Sigma Aquilae
Sigma Aquilae is an eclipsing binary star system in the equatorial constellation of Aquila. Its name is a Bayer designation that is Latinized from σ Aquilae, and abbreviated Sigma Aql or σ Aql. The baseline apparent visual magnitude of the pair is +5.17, which, according to the Bortle Dark-Sky Scale, is bright enough to be seen with the naked eye from suburban skies. Because of the Earth's orbit about the Sun, this system has an annual parallax shift of. This provides a distance estimate of approximately. The system is drifting closer to the Sun with a radial velocity of −5 km/s.
File:SigAqlLightCurve.png|thumb|left|A light curve for Sigma Aquilae, plotted from Hipparcos data
Sigma Aquilae is a double-lined spectroscopic binary system consisting of two intermediate-mass B-type [main sequence star]s; each has a stellar classification of B3 V. They are detached components, which means the two stars are sufficiently distant from each other that neither fills its Roche lobe. Their close, circular orbit has a period of 1.95 days with a semimajor axis of 14.3 times the radius of the Sun.
Because the orbital plane lies close to the line of sight with the Earth, they form an eclipsing binary system. The two components are each distorted by the gravity of the other star, and their shapes mean that the magnitude of the star system varies constantly even outside of the eclipses, an arrangement known as a Beta Lyrae variable. The brightness of the pair decreases during each eclipse, which occurs with a frequency determined by their orbital period of 1.95028 days. During the eclipse of the primary component the magnitude decreases by 0.20 to a net of 5.37; the eclipse of the secondary component results in a magnitude decrease of 0.10 to a net of 5.27.
The primary component has 5.8 times the mass of the Sun and 3.7 times the Sun's radius. It is radiating 1,862 times the luminosity of the Sun from its photosphere at an effective temperature of 18,493 K. The smaller secondary has 4.6 times the mass, 3.3 times the radius, and 524 times the luminosity of the Sun. Its outer atmosphere has an effective temperature of 15,848 K. This star has a high rate of spin, showing a projected rotational velocity of 120 km/s.