M16, also known as the Eagle Nebula, is a young open cluster of stars embedded within an extremely large cloud of interstellar gas and dust in the constellation of Serpens (Cauda). It's located in the next inner spiral arm of the Milky Way, 7,000 light-years distant. The emission part of the nebula or HII region is catalogued as IC 4703 and is an active star-forming region, which has already created a significant cluster of young stars. The cluster itself lies at the heart of the Eagle Nebula and is known as NGC 6611. M16 was discovered by Philippe Loys de Chéseaux in 1745-6, but Charles Messier was the first to record the associated nebulosity on June 3, 1764.
The constellation of Serpens is faint but unique as its split into two separate sections. One half, named Serpens Caput, lies to the west of Ophiuchus and the other half, Serpens Cauda, lies on the eastern side of Ophiuchus. At the very southern tip of Serpens Cauda close to the Scutum and Sagittarius border is M16. It can be found 2.5 degrees west of γ Sct (mag. +4.7) and a few degrees north of the Omega Nebula (M17), M18 and the Sagittarius Star Cloud (M24). This beautifully rich area of the sky is a delight to scan with binoculars.
The Eagle Nebula was immortalised in 1995 when imaged several times by the Hubble Space Telescope. The resulting iconic photograph, titled the Pillars of Creation, showed three magnificent columns of interstellar gas and dust displayed in sensational detail.
M16 has an apparent magnitude of +6.2, which places it at the very edge of naked eye visibility, but easily within the range of binoculars or small telescopes. A pair of 7x50 or 10x50 models show a faint triangular shaped patch of light along with the brightest cluster stars. Through a 100mm (4-inch) scope roughly 20 stars are revealed, but spotting the emission nebula is much more challenging. Under dark skies the nebula hints at visibility, but due to its low surface brightness, medium to large amateur scopes are better suited to the task. Through 10-inch (250-mm) scopes at low powers, the nebula appears wispy with subtle details including dark obscuring matter to the north along with many more stars visible. To spot the famous Pillars of Creation an instrument of at least 300mm (12-inch) aperture is recommended.
In total, the nebula part of M16 covers 65 x 50 arc minutes of apparent sky with the open cluster spanning 7 arc minutes. This corresponds to spatial diameters of 130 x 100 light-years and 15 light-years respectively. The open cluster is about 5.5 million years old and contains at least 450 stars of which the brightest member shines at mag. +8.2.
M16 is best seen during the months of June, July and August.
M16 Data Table
|Object Type||Emission nebula with open cluster|
|RA (J2000)||18h 18m 48s|
|DEC (J2000)||-13d 48m 26s|
|Apparent Size (arc mins)||7.0 x 7.0 (cluster), 65 x 50 (nebula)|
|Radius (light-years)||7.5 (cluster), 65 x 50 (nebula)|
|Other Names||Collinder 375, Sharpless 49|
|Notable Feature||Subject of the famous Hubble Telescope Pillars of Creation photograph|