12 BOO (12 Bootis). There was a time, not long ago, when astronomers told of three kinds of double stars. "Visual binaries" were observed as distinctly separate stars through the telescope. "Spectroscopic binaries" were too close together to be split in two, but their closeness gave them sufficiently high orbital speeds that they could be observed through Doppler shifts in their combined spectra (though if one star dominates, only its spectrum will be observed to shift). "Eclipsing binaries," essentially a subset of the spectroscopic, have orbits that lie in the line of sight such that the stars eclipse each other. Modern technology has not just blurred the distinction between visual and spectroscopic doubles, it has destroyed the distinction, the revolution epitomized by 12 Bootis (as well as by Mizar and a few others). 12 Boo is a modest fifth magnitude (4.83) class F (F9) subgiant in Bootes, the Herdsman, not far from brilliant Arcturus, and is not one star, but two, the brighter the visually dominant F9 subgiant (6130 Kelvin), the fainter an F8 subgiant (6230 Kelvin). The two stars tell powerful stories of modern observational precision and of stellar evolution, "subgiant" implying the end of normal hydrogen-fusing life. Though they cannot be separated by eye through the telescope, their mutual orbits can be examined in great detail through "interferometry," in which we make use of the ability of light waves to interfere with one another in order to resolve very fine detail.
The binary orbit is constructed with data from the Palomar Test Bed Interferometer, the Navy Prototype Optical Interferometer, and spectroscopic observations, and is presented as seen with 12 Boo B (in blue) going about 12 Boo A (red); in reality the two stars go about a common center of mass that lies between them. "RA" is "right ascension (east-west position), "Dec" is "declination," north-south position. You are looking at the orbit as seen from the "underside," with 12 Boo B in the back, moving to the right, and at "periastron," the two stars actually closest together although it does not seem that way as the orbit is viewed as projected against the plane of the sky. The "line of nodes" indicates the points at which 12 Boo B seems to cross the sky's apparent plane. "a" is the size of the orbit in thousandths of a second of arc (the scale of the axes), "e" is the eccentricity of the true (unprojected) ellipse, and "i" is the orbital tilt. (Thanks to A. F. Boden, G. Torres, and C. A. Hummel as published in the Astrophysical Journal.)
Combination of this technique with spectroscopic data tells us just about everything we want to know about the stars with great accuracy, even the distance (118 light years, in close agreement with the slightly less-accurate distance found from direct parallax). The components of 12 Boo orbit each other with a period of 9d 14h 30m 33s at an average separation of 0.1245 Astronomical Units (just a third Mercury's distance from the Sun). A modest eccentricity swings the stars as far as 0.1485 AU to as close as 0.1006 AU. The brighter F9 star (12 Boo A) shines with a luminosity 7.76 times that of the Sun, its radius 2.474 times solar. 12 Boo B is notably fainter, 4.69 solar, the radius 1.86 solar. The stars have near-solar chemical compositions and appear to rotate in synchrony with their orbital revolution. The large luminosity difference is caused by a slight mass difference (the brighter at 1.416 solar masses, the fainter at 1.374 solar, possibly a record in precision) combined with a sudden change caused by the aging process. This one-of-a-kind binary has been caught at a rare moment in time. At an age of 3.2 billion years, the stars, both subgiants, are at the ends of their main hydrogen-fusing stage. As such they are subject to very rapid changes. Theory shows that just before the cessation of fusion a star suddenly brightens. 12 Boo B is still fusing its remnant core hydrogen, while 12 Boo A appears to be done and is preparing to become a true red giant with a dead helium core. 12 Boo B is evolving just behind it and will soon follow its mate. The pair, which provide a marvelous test of theory, shows that we do indeed understand the basic internal processes that take place in stars, and in turn reveal that our outlook on the fate of the Sun itself is accurate, the Sun following 12 Boo into gianthood in about 5 billion years. (From work by A. F. Boden, G. Torres, and C. A. Hummel as published in the Astrophysical Journal.)
Written by Jim Kaler. Return to STARS.