Lambda Aquilae

Lambda Aquilae is a binary star, with two stars orbiting a common center of mass. This system has been resolved with interferometry at the Very Large Telescope, which led to the discovery of the secondary. The projected separation was of 0.18 astronomical units, resulting in an estimated orbital period of 15 days. Before this, there was evidence for a companion via spectroscopy and adaptive optics, but a sequence of detections and non-detections by different studies made the presence of any companions unclear.^[8]

The primary star is a main sequence star with a stellar classification of B9Vn,^[5] which means that, like the Sun, it is generating energy at its core through the nuclear fusion of hydrogen. It is more massive than the Sun, with about three times its mass,^[10] and has 2.3 times the Sun's radius.^[6] The star is radiating about 55 times the Sun's luminosity from its photosphere^[11] at a higher effective temperature of 11,780 K.^[15] This temperature gives Lambda Aquilae the blue-white hue that is a characteristic of B-type stars.^[16] Lambda Aquilae was one of the least variable stars observed by the Hipparcos satellite.^[7] It has been classified as a suspected Lambda Boötis star,^[17] although examination of the ultraviolet spectrum shows that to be unlikely.^[18] The star is rotating rapidly with a projected rotational velocity of 103 km s^−1.^[12]

The secondary is a red dwarf star, and as well as its primary, a main sequence star. It is smaller and cooler than the primary, with 60% of the Sun's radius and an effective temperature of 4,250 K. The estimated magnitude difference is 7.4 magnitudes. This star explain the X-ray emission coming from its sky position; such emission is unexpected for a B-type star, but is typical of a young low-mass star â€“ the estimated age of Lambda Aquilae is 90 million years.^[8]

When the primary start to evolve and expand into a red giant, around 300 million years in the future, mass transfer between both stars is expected to occur; the end product will be a common envelope system.^[8]

This star lies about 5° from the galactic plane and about 30° from the line of sight to the Galactic Center. This region of the sky is crowded with other objects along the line of sight, with at least 55 located within 10 arcseconds of the star.

NASA's Pioneer 11 space probe, launched in April 1973, exited the Solar System in 1990 and continued in the direction of Lambda Aquilae. It is estimated that it will take around 4 million years^[19] for the probe to make its closest approach to its destination, assuming it will remain intact. However, NASA stopped communicating with Pioneer 11 in March 1997 because the space probe's power was already too weak to transmit any data.^[20]

In Chinese, 天弁 (Tiān Biàn), meaning Market Officer, refers to an asterism consisting of λ Aquilae, α Scuti, δ Scuti, ε Scuti, β Scuti, η Scuti, 12 Aquilae, 15 Aquilae and 14 Aquilae.^[21] Consequently, the Chinese name for λ Aquilae itself is 天弁七 (Tiān Biàn qī, English: the Seventh Star of Market Officer).^[22]

This star, together with η Aql, θ Aql, δ Aql, ι Aql and κ Aql made up the obsolete constellation Antinous.^[23]