ETA AQL (Eta Aquilae). Lying almost exactly one degree north of the celestial equator, this quite- wonderful mid-fourth magnitude (nominally 3.90) star in Aquila (the Eagle) glides across the sky just 8 degrees to the south of much brighter first magnitude Altair. Unfortunately given no proper name by the ancients, it is now known principally as Eta Aquilae, Eta the seventh letter of the Greek alphabet. Near one of the Eagle's talons, Eta also represents the head of the now-defunct no-longer-recognized constellation Antinous, who was honored in the sky by the Roman emperor Hadrian, and depicted as being carried by Aquila. The star seems relatively dim only because it is so far away, by parallax an uncertain 1170 light years. It remains quite visible to us only because it is a luminous yellow-white class F (nominally F6) supergiant that shines 3400 times more brightly than the Sun. While such brilliance may pale beside that of the blue hydrogen- fusing dwarfs or the great red supergiants, the star's moderate temperature of 5600 Kelvin assures that nearly all of its radiation pours out in the visual where we can see it rather than hidden in the invisible ultraviolet (as it is for hot stars) or the infrared (cool stars). The luminosity and temperature conspire to give a radius 61 times that of the Sun, while direct measures of angular diameter give a close 65 times. But these characteristics are only an aside compared with the star's status as one of the sky's most prominent Cepheid variables (the first known, the variability discovered long ago, in 1784), Eta Aquilae comparable to the prototype Delta Cephei (the name "Cepheid" taken from the constellation "Cepheus"), Mekbuda (Zeta Geminorum), and the southern hemisphere's Beta Doradus and W Sagittarii. Polaris is actually the brightest of all Cepheids, but its small variations are not sensible to the eye. The variations of Eta Aquilae, however (as they are for the others listed here), are obvious, the star changing its brightness from magnitude 3.5 to 4.3 and back again over precisely determined period of 7.176641... days (7d 4h 14m 22s). As Eta dims, it dips
The top graph shows the visual (V) light curve of Eta Aquilae over two cycles, where "phase" is the relative period, which begins with maximum light at 0 and ends with maximum light at 1. The middle graph gives the relative change in "color," the blue magnitude (B) minus the usual visual magnitude (V). Color correlates with temperature and spectral class, wherein the more positive values indicate cooler temperature. (These are not true B-V colors, but show only relative change.) The bottom graph shows the "radial velocity" of the star, that is, how fast the star appears to be moving along the line of sight as determined from the Doppler shift (positive values indicating recession). The variation is coming from the expansion and contraction of the stellar surface during the pulsation cycle. The star is brightest not when it is biggest or smallest, but when it expanding the fastest, and dimmest when it is contracting the fastest. From an article by D. Bersier in the Astrophysical Journal Supplement Series, vol. 140, p. 465, 2002.
to spectral class G, the temperature falling from a high of 6200 Kelvin to 5300. Like all Cepheids, Eta Aquilae is in the process of dying. Having given up core hydrogen fusion, it is probably fusing helium in its deep core. In its current structure, it has become unstable, a deep layer that valves radiation making it pulsate and change its surface temperature and radius. Cepheid variables are among the most important stars of the sky, as their variation periods are strictly related to their luminosities. Once the period of a Cepheid is found, we thus know the luminosity, which through comparison with apparent brightness gives the distance. The relation gives 1100 light years for Eta Aquilae, very close to the parallax value. Cepheids are so luminous they are easily seen in other galaxies, and are thus the chief means by which we can determine their distances. Eta's luminosity and general characteristics lead to a mass about seven times that of the Sun. As it evolves, it will someday cease its pulsations, lose most of its outer envelope, and die as a tiny white dwarf smaller than the Earth.
Written by Jim Kaler. Return to STARS.