The η Aquilae system contains at least two stars, probably three. 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 given 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.[4] Radial velocity measurements could not find a satisfactory fit, which suggests that the orbit of η Aql B may be face-on, or very large.[12]
A companion has been resolved visually 0.66" distant, but measurements give this a spectral type of F1 - F5. It seems likely that the hot star detected in the spectrum is closer and unresolved. The resolved companion has not been shown to be physically associated, but it is estimated that it would have a period of nearly a thousand years. Measurements with the HSTfine guidance sensors show variations likely to be due to orbital motion on a scale of two years, so η Aql would appear to be a triple system.[4][5]
η Aquilae A is a Cepheid variable star, discovered by Edward Pigott in 1784.[14][3] It has an apparent magnitude that ranges from 3.49 to 4.3 over a period of 7.177 days.[3] Along with Delta Cephei, Zeta Geminorum and Beta Doradus, it is one of the most prominent naked eye Cepheids;[15] 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,[9] has burned through the hydrogen fuel at its core and evolved into a supergiant, giving it a baseline stellar classification of F6 Iab.[4] The periodic pulsations of this star actually cause the stellar class to vary between F6.5Ib to G2Ib over the course of each cycle.[16]
Compared to the Sun, Eta Aquilae has around 6 times the mass,[4] 60 times the radius, and is radiating 3,400 times as much luminosity.[10] This energy is being emitted from the outer envelope at an effective temperature of 5,700 K,[10] giving it the yellow-whitish hued glow of a G-type star. The radius of the star varies by 4.59 × 106km (0.007 R☉) over the course of a pulsation cycle.[17] Compared to its neighbors, this star has a high peculiar velocityof16.7 ± 6.9 km s−1.[18]
This star, along with δ Aql and θ Aql, were Al Mizān (ألميزان), the Scale-beam.[21]According to the catalogue of stars in the Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars, Al Mizān was the title for three stars: δ AqlasAl Mizān I, η Aql as Al Mizān II and θ AqlasAl Mizān III.[22]
η Aquilae, together with θ Aql, δ Aql, ι Aql, κ Aql and λ Aql, was part of the obsolete constellation Antinous.[23]
^Wielen, R.; et al. (1999), "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions", Veröff. Astron. Rechen-Inst. Heidelb, 35 (35): 1, Bibcode:1999VeARI..35....1W.
^ abNicolet, B. (1978), "Photoelectric photometric Catalogue of homogeneous measurements in the UBV System", Astronomy and Astrophysics Supplement Series, 34: 1–49, Bibcode:1978A&AS...34....1N.
^ abcBaines, Ellyn K.; Armstrong, J. Thomas; Schmitt, Henrique R.; Zavala, R. T.; Benson, James A.; Hutter, Donald J.; Tycner, Christopher; Van Belle, Gerard T. (2018), "Fundamental Parameters of 87 Stars from the Navy Precision Optical Interferometer", The Astronomical Journal, 155 (1): 30, arXiv:1712.08109, Bibcode:2018AJ....155...30B, doi:10.3847/1538-3881/aa9d8b, S2CID119427037.
^ abcdeGallenne, A.; Mérand, A.; Kervella, P.; Pietrzyński, G.; Gieren, W.; Hocdé, V.; Breuval, L.; Nardetto, N.; Lagadec, E. (May 25, 2021). "Extended envelopes around Galactic Cepheids. V. Multi-wavelength and time-dependent analysis of IR excess". Astronomy & Astrophysics. 651: A113. arXiv:2105.12197. Bibcode:2021A&A...651A.113G. doi:10.1051/0004-6361/202140350. ISSN0004-6361.
^Pigott, Edward (1785). "Observations of a New Variable Star. In a Letter from Edward Pigott, Esq. to Sir H. C. Englefield, Bart. F. R. S. and A. S.". Philosophical Transactions of the Royal Society of London. 75: 127–136. Bibcode:1785RSPT...75..127P. doi:10.1098/rstl.1785.0007. S2CID186212958.
^Wiktorowicz, Sloane J.; Matthews, Keith (December 2008), "A High-Precision Optical Polarimeter to Measure Inclinations of High-Mass X-Ray Binaries", The Publications of the Astronomical Society of the Pacific, 120 (874): 1282–1297, arXiv:0810.5561, Bibcode:2008PASP..120.1282W, doi:10.1086/595966, S2CID14883175.
^Gray, David F.; Stevenson, Kevin B. (April 2007), "Spectroscopic Determination of Radius Changes of Cepheid Variable Stars", The Publications of the Astronomical Society of the Pacific, 119 (854): 398–406, Bibcode:2007PASP..119..398G, CiteSeerX10.1.1.495.3147, doi:10.1086/518128
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