Aquila the Eagle, by Kim Touzel


Northwest of Capricornus Aquila the Eagle flies northwest across the Milky Way as depicted on my map. Depending on which myth you read it may be flying southeast.

In Greek mythology the Eagle was the king of birds who retrieved the thunderbolts  hurled by the great sky-god Zeus. One myth is depicted pictorially with Aquila flying southeast with the boy Ganymede clutched in his claws. Alpha is the eagle’s chest and Beta the boys head, Zeta the eagle’s tail.  Myth has the boy being abducted and seduced by Zeus.

Alpha α Aquilae,  Altair  (Arabic for Eagle) the brightest star, mag. 0.77 is a white star, the 12th brightest in the sky, fairly close to us at 16 ly. Altair forms one of the three points of the Northern Hemisphere Summer Triangle. Vega in Lyra and Deneb in Cygnus form the other 2 points of the triangle and are still visible near the northern horizon in the early evening. Of interest Altair has a very rapid rotation of approximately 100km/sec completing one turn in 6.5 hours…compared to our sun with a rotation period of 25.4 days.

Beta β, Alshain 3.7 mag. yellow star and Gamma γ, Tarazed 2.7 mag. yellow star form a close group of stars with Altair known as the “Family of Aquila”.  They form a distinct line of approximately 5 ° of arc. Aquila is easily located in the north-western sky by finding this line of three naked eye stars south of the bright Vega in Lyra.

Eta η a yellow-white supergiant 1200 ly away is one of the brightest Cepheid  variables. It fluctuates between magnitude 3.5 and 4.4 in 7.1 days. Using nearby Iota magnitude 4.4 and Theta magnitude 3.2 as comparison stars the variation in brightness can be appreciated. You don’t need a telescope for this exercise as all stars are naked eye. You may want to also use your binoculars.

What is a Cepheid Variable?  One of a class of variable stars that pulsate in a regular manner with changes in luminosity. They periodically expand and contract changing in size as much as 30 %. The surface temperature typically changes from 6000 K to 7500 K.  Average luminosity is 10,000 times that of the sun.  They have an important use in cosmology.  In 1912 Henrietta Leavitt discovered they could be used to determine the distance of stars in our and other Galaxies by the period-luminosity law – a simple relationship between the period of light variation and the absolute magnitude of the Cepheid.

Although in the Milky Way Aquila is not rich in objects for small telescopes.  There are no Messier objects.  One of a few open clusters NGC 6709 a loose cluster of about 40 stars of magnitude 9 -11 is not too impressive in small scopes.  A large number of Supernovae occurred in Aquila, approximately 100 have been recorded.  The constellation is rich in planetary nebulae. I located two of these with the 20 cm Schmidt-Cassegrain (C8) and Argo Navis.

NGC 6781 is a large moderately bright grey planetary nebula about 2 arc minutes across.  In the C8 at 78 x magnification it was large, quite faint and appeared as a grey smudge.  OIII filter improves the image bringing out the annular character.  This should be just visible in a dark sky with a 10 cm scope.

NGC 6790 is a very tiny 0.10 arc minute pale blue planetary nebula. With the C8 at   78 x magnification it appeared as a tiny dull star, blinking with averted vision.  It should just be visible with a 7.5 cm scope, however will be difficult to distinguish from surrounding stars.

Sources: Philips Astronomy Dictonary , 2nd Ed 1999; Hartung’s, D. Malin & D. Frew 1995; The Southern Sky Guide, D Ellyard & W. Tirion, 2001; Collins Pocket Guide – Stars & Planets 1993; The Starlore Handbook, Geoffrey Cornelius,1997;  Burnham’s Celestial Handbook 1966.