Summer Constellation Spotlight: Aquila

Aquila is the Latin word for eagle. In Greek mythology, this constellation depicted the eagle that carried Zeus’ lightning bolts. Aquila is a Northern Hemisphere constellation located on the celestial equator; it soars within the stars of the Milky Way. Aquila is easy to spot during the mid-summer and early fall months from July through October thanks to its distinctive shape, which resembles a bird in flight.

Second-century Greek astronomer Ptolemy cataloged Aquila as one of the original 48 constellations . It’s the 22nd-largest constellation in the sky, surrounded by the constellations Delphinus, Sagitta, Vulpecula, Hercules, Ophiuchus, Scutum, Equuleus, Capricornus, Cygnus, and Lyra.

Although Aquila lies in the Milky Way, interestingly, it contains no Messier objects. However, a few often-overlooked celestial objects reside within the constellation, including colorful stars, clusters, and planetary nebulae. Let’s look at some of them.

Aquila’s brightest star, Altair, forms part of the famous Summer Triangle, an easily recognizable asterism of three luminous stars. The other vertices of the triangle include Vega in Lyra and Deneb, which lies within another bird constellation, Cygnus, the swan.

Altair is an Arabic phrase that means “the flying eagle.” Also known as α Aquilae (Alpha Aquilae), it is one of the closest stars to Earth and is the twelfth-brightest star in the night sky, with an apparent visual magnitude of +0.77. This variable whitish-blue star is just 16.8 light-years from Earth. It’s a fine binocular or telescope target, as it stands out compared to nearby stars. Altair is 1.86 times the Sun’s mass and about 10.6 times more luminous. It spins around its axis very quickly—about once every nine hours, which gives it an oblong shape! It has three faint visual companion stars, B, C, and D.

Tarazed, or γ Aquilae (Gamma Aquilae), is the second brightest star in Aquila. It is a relatively cool double giant orange star shining at an apparent visual magnitude of +2.72. It lies 460 light-years from Earth. Tarazed is over 2,500 times as bright as our Sun and is visible even from light-polluted skies. It is one of a few stars without an Arabic-derived name. Tarazed comes from a Persian phrase meaning “the beam of the scale” or “the balance.” Early astronomers initially gave this name to a group of three stars that reminded them of a scale in the night sky. Tarazed retained the name, while the other two stars became known as Altair and Alshain.

Alshain, or β Aql (Beta Aquilae), is a variable yellow triple star system with an apparent visual magnitude of +3.87 that lies 44.7 light-years from Earth. The primary star is a subgiant 30% more massive than our Sun, and its smaller companion is a red dwarf.

Eta Aquilae, or η Aquilae, is a dying variable yellow-white supergiant that shines 3,400 times brighter than the Sun. Discovered in 1784, it is the sky’s most prominent Cepheid variable—a star that brightens and dims periodically. Eta Aquilae changes its magnitude between magnitude +3.5 and +4.3 and back again every seven days.

NGC 6709 is an open star cluster located towards the galaxy’s center, 3,510 light-years from Earth. The stars in the cluster appear loosely arranged into a diamond-like shape. The cluster is easily resolved in a small telescope, with an apparent magnitude of +6.7. German-born British Astronomer William Herschel discovered NGC 6709 on July 19, 1828.

NGC 6751, commonly known as the Glowing Eye Nebula, is a planetary nebula that resembles a cosmic eye with clouds of gas ejected thousands of years ago from its centralized star. It lies approximately 6,500 light-years from Earth and has a visual magnitude of +11.9. The Hubble Heritage Project released an impressive, detailed image of this nebula to commemorate the Hubble Space Telescope’s tenth anniversary. To see the Glowing Eye for yourself, use a telescope with an aperture of at least 8” under dark skies.

German-born British astronomer William Herschel discovered the open cluster NGC 6755 in 1785. This cluster of stars has a visual magnitude of +7.5 and is approximately 8,060 light-years away. Another smaller open cluster, NGC 6756, lies close to NGC 6755. Its apparent magnitude is +10.6, and it lies 6,363 light-years away. Together, they form a visual double cluster, although they are not a true binary cluster system since they are not the same age and are far apart.

NGC 6781 is a planetary nebula resembling the famous Owl Nebula, Messier 97. It was discovered in 1788 by German-born British astronomer William Herschel. NGC 6781 has an apparent magnitude of +11.4 and lies approximately 3,500 light-years from Earth. The planetary nebula shows plenty of structure within its ring at high powers in a large aperture telescope. Its faint central star is a bluish-white dwarf with a magnitude of +16.8.