Messier 17

M17, also known as the Omega Nebula, NGC 6618, Sharpless 45, RCW 160, Gum 81, Collinder 377, LBN 60, Swan Nebula, Lobster Nebula, Horseshoe Nebula, Checkmark Nebula, is a famous star-forming nebula located in the southern constellation Sagittarius. The diffuse emission nebula lies near the constellation’s northern border with Scutum, at a distance of 5,000 to 6,000 light-years from Earth at right ascension 18h 20m 26s and declination -16°10’36”. It has an apparent magnitude of 6.0. The Omega Nebula is one of the brightest star-forming nebulae of the Milky Way. With a total mass of roughly 800 solar masses, it is also one of the most massive H II regions of our galaxy. The nebula occupies an area roughly 15 light-years in diameter and is part of a larger cloud, which is about 40 light-years in diameter. Messier 17 is located in the Sagittarius Arm of the Milky Way, the next inward spiral arm to our own, and maybe part of the same giant cloud complex as its close neighbour, Messier 16 (the Eagle Nebula), located in the constellation Serpens.

This image of the rose-coloured star-forming region Messier 17 was captured by the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile. It is one of the sharpest images showing the entire nebula and not only reveals its full size but also retains fine detail throughout the cosmic landscape of gas clouds, dust and newborn stars.

The Omega Nebula lies just to the south of the Eagle Nebula. Two other famous nebulae – the Trifid Nebula (Messier 20) and the Lagoon Nebula (Messier 8) are located in the same part of the sky. Messier 17 can also be found by star-hopping from the bright Altair in the constellation Aquila down the eagle’s back toward Gamma Scuti in the constellation Scutum. M16 and M17 lie just to the southwest of the star. The best time of year to observe the Omega Nebula and its neighbours is in the summer months when Sagittarius hovers over the southern horizon in the evening.

The Omega Nebula was discovered by the Swiss astronomer Jean-Philippe Loys de Chéseaux in 1745. It was independently discovered by Charles Messier on June 3, 1764. John Herschel, William Herschel’s son, was the one who noted the nebula’s resemblance to the Greek capital letter omega (Ω). He was the first to accurately draw M17 in 1833 and published his drawing as part of a series of sketches of nebulae in 1836.

Astronomers using data from ESO’s Very Large Telescope (VLT), at the Paranal Observatory in Chile, have made an impressive composite of the nebula Messier 17, also known as the Omega Nebula or the Swan Nebula. The painting-like image shows vast clouds of gas and dust illuminated by the intense radiation from young stars. The image shows a central region about 15 light-years across, although the entire nebula is even larger, about 40 light-years in total. Messier 17 is in the constellation of Sagittarius (the Archer), about 6000 light-years from Earth. It is a popular target for amateur astronomers, who can obtain good quality images using small telescopes. These deep VLT observations were made at near-infrared wavelengths with the ISAAC instrument. The filters used were J (1.25 µm, shown in blue), H (1.6 µm, shown in green), and K (2.2 µm, shown in red). In the centre of the image is a cluster of massive young stars whose intense radiation makes the surrounding hydrogen gas glow. To the lower right of the cluster is a huge cloud of molecular gas. At visible wavelengths, dust grains in the cloud obscure our view, but by observing in infrared light, the glow of the hydrogen gas behind the cloud can be seen shining faintly through. Hidden in this region, which has a dark reddish appearance, the astronomers found the opaque silhouette of a disc of gas and dust. Although it is small in this image, the disc has a diameter of about 20 000 AU, dwarfing our Solar System (1 AU is the distance between the Earth and the Sun). It is thought that this disc is rotating and feeding material onto a central protostar — an early stage in the formation of a new star. Credit: ESO/R. Chini

Messier 17 contains an open cluster of 35 hot, young stars which are responsible for the nebula’s glow. The cluster is only 1 million years old, which makes it one of the youngest known clusters in our galaxy. It is home to up to 800 stars, including nine stars of spectral type O and about 100 of spectral type earlier than B9. Additionally, there are more than a thousand stars in the process of forming in the nebula’s outer regions. The young stars in M17 do not readily appear in visible light images but are hidden within the nebula’s dust and gas.  Even the light of the 35 brightest stars is obscured by the surrounding nebulosity. Messier 17 has a similar geometry to Messier 42, the famous Orion Nebula, but unlike M42, which appears face-on, M17 is seen edge-on. The Omega Nebula is considerably richer in star-forming material and young stars than M42, and it is also much more distant. The Orion Nebula lies some 1,300 light away from Earth and is located in the Orion Arm of the Milky Way, the same spiral arm that is home to our solar system.

NASA’s Spitzer Space Telescope has captured a new, infrared view of the choppy star-making cloud called M17, also known as the Omega Nebula or the Swan Nebula. The cloud, located about 6,000 light-years away in the constellation Sagittarius, is dominated by a central group of massive stars — the most massive stars in the region (see yellow circle). These central stars give off intense flows of expanding gas, which rush like rivers against dense piles of material, carving out the deep pocket at centre of the picture. Winds from the region’s other massive stars push back against these oncoming rivers, creating bow shocks like those that pile up in front of speeding boats. Three of these bow shocks are labelled in the magnified inset. They are composed of compressed gas in addition to dust that glows at infrared wavelengths Spitzer can see. The smiley-shaped bow shocks curve away from the stellar winds of the central massive stars. This picture was taken with Spitzer’s infrared array camera. It is a four-colour composite, in which light with a wavelength of 3.6 microns is blue; 4.5-micron light is green; 5.8-micron light is orange, and 8-micron light is red. Dust is red, hot gas is green and white is where gas and dust intermingle. Foreground and background stars appear scattered through the image. Credit: NASA/JPL-Caltech/M. Povich (Univ. of Wisconsin)

Two blue hypergiants are believed to be associated with M17: HD 168607, a luminous blue variable (LBV), has a visual magnitude of 8.25. The star forms a pair with another blue hypergiant and suspected luminous blue variable, HD 168625, that lies just southeast of the nebula. HD 168607 is 240,000 times more luminous than the Sun, while HD 168625 has 220,000 solar luminosities. The nebula’s O-type stars and blue hypergiants will likely end their life in Type II supernova explosions in the not-too-distant future, astronomically speaking. Credits: Messier Objects, NASA, Universe Today, Wikipedia.

The photograph, taken by NASA’s Hubble Space Telescope, captures a small region within M17, a hotbed of star formation. M17, also known as the Omega or Swan Nebula, is located about 5500 light-years away in the constellation Sagittarius. The wave-like patterns of gas have been sculpted and illuminated by a torrent of ultraviolet radiation from young, massive stars, which lie outside the picture to the upper left. The glow of these patterns accentuates the three-dimensional structure of the gases. The ultraviolet radiation is carving and heating the surfaces of cold hydrogen gas clouds. The warmed surfaces glow orange and red in this photograph. The intense heat and pressure cause some material to stream away from those surfaces, creating the glowing veil of even hotter greenish gas that masks background structures. The pressure on the tips of the waves may trigger new star formation within them. The image, roughly 3 light-years across, was taken May 29-30, 1999, with the Wide Field Planetary Camera 2. The colours in the image represent various gases. Red represents sulfur; green, hydrogen; and blue, oxygen. Image: NASA, ESA and J. Hester (ASU)