Eta Aquilae

System

The η Aquilae system contains three stars. The primary star η Aql A is by far the brightest and dominates the spectrum. An ultraviolet excess in the spectral energy distribution suggest the presence of a faint hot companion, η Aql B, which has been fitted to a spectral type of B8.9 V. The fractional spectral type is an artefact of the mathematics used to model the spectrum, not an indication of any specific spectral features that would be intermediate between B8 and B9. A 2022 study using radial velocity measurements could not find a satisfactory fit for η Aql Ab's orbit, which suggests that it may be face-on, or very large. A 2024 study including archival observations did show a change in radial velocity, consistent with an orbital period of 50 years, although it is not known which of the stellar companions are responsible for the variations.

An outer stellar companion has been visually resolved at an angular separation of 0.7 arc seconds, corresponding to a projected separation of 191 astronomical units. The companion's absolute magnitude, assuming it to be a main sequence star, is consisent with a A5V-F3V spectral type and a mass of 1.4–1.9 M_☉. Measurements with the HST fine guidance sensors show variations likely to be due to orbital motion on a scale of two years, so the companion appears to be bound, making η Aql a triple system. The star was later shown to have shifted 4.1° over a timescale of 7.5 years, suggesting an orbital period of at least 659 years.

At Eta Aquilae's distance (272 pc), its apparent brightness is diminished by 0.74 magnitudes due to extinction caused by interstellar dust between Earth and the star.

Cepheid variable

η Aquilae A is a Cepheid variable star, discovered by Edward Pigott in 1784. It has an apparent magnitude that ranges from 3.49 to 4.3 over a period of 7.177 days. Along with Delta Cephei, Zeta Geminorum and Beta Doradus, it is one of the most prominent naked eye Cepheids; that is, both the star itself and the variation in its brightness can be distinguished with the naked eye. Some other Cepheids such as Polaris are bright but have only a very small variation in brightness.

This massive star, being 100–200 million years old, has burned through the hydrogen fuel at its core and evolved into a supergiant, giving it a baseline stellar classification of F6 Iab. The periodic pulsations of this star actually cause the stellar class to vary between F6.5Ib to G2Ib over the course of each cycle.

Compared to the Sun, Eta Aquilae has around six times the mass, 52 times the radius, and is radiating 2,850 times as much luminosity. This energy is being emitted from the outer envelope at an effective temperature which varies between 5,410 and 6,310 K, giving it the yellow-whitish hued glow of a G-type star. The radius of the star varies by 4.59×10⁶ km (0.007 R_☉) over the course of a pulsation cycle. Compared to stars in its neighborhood, this star has a high peculiar velocity of 16.7±6.9 km s⁻¹.