Star of the Month
Eta Aquilae
AKA: 55 Aquilae
Position: 19 hrs 52 min 28.4 sec +01 degrees 00 min 20 sec
Due south at 23:16 (BST) on 15 August 2022
Eta Aquilae
Image: Simbad (http://simbad.u-strasbg.fr/simbad/)
Eta Aquilae is a yellow F6 supergiant which has already used up its hydrogen although it is a mere 26 million years old. It lies 890 light years away. Hence it is similar to the F supergiants which comprise the optical double 15 Aquilae (see the Double Star for this month). It pulsates periodically, but that is not why it is interesting. Eta Aquilae has a companion which is also a F supergiant of a similar age, which has a separation of only 0.7 arcseconds. Crucially it also has an invisible companion which is an even hotter B type star. Thanks to this unseen star, Eta Aquilae is a classic Cepheid variable similar to the prototype Delta Cephei (see the Double Star of the Month for October 2019). Its magnitude varies between 3.5 and 4.3 in just 7.18 days, so its variability can be observed with the naked eye or binoculars over the space of a couple weeks. It is also easy to find, it is the first fairly bright star below Altair and lies between Theta Aquilae (mag. 3.2) and Delta Aquilae (mag. 3.4). A useful comparison star is 71 Aquilae which lies 40 arcminutes to the south-east as it has a magnitude of 4.3. At its brightest, Eta Aquilae is comparable to Delta Aquilae. The variability of Eta Aquilae was discovered by Edward Pigott in 1784 just before his collaborator and relative John Goodricke discovered the variability of Delta Cephei. So you will be observing the very first Cepheid discovered.