GAMMA CRB (Gamma Coronae Borealis). To the northeast of Arcturus in Bootes lies one of the most graceful of celestial figures, the semi-circle that makes Corona Borealis, the Northern Crown, in mythology the Crown of Ariadne. Set into the center of the curve, like a genuine jewel, is the luminary, third magnitude Alphecca. Immediately to the east of it lies fourth magnitude (3.81) Gamma Coronae Borealis (of no proper name). It is not one star, but two, a double with components in tight orbit about each other. The brighter (Gamma CrB A) is a fourth magnitude class B (on the cool side, B9) subgiant (suggesting the cessation of core hydrogen fusion, but see below); the fainter (Gamma B) is a sixth magnitude (5.60) class A3 hydrogen-fusing dwarf. The two are always less than a second of arc apart and quite difficult to separate through the telescope. The discovery of duplicity in 1826 by F. G. W. Struve is thus quire remarkable. At a distance of 145 light years, they orbit every 92.94 years at an average separation of 32.7 Astronomical Units, roughly the distance between Neptune and the Sun, a relatively high eccentricity of 0.48 taking the physical separation between 49 and 17 AU. The next close approach will take place in 2024.
Gamma Coronae
Borealis Gamma CrB B goes around Gamma A (the brighter and more massive of the two, placed at the cross) with an orbital period of 92.94 years at an average separation of 32.7 Astronomical Units. North is down, and the scale around the edges is in seconds of arc. Note the closeness of the stars, which are always less than a second of arc apart, making separation difficult and the orbit somwhat uncertain. The dot-dash line is the orbit's major axis. The orbital plane is nearly in the line of sight, resulting in a severe distortion of the elliptical orbit such that Gamma A does not appear anywhere near the focus (where it actually is). When the stars are physically closest together, they actually appear to us to be relatively far apart. In truth, the stars go about each other, orbiting around a common center of mass, which is not determined for this pair. (From W. I. Hartkopf, H. A. McAlister, and O. G. Franz, Astronomical Journal, vol. 98, p. 1014, 1989 in the US Naval Observatory Washington Double Star Catalogue, B. D. Mason, G. L. Wycoff, and W. I. Hartkopf.)
The orbital plane is tilted almost into the line of sight, just 5 degrees off, so that the stars are not that far from eclipsing one another. Analysis of the orbit via Kepler's Laws yield a combined mass of 4.1 times that of the Sun. The individual masses are not available from the orbital data, so we use the theory of stellar structure. From the spectral classes, the respective temperatures for Gamma A and B are 11,000 and 8800 Kelvin. Distance and apparent brightness (with small corrections for ultraviolet radiation) give respective luminosities of 48 and 9 times solar, radii of 1.9 and 1.3 solar, and masses 2.6 and 1.85 times that of the Sun. The combined "evolutionary mass" is then 4.45 time that of the Sun, not far off that derived from the binary orbit, showing that all the parameters are in pretty good shape. The hotter one is sometimes classed as an A0 or A1 star, but that lowers the mass by only a tenth of a Sun or so. Theory shows that the stars are both quite young, and that Gamma CrB A is not a subgiant at all but a dwarf, although a greater one than its mate. Another point of contention is that Gamma A is listed as a Delta Scuti-type variable with a brightness range of about six percent over a period of 0.7 hours, but it seems too hot for that category. Eventually, in a few hundred million years, Gamma A's internal fire will indeed go out, and Gamma B will watch it turn into a distended giant, but one far enough away that Gamma B should remain unaffected, the pair eventually dying as a double white dwarf.
Written by Jim Kaler 7/25/08. Return to STARS.