SIGMA AQL (Sigma Aquilae). Not one star, but two, not that you could tell by a quick glance through the telescope, as the components of this fifth magnitude (5.17) binary star in central Aquila (the Eagle) are so close as to be visually inseparable. We know that Sigma Aql is double because two stars appear in the spectrum (each showing their Doppler shifts) and because every 1.95026 days they eclipse each other, the main eclipse yielding a dip of about 0.2 magnitudes. If you like blue stars, this one is for you, as the double consists of nearly identical hot class B (B3) hydrogen-fusing dwarfs with estimated temperatures of 18,500 Kelvin (though one source gives them as B3 and B4). From the ratio of the depth of the two eclipses (each getting partially in front of the other every orbital revolution), one is about ten percent brighter than the other. In the Milky Way, at 680 light years, the stellar double is far enough to suffer 0.77 magnitudes of dimming by interstellar dust. Were it absent, the star would shine at fourth magnitude (4.40). That correction, the distance, and allowance for ultraviolet light gives a combined luminosity of 2940 times that of the Sun, and individual values of 1540 and 1400 Suns for Sigma Aql A and B, which in turn lead to radii of 3.8 and 3.7 times that of the Sun and (from the theory of stellar structure and evolution) individual masses of 6 solar. Kepler's third law then gives us an average separation of 0.07 Astronomical Units, or just 15 solar radii, not that much larger than the sizes of the stars themselves. Analysis of the eclipses and orbital velocities show a circular orbit that is tilted to the plane of the sky by 72 degrees (to the line of sight by 18 degrees) and again a separation of 0.07 AU. As a result of the tilt, the eclipses are not total, but partial. Equatorial rotation speeds of 79 and 101 kilometers per second (corrected for the tilt, assuming that the rotation and orbital axes are aligned) give rotation periods of 2.3 and 1.9 days, close enough (given the inevitable errors of measurement) to imply that the rotations are synchronized with the orbital period, the stars always presenting the same "face" to each other as a result of mutual tides. The tides and the rapid rotation make the stars so oblate that, as they orbit, they present different apparent disk sizes to us, enough so that their combined brightness varies continuously even outside of eclipse and makes the derivation of masses (a consistent 6.9 and 5.5 solar) much more difficult. As close as they are, however, they are not sufficiently close that they transfer mass (that is, they do not fill their zero-gravity tidal surfaces). That nice state will end with evolution. The stars are about halfway through their 30-million-year hydrogen-fusing lifetime. When the more massive of the two gives up core hydrogen fusion and begins to expand as a red giant, it will encroach on its companion. If the dominant member were single, it would eventually grow to a radius of 200 times solar, far larger than the orbital size. As a result, one star will pass mass to the other, and may even absorb the other in a merger in which much mass will be lost to the system as well, the final result not really very predictable. Nearly a minute of arc out from the system is a 12th magnitude "companion." It won't be watching the action, however, because the apparent companionship is merely a line-of- sight coincidence.
Written by Jim Kaler 11/02/07. Return to STARS.