Messier 16

M16, the famous Eagle Nebula, is a star-forming diffuse emission nebula or H II region, with a young open star cluster located in Serpens. M16 lies near the borders with the constellations Sagittarius and Scutum at right ascension 18h 18m 48s, declination -13°49′. The nebula is best known for the Pillars of Creation region, three large pillars of gas famously photographed by Hubble in 1995. Also known as the Star Queen Nebula, M16 lies at a distance of 7,000 light-years from Earth and has an apparent magnitude of 6.0. The cluster’s designation in the New General Catalogue is NGC 6611, while the nebula is also referred to as IC 4703, The Spire, Sharpless 49, RCW 165, Gum 83, Collinder 375, C 1816-138, and CTB 51. The Eagle Nebula occupies an area 70 by 55 light-years in size, while the open cluster has a radius of 15 light-years. The name Eagle comes from the nebula’s shape, which is said to resemble an eagle with outstretched wings. American astronomer Robert Burnham, Jr. introduced the name Star Queen Nebula because the nebula’s central pillar reminded him of a silhouette of the Star Queen. IC 4703, the H II region that surrounds the open cluster, is a diffuse emission nebula, a vast region of active star formation with a visual magnitude of 8, located in the Sagittarius Arm of the Milky Way Galaxy, the next inner spiral arm from our own.

Three-colour composite mosaic image of the Eagle Nebula (Messier 16, or NGC 6611), based on images obtained with the Wide-Field Imager camera on the MPG/ESO 2.2-metre telescope at the La Silla Observatory. At the centre, the so-called “Pillars of Creation” can be seen. This wide-field image shows not only the central pillars, but also several others in the same star-forming region, as well as a huge number of stars in front of, in, or behind the Eagle Nebula. The cluster of bright stars to the upper right is NGC 6611, home to the massive and hot stars that illuminate the pillars. The “Spire” — another large pillar — is in the middle left of the image. Image: ESO

Messier 16 is home to several regions of active star formation. These include the famous Pillars of Creation in the central part of the nebula and the Stellar Spire, located just to the left of the pillar structure. The largest of the three Pillars of Creation is approximately 4 light-years high.  The Stellar Spire, a large tower of gas that appears to be coming off the region of nebulosity, is about 9.5 light-years high, corresponding to a length of 90 trillion kilometres.  This is roughly twice the distance from the Sun to the nearest star system, Alpha Centauri. The Stellar Spire is being eroded by the ultraviolet light of the young, massive, extremely hot stars seen at the top of the image.  The stars are also responsible for illuminating the surface of the spire. The cluster associated with the nebula has approximately 8100 stars, which are mostly concentrated in a gap in the molecular cloud to the north-west of the Pillars. The brightest star, HD 168076, has an apparent magnitude of +8.24. It is actually a binary star formed of an O3.5V star plus an O7.5V companion. This star has a mass of roughly 80 solar masses and a luminosity up to 1 million times that of the Sun. The cluster’s age has been estimated to be 1–2 million years.

Stellar Spire in the Eagle Nebula, Image: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)

Messier 16 was discovered by the Swiss astronomer Jean-Philippe Loys de Chéseaux in 1745-46. The nebula itself was discovered by Charles Messier on June 3, 1764. William Herschel observed the cluster on July 30, 1783. In 1895, the American astronomer Edward Emerson Barnard was the first to produce a photograph of the nebula, and it was the image taken by amateur astronomer and astrophotography pioneer Isaac Roberts in 1897 that brought the Eagle Nebula into the IC catalogue of 1908.

This image composite highlights the pillars of the Eagle nebula, as seen in infrared light by NASA’s Spitzer Space Telescope (bottom) and visible light by NASA’s Hubble Space Telescope (top insets). The top right inset focuses on the three famous pillars, dubbed the “Pillars of Creation,” which were photographed by Hubble in 1995. Hubble’s optical view shows the dusty towers in exquisite detail, while Spitzer’s infrared eyes penetrate through the thick dust, revealing ghostly transparent structures. The same effect can be seen for the pillar outlined in the top left box. In both cases, Spitzer’s view exposes newborn stars that were hidden inside the cocoon-like pillars, invisible to Hubble. These stars were first uncovered by the European Space Agency’s Infrared Satellite Observatory. In the Spitzer image, two embedded stars are visible at the tip and the base of the left pillar, while one star can be seen at the tip of the tallest pillar on the right. Image: NASA/JPL-Caltech/N. Flagey (IAS/SSC) & A. Noriega-Crespo (SSC/Caltech)

The Eagle Nebula is easy to find in the sky. It is located about 2.5 degrees west of the naked eye star Gamma Scuti in the constellation Scutum and a few degrees north of Messier 17, the Omega Nebula, in Sagittarius. Gamma Scuti, a white giant with a visual magnitude of 4.70, lies along the imaginary line drawn from Altair, the brightest star in Aquila, down the eagle’s back to Lambda Aquilae, in the direction of Sagittarius. M16 is located at the southern tip of Serpens Cauda, the Serpent’s Tail, the eastern part of the constellation. The constellation Serpens is divided into two separate sections – Serpens Caput or the Serpent’s Head and Serpens Cauda or the Serpent’s Tail – lying on each side of Ophiuchus, the Serpent Bearer. Messier 16 can also be found using the Teapot in Sagittarius. It is located along the line drawn from Kaus Australis to Kaus Media. The best time of year to see Messier 16 from northern latitudes is summer when Serpens and Sagittarius can be seen above the horizon in the southern sky after sunset.

Using the infrared multi-mode ISAAC instrument on the 8.2-m VLT ANTU telescope, European astronomers were able to image the Eagle Nebula at near-infrared wavelength. The ISAAC near-infrared images cover a 9 x 9 arcmin region, in three broad-band colours and with sufficient sensitivity to detect young stars of all masses and – most importantly – with an image sharpness as good as 0.35 arcsec. The wide-field view of M16 shows that there is much happening in the region. The first impression one gets is of an enormous number of stars. Those which are blue in the infrared image are either members of the young NGC 6611 cluster – whose massive stars are concentrated in the upper right (north-west) part of the field – or foreground stars which happen to lie along the line of sight towards M16. Most of the stars are fainter and more yellow. They are ordinary stars behind M16, along the line of sight through the galactic bulge, and are seen through the molecular clouds out of which NGC 6611 formed. Some very red stars are also seen: these are either very young and embedded in gas and dust clouds, or just brighter stars in the background shining through them. This photo is the result of a three-colour composite mosaic image of the Eagle Nebula (Messier 16), based on 144 individual images obtained with the infrared multi-mode instrument ISAAC on the ESO Very Large Telescope (VLT) at the Paranal Observatory. At the centre, the so-called “Pillars of Creation” can be seen. This wide-field infrared image shows not only the central three pillars but also several others in the same star-forming region, as well as a huge number of stars in front of, in, or behind the Eagle Nebula. The cluster of bright blue stars to the upper right is NGC 6611, home to the massive and hot stars that illuminate the pillars. Image: ESO

There are several other Messier objects in the same field of view, just above the Teapot: Messier 17 (the Omega Nebula), the open cluster Messier 18, Messier 20 (the Trifid Nebula), the open star clusters Messier 21, Messier 23 and Messier 25, Messier 8 (the Lagoon Nebula), and the globular clusters Messier 22 and Messier 28. The brightest star in the Eagle Nebula catalogued as HD 168076. It is really a binary star system composed of an O3.5V star and an O7.5 companion. The system has an apparent magnitude of 8.24. The open cluster NGC 6611 contains about 460 stars. The brightest members are only 1 to 2 million years old and belong to the spectral class O. These young stars are about 80 times as massive as the Sun and up to 1 million times more luminous.  NGC 6611 was classified by Shapley as a type ‘c’ cluster, which means that it is very loose and irregular. Credit: Messier Objects, NASA, Universe Today, Wikipedia.

Credits: NASA, ESA and the Hubble Heritage Team (STScI/AURA)