Brian Jones: The Eagle
As we saw in last month’s blog, the bright star Altair in Aquila is the southernmost member of the Summer Triangle. To spot Altair, and thereby locate the constellation Aquila, look into the southern sky during evenings in August, once the sun has set and the stars become visible. Provided the sky is reasonably clear, you should see the prominent constellation Aquila, its brightest star Altair easily visible around half way between the south eastern horizon and the overhead point.
Altair is flanked by two slightly fainter stars, Tarazed and Alshain, which together form a distinctive trio. The rest of Aquila can be seen extending to the lower right of these three stars, the group taking the form of a large cross which can be easily likened to a bird in flight. Down to the south-west of this trio of stars is the blue-white star Delta, which marks the central point of the constellation.
Aquila represents the eagle which, according to a rather gruesome Greek legend, preyed upon the vitals of the hapless Prometheus! The constellation was important to the Romans and was depicted on many Roman coins. The Roman poet Caesius, who lived during the reign of Nero, referred to the group as the Eagle of Military Rome or the Eagle of St John.
Altair is the 12th brightest star in the sky and shines from a distance of 16 light years. Its colour is white and measurements show that its actual brightness is nine times that of our Sun. Alshain, seen just to the south of Altair, has a slightly yellowish tint that is unlikely to be spotted in the presence of any horizon glow. Tarazed, on the opposite side of Altair, is orange-yellow in colour and lies at a distance of over 300 light years.
If the night is dark and clear you may see the Milky Way running from the northern horizon right across the sky, through the constellation Cygnus (read more in my post on Cygnus) and down towards the south, passing through Aquila on its way. As a result, the area of sky around Aquila is rich in star fields and will repay sweeping with binoculars on really dark, clear nights.
The tail of the eagle is represented by the two stars Zeta and Epsilon, the yellow-orange tint of Epsilon contrasting with the blue-white Zeta. Many of these colour comparisons should be seen in binoculars given clear and transparent skies.
Zeta and Epsilon act as guide stars for the faint but interesting open star cluster NGC 6709. This cluster lies well over 3,000 light years away and can be found a little to the southwest of the stars Zeta and Epsilon. It contains around 40 stars and binoculars will show it as a very faint misty patch of light which may be a challenge to pick out against the background of the Milky Way. The accompanying finder chart shows other stars in the same area, all visible in binoculars, and which should help you track down the cluster.
The late-summer evenings are still fairly warm, so why not seek out the celestial eagle and try to spot its faint open star cluster? Happy stargazing!