, which may contain as many as 7 individual stars, the brightest of which is a curious one in its own right. For the backyard observer, Zeta breaks into a brighter fourth magnitude (4.36) class A hydrogen- fusing dwarf and a fainter sixth magnitude class F (F0) "subgiant" (so-called though really a dwarf) 44 seconds of arc apart that together make the naked-eye star shine at magnitude 4.08 and that lie at a distance of 152 light years. The brighter cannot easily be classified because it is a "metallic line star" with a very odd surface composition that distorts its spectrum. A slow rotation (for an A star, at least 47 kilometers per second, period under 3 days) allows some chemical elements to fall under the force of gravity, while others are lifted up by radiation, the action enhancing iron by a factor of 5 over the Sun's iron content, while elements like yttrium, zirconium, and barium are up by factors of 20 or more. From its 8150 Kelvin surface (which corresponds to class A5), it radiates energy at a rate 31 times that of the Sun, the mass about 2.2 times solar, the radius 2.8 solar. The fainter companion, at least 2000 Astronomical Units away from "Zeta Lyrae A," is a more ordinary, cooler (about 7500 Kelvin) star, but one that spins much faster, at least 212 kilometers per second, giving it a period less than 0.4 days, the difference rather odd given that the two stars are are paired. It has a luminosity 9 times that of the Sun, and a mass and radius both 1.7 solar. The separation and masses imply an orbital period of at least 47,000 years. From the F star, the A star would appear 8 times brighter than the full Moon, while from the A star, the F star would appear 3 times brighter. Now things get more complicated. The A star (also the "A" component) is a close spectroscopic binary with a period of only 4.3 days, implying (assuming a total system mass of 3 solar) a separation of but 0.07 AU, less than 20 percent Mercury's distance from the Sun. The F star might be double too, though its spectral variation might well be intrinsic. Now add three more faint stars: 26 seconds of arc away at magnitude 12.5, 46 seconds at magnitude 10.0, and 62 seconds away at magnitude 11.5. The components of this system are lettered outward by distance, so the F star becomes "Zeta Lyrae D." If they are all together, they are in an unstable configuration, and will eventually be ejected (only hierarchical multiples are stable, that is double-single, double-double like Epsilon Lyrae, double-double single like Mizar and Alcor), which argues against their all belonging together.
Written by Jim Kaler. Return to STARS.