Eta Aquilae

η Aquilae
Location of η Aql (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Aquila
Right ascension 19h 52m 28.369s[1]
Declination +01° 00′ 20.37″[1]
Apparent magnitude (V) 3.49–4.3[2]
Characteristics
Spectral type F6 Iab[3] (or F6Ib–G4Ib[2]) + B9.8 V[3] + A5V-F3V[4]
U−B color index +0.51[5]
B−V color index +0.89[5]
Variable type δ Cep[6]
Astrometry
Radial velocity (Rv)10.82±0.55[1] km/s
Proper motion (μ) RA: +8.890 mas/yr[1]
Dec.: −8.322 mas/yr[1]
Parallax (π)3.6715±0.1942 mas[1]
Distance885.8+41.7
−45.0 ly
(271.6+12.8
−13.8 pc)[7]
Absolute magnitude (MV)−3.70[3]
Details
Aa
Mass6.15+0.21
−0.41[1] M☉
Radius52.3+2.6
−2.3[8] R☉
Luminosity2,850±300[8] L☉
Surface gravity (log g)1.834[9] cgs
Temperature5,747±19[10] K
Metallicity [Fe/H]+0.08±0.08[11] dex
Age150±50[12] Myr
Ab
Mass2.3[3] M☉
B
Mass1.4–1.9[4] M☉
Other designations
Eta Aql, η Aql, η Aquilae, 55 Aquilae, AAVSO 1947+00, BD+00°4337, FK5 746, GC 27517, HD 187929, HIP 97804, HR 7570, SAO 125159, PPM 168843, 2MASS J19522835+0100203[13]
Database references
SIMBADdata

Eta Aquilae is a multiple star system in the equatorial constellation of Aquila, the eagle. Its name is a Bayer designation that is Latinized from η Aquilae, and abbreviated Eta Aql or η Aql. This star was once part of the former constellation Antinous. Its apparent visual magnitude varies between 3.49 and 4.3,[2] making it one of the brighter members of Aquila. Based upon parallax measurements made by the Gaia spacecraft on its third data release (DR3), this star is located at a distance of approximately 272 parsecs (890 light-years).[1] The primary component is a Classical Cepheid variable.[6]

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.[3] 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.[14] 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.[15]

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.[16] 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☉.[4] 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.[3][17] 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.[4]

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.[1]

Cepheid variable

A visual band light curve for η Aquilae, adapted from Kiss (1998)[18]

η Aquilae A is a Cepheid variable star, discovered by Edward Pigott in 1784.[19][2] It has an apparent magnitude that ranges from 3.49 to 4.3 over a period of 7.177 days.[2] Along with Delta Cephei, Zeta Geminorum and Beta Doradus, it is one of the most prominent naked eye Cepheids;[20] 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,[12] has burned through the hydrogen fuel at its core and evolved into a supergiant, giving it a baseline stellar classification of F6 Iab.[3] The periodic pulsations of this star actually cause the stellar class to vary between F6.5Ib to G2Ib over the course of each cycle.[21]

Compared to the Sun, Eta Aquilae has around six times the mass,[1] 52 times the radius, and is radiating 2,850 times as much luminosity.[8] 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 4.59×106 km (0.007 R☉) over the course of a pulsation cycle.[22] Compared to stars in its neighborhood, this star has a high peculiar velocity of 16.7±6.9 km s−1.[23]

Name

In Chinese, 天桴 (Tiān Fú), meaning Celestial Drumstick, refers to an asterism consisting of η Aquilae, θ Aquilae, 62 Aquilae and 58 Aquilae.[24] Consequently, the Chinese name for η Aquilae itself is 天桴四 (Tiān Fú sì, English: the Fourth Star of Celestial Drumstick.)[25]

This star, along with δ Aql and θ Aql, were Al Mizān (ألميزان), the Scale-beam.[26] 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 Aql as Al Mizān I, η Aql as Al Mizān II and θ Aql as Al Mizān III.[27]

η Aquilae, together with θ Aql, δ Aql, ι Aql, κ Aql and λ Aql, was part of the obsolete constellation Antinous.[28]

References

  1. ^ a b c d e f g h i Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
  2. ^ a b c d e "VSX : Detail for eta Aql", AAVSO, retrieved 2024-05-25.
  3. ^ a b c d e f g Remage Evans, Nancy; et al. (2013), "Binary Cepheids: Separations and Mass Ratios in 5 M ⊙ Binaries", The Astronomical Journal, 146 (4): 93, arXiv:1307.7123, Bibcode:2013AJ....146...93E, doi:10.1088/0004-6256/146/4/93, S2CID 34133110.
  4. ^ a b c d Gallenne, A.; Kervella, P.; Evans, N. R.; Milli, J.; Sivkova, E.; Gieren, W.; Pietrzyński, G.; Bras, G.; Hocdé, V. (2026-04-07), "High-contrast imaging of Galactic Cepheids with VLT/SPHERE", Astronomy and Astrophysics, arXiv:2604.05287v1, doi:10.1051/0004-6361/202658849.
  5. ^ a b Nicolet, 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.
  6. ^ a b GCVS Query=eta Aql, General Catalogue of Variable Stars @ Sternberg Astronomical Institute, Moscow, Russia, retrieved 2010-11-24.
  7. ^ Bailer-Jones, C. A. L.; et al. (March 2021), "Estimating distances from parallaxes. V: Geometric and photogeometric distances to 1.47 billion stars in Gaia Early Data Release 3", The Astronomical Journal, 161 (3): 147, arXiv:2012.05220, Bibcode:2021AJ....161..147B, doi:10.3847/1538-3881/abd806, ISSN 0004-6256. Data about this star can be seen here.
  8. ^ a b c Baines, Ellyn K.; Clark, James H.; Kingsley, Bradley I.; Schmitt, Henrique R.; Stone, Jordan M. (2025-05-07), "Vintage NPOI: New and Updated Angular Diameters for 145 Stars", The Astronomical Journal, 169 (6): 293, arXiv:2506.02912, Bibcode:2025AJ....169..293B, doi:10.3847/1538-3881/adc930, ISSN 1538-3881.
  9. ^ Groenewegen, M. A. T. (March 2020), "Analysing the spectral energy distributions of Galactic classical Cepheids", Astronomy and Astrophysics, 635: A33, arXiv:2002.02186, Bibcode:2020A&A...635A..33G, doi:10.1051/0004-6361/201937060, ISSN 0004-6361. Eta Aquilae's database entry at VizieR.
  10. ^ a b Gallenne, A.; et al. (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, ISSN 0004-6361.
  11. ^ Trahin, B.; Breuval, L.; Kervella, P.; Mérand, A.; Nardetto, N.; Gallenne, A.; Hocdé, V.; Gieren, W. (2021-12-01). "Inspecting the Cepheid parallax of pulsation using Gaia EDR3 parallaxes - Projection factor and period-luminosity and period-radius relations". Astronomy & Astrophysics. 656: A102. arXiv:2111.09125. Bibcode:2021A&A...656A.102T. doi:10.1051/0004-6361/202141680. ISSN 0004-6361..
  12. ^ a b Baines, Ellyn K.; et al. (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, S2CID 119427037.
  13. ^ "eta Aql", SIMBAD, Centre de données astronomiques de Strasbourg, retrieved 2025-04-17.
  14. ^ Benedict, G. Fritz; et al. (2022), "The η Aquilae System: Radial Velocities and Astrometry in Search of η Aql B", The Astronomical Journal, 163 (6): 282, arXiv:2204.09759, Bibcode:2022AJ....163..282B, doi:10.3847/1538-3881/ac68ed, S2CID 248299714.
  15. ^ Shetye, Shreeya S.; Viviani, Giordano; Anderson, Richard I.; Mowlavi, Nami; Eyer, Laurent; Evans, Nancy R.; Szabados, László (2024-10-01), "VELOcities of CEpheids (VELOCE) - II. Systematic search for spectroscopic binary cepheids", Astronomy & Astrophysics, 690: A284, arXiv:2405.19840, Bibcode:2024A&A...690A.284S, doi:10.1051/0004-6361/202450185, ISSN 0004-6361.
  16. ^ Evans, Nancy Remage; Günther, H. Moritz; Bond, Howard E.; Schaefer, Gail H.; Mason, Brian D.; Karovska, Margarita; Tingle, Evan; Wolk, Scott; Engle, Scott; Guinan, Edward; Pillitteri, Ignazio; Proffitt, Charles; Kervella, Pierre; Gallenne, Alexandre; Anderson, Richard I. (2020-12-01). "Hubble Space Telescope Snapshot Survey for Resolved Companions of Galactic Cepheids: Final Results". The Astrophysical Journal. 905 (1): 81. arXiv:2010.07996. Bibcode:2020ApJ...905...81E. doi:10.3847/1538-4357/abc1f1. ISSN 0004-637X.
  17. ^ Gallenne, A.; et al. (2014), "Searching for visual companions of close Cepheids", Astronomy & Astrophysics, 567: A60, arXiv:1406.0493, Bibcode:2014A&A...567A..60G, doi:10.1051/0004-6361/201423872, S2CID 55702630.
  18. ^ Kiss, Laszlo L. (July 1998), "A photometric and spectroscopic study of the brightest northern Cepheids - I. Observations", Monthly Notices of the Royal Astronomical Society, 297 (3): 825, Bibcode:1998MNRAS.297..825K, doi:10.1046/j.1365-8711.1998.01559.x, S2CID 121699816.
  19. ^ 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, S2CID 186212958.
  20. ^ Basu, Baidyanath (2003), An Introduction to Astrophysics, PHI Learning Pvt. Ltd., p. 171, ISBN 978-81-203-1121-3.
  21. ^ 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, S2CID 14883175.
  22. ^ 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, CiteSeerX 10.1.1.495.3147, doi:10.1086/518128
  23. ^ Tetzlaff, N.; et al. (January 2011), "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun", Monthly Notices of the Royal Astronomical Society, 410 (1): 190–200, arXiv:1007.4883, Bibcode:2011MNRAS.410..190T, doi:10.1111/j.1365-2966.2010.17434.x, S2CID 118629873.
  24. ^ Ridpath, Ian, "Aquila", Star Tales, retrieved 2025-04-17.
  25. ^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 3 日 Archived 2011-05-21 at the Wayback Machine
  26. ^ Allen, R. H. (1963), Star Names: Their Lore and Meaning (Reprint ed.), New York, NY: Dover Publications Inc., p. 61, ISBN 978-0-486-21079-7, retrieved 2010-12-12.{{citation}}: CS1 maint: ignored ISBN errors (link)
  27. ^ Rhoads, Jack W. (November 15, 1971), Technical Memorandum 33-507-A Reduced Star Catalog Containing 537 Named Stars, California Institute of Technology: Jet Propulsion Laboratory, retrieved 2025-04-17.
  28. ^ Ridpath, Ian, "Antinous", Ian Ridpath's Star Tales, retrieved 2025-04-17.
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