This magnitude 3.90 star in Aquila lies 8 degrees to the south of much brighter Altair, and almost exactly one degree north of the celestial equator. Near one of the Eagle's talons, η Aql also represents the head of the now-defunct constellation Antinous, who was honored in the sky by the Roman emperor Hadrian, depicted as being carried by Aquila.
Properties
η Aql seems relatively dim because it is so far away - roughly 1170 light years, as computed from its parallax. It is visible to us only because it is a luminous yellow-white class F6 supergiant that shines 3400 times more brightly than the Sun. The star's moderate temperature of 5600 K means that nearly all of its radiation pours out in the visible part of the spectrum. Its luminosity and temperature imply a radius 61 times the Sun's, while direct measures of its angular diameter give 65 solar radii.
Variability
Eta Aquilae is one of the sky's most prominent Cepheid variables, comparable to the prototype Delta Cephei, both discovered in 1784. The variations of η Aql are obvious. It changes from magnitude 3.5 to 4.3 and back again over a period of 7.176641 days. As η Aql dims, it dips to spectral class G, and its temperature falls from 6200 K to 5300 K.
The periods of Cepheid variables are strictly related to their luminosities; once the period of a Cepheid is found, we thus know its luminosity, which through comparison with its apparent brightness gives the distance. The relation gives 1100 light years for Eta Aquilae - very close to the parallax value. Eta's luminosity and general characteristics imply a mass about seven times that the Sun's.
Like all Cepheids, Eta Aquilae is dying. Hydrogen fusion has ended, and it is probably fusing helium in its deep core. Its current structure has become unstable, making it pulsate and change surface temperature. As it evolves, it will someday cease its pulsations, lose most of its outer envelope, and become a tiny white dwarf.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]