Aquila the Eagle

Aquila the Eagle
Aquila the Eagle and some neighboring constellations against the cloudy background of the Milky Way [John Nanson / Stellarium]

An ancient eagle straddles the celestial equator – it's the constellation Aquila.

History and mythology
Origins
Aquila is a Roman name, but the Romans adopted the constellation from the Greeks. The Greeks, in turn, had borrowed it from the Babylonians at some time before the 4th century BC when it was mentioned in the writings of Eudoxus. Babylonia was a Middle Eastern civilization that flourished over three thousand years ago.

Myths
There were a number of classical myths surrounding the constellation, all associated with the Greek god Zeus or his Roman counterpart Jupiter. Sometimes the eagle carried the god's thunderbolts. He also carried the handsome Trojan boy Ganymede to Olympus where the boy served as cup bearer to the gods. The nearby constellation Aquarius (the water bearer) is often associated with Ganymede. Yet in other stories, Zeus himself in the form of an eagle brought Ganymede to Olympus.

Aquila and Antinous
In the 2nd century AD, Greek astronomer Ptolemy listed Aquila in his work The Almagest. Interestingly, he also included Antinous as part of Aquila, not listing it as a separate constellation. Antinous had been the lover of the emperor Hadrian (76-136 AD). When he died, Hadrian declared Antinous a god and made a constellation for him from stars near Aquila. It was sometimes included in star atlases, but didn't make the cut when the modern 88 constellations were set by the International Astronomical Union (IAU).

Stars and planets
Altair
Altair, named from the Arabic for flying eagle, is the brightest star in Aquila. Yet it may be best known for being one vertex of the prominent Summer Triangle asterism. (An asterism is a pattern of stars that isn't an official constellation.) Altair is younger and hotter than the Sun, but unlike the spherical Sun, it's notably egg shaped. The Sun takes 25 days to rotate once. Altair however takes just nine hours, and has a considerable bulge at the equator. If it rotated much faster, it would have blown apart.

Beta Aquilae and Gamma Aquilae
Seeming to flank Altair are the stars Beta and Gamma Aquilae, as shown in the header image. According to Ptolemy's description, Beta lies in the eagle's neck and Gamma in its left shoulder. Their traditional names are Alshain and Tarazed.

Tarazed is noticeably brighter than Alshain. In fact, it's about 3000 times more luminous than the Sun and over 100 times bigger. It's 460 light years away, but if it were as close to us as Sirius, it would be by far the brightest star in our night sky.

Exoplanets – planets circling stars beyond the Solar System
Astronomers are discovering exoplanets at a rapid rate. As of June 2020, the Universe Guide listed 18 confirmed planets orbiting 15 stars in Aquila. The planets are big ones, usually Jupiter-sized or more larger. The smallest ones are Neptune-sized, including the two circling HD 176986.

The international Astronomical Union (IAU) system for exoplanet names is usually the star name with a lower case letter. For example, the two planets of HD 176986 are HD 176986b and HD 176986c. Nonetheless, in recent years the IAU has occasionally had some of the stars and planets named by the public.

2019 saw the celebration of the IAU's 100th anniversary, and 112 countries each named a star and its planet. In Aquila, HD 192263 was named Phoenicia by Lebanon and HD 192263b got the name Beirut. (The Phoenicians were an ancient people of Lebanon, and Beirut is the modern capital.) Chechia was the name Tunisia chose for HD 192699. The chechia is the traditional hat of Tunisia. Chechia's planet is Khomsa. A khomsa is a special piece of jewelry sometimes pinned to clothing for ceremonies.

Deep-sky objects
Aquila is rich in nebulae and star clusters, and 18th century Anglo-German astronomer William Herschel discovered a number of them, including the planetary nebula NGC 6804. [image: Jacoby, NOAO] Herschel described such objects as planetary because what he saw in his telescope looked like the disc of a planet. In fact, these nebulae form when a dying sunlike star sloughs off its outer layers. Many of them are highly complex, as you can see in NGC 6751, also called the Glowing Eye Nebula. It's about six times the size of our Solar System and some 6500 light years away. [image: Hubble Legacy Archive, processing: Donald Waid]

There are also dark nebulae – these stellar nurseries are so opaque they can only be seen by their shapes against a bright background. William Herschel and others thought they were holes in the sky.

Aquila also has several open star clusters and the globular cluster NGC 6760. Star clusters are made up of stars that all formed from the same massive nebula. Globular clusters contain some of the oldest known stars. They are big, ranging from tens of thousands to over a million stars. The mutual gravitational pull of the stars holds the clusters in a spherical shape between 100-300 light years in diameter.

The open clusters are much smaller and shapeless. Not having enough stars to pull the cluster together, over time it breaks up. Our Sun probably formed in such a cluster.



You Should Also Read:
Summer Triangle
William Herschel
What Is a Nebula

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