Object Name: Messier 20
Alternative Designations: M20, NGC 6514, Trifid Nebula
Object Type: Emission Nebula and Reflection Nebula with Open Star Cluster
Right Ascension: 18 : 02.6 (h:m)
Declination: -23 : 02 (deg:m)
Distance: 5.2 (kly)
Visual Brightness: 9.0 (mag)
Apparent Dimension: 28.0 (arc min)
Locating Messier 20: Once you have become familiar with the Sagittarius region, finding Messier 20 is an easy 2 degrees northwest of Messier 8, the “Lagoon” Nebula. However, at magnitude 9, it isn’t an easy object for small binoculars and it isn’t always easy for a small telescope. Because we often see it depicted in pictures as bright and beautiful, we simply assume M20 will jump out of the sky, but you’re going to find that its a lot fainter and more elusive than you might think. If you are just beginning to astronomy, try starting at the teapot’s tip star (Lambda) Al Nasl and starhopping in the finderscope northwest to the Lagoon. While the nebulosity might not show in your finder, optical double 7 Sagittari, will. From there you will spot a bright cluster of stars two degrees due north. These are the stars embedded withing the Trifid and the small, compressed area of stars to its northeast is open star cluster M21. Center your finderscope on the north and south oriented pair of stars and observe. Remember that you will need a moonless night and that sky conditions will need to be right to see the dark dustlanes!
What You Are Looking At: Almost everyone who is familiar with space images has seen this beautiful and color emission and reflection nebula. When looking at M20 through a telescope, what you will see will match the photo above, while what is normally presented in science journals is colorful. Why? Well, when it comes to photographs, exposure times and wavelengths causes the different colors you see. Photographically, the red emission nebula contained within Messier 20 has a bright blue star cluster in it central portion. It glows red because the ultraviolet light of the stars ionizes the hydrogen gas, which then recombines and emits the characteristic red hydrogen-alpha light captured on film. Further away, the radiation from these hot, young stars becomes too weak to ionize the hydrogen. Now the gas and dust glows blue by reflection! No matter how it is observed, the Trifid – or “three lobed” nebula has a distinctive set of dark dust lanes which divide it. These also have a classification of their own and were cataloged by E.E. Barnard as dark nebula Barnard 85 (B 85).
In 1999 the Hubble Space Telescope took a look deep into the Trifid nebula at some of its star forming regions and found a stellar jet poking its way into the cloud, like a fabulous twisted antenna. Inside the exhaust column is a new star waiting to be born, yet sometime over the next 10,000 years the central massive star will probably erode away all of its material before it can fully form. Nearby a stalk stands waiting… Like the jet, it is also a stellar nursery – one with an EGG (evaporating gaseous globule) at its tip – a condensed cloud of gas able to survive so far. “If our interpretation is correct, the microjet may be the last gasp from a star that was cut off from its supply lines 100,000 years ago.” says Jeff Hester of the Department of Physics & Astronomy, “The vast majority of stars like our sun form not in isolation, but in the neighborhood of massive, powerful stars. HST observations of the Trifid Nebula provide a window on the nature of star formation in the vicinity of massive stars, as well as a spectacular snapshot of the “ecology” from which stars like our sun emerge.”
We know that Messier 20 contains new stars, but what of old? Are there surprises buried within these bright folds that still await discovery? The answer is yes. “We report the discovery of a new candidate barrel-shaped supernova remnant (SNR) lying adjacent to
M20 and two shell-type features to the north and east of SNR W28.” says F. Yusef-Zadeh (et al), ” Future observations should clarify whether the nonthermal shell fragment is either part of W20 or yet another previously unidentified shell-type SNR.”
History: Charles Messier discovered this object on June 5, 1764 and from his notes he states: “In the same night I have determined the position of two clusters of stars which are close to each other, a bit above the Ecliptic, between the bow of Sagittarius and the right foot of Ophiuchus: the known star closest to these two clusters is the 11th of the constellation Sagittarius, of seventh magnitude, after the catalog of Flamsteed: the stars of these clusters are, from the eighth to the ninth magnitude, environed with nebulosities. I have determined their positions. The right ascension of the first cluster, 267d 4′ 5″, its declination 22d 59′ 10″ south. The right ascension of the second, 267d 31′ 35″; its declination, 22d 31′ 25″ south.”
While Messier did separate the two star clusters, he did not note so many different portions to the nebula – but, he did note nebulosity. In this circumstance, we cannot fault him, for his job was to locate comets and the purpose of his catalog was to merely not objects that were not. In later years, it would be Sir William Herschel who would take a much closer look at Messier 20 and discover much more: “If it was supposed that double nebulae at some distance from each other would frequently be seen, it will now on the contrary be admitted that an expectation of finding a great number of attracting centers in a nebulosity of no great extent is not so probable; and accordingly observation has shewn that greater combinations of nebular than those of the foregoing article are less frequently to be seen. The following list however contains 20 treble, 5 quadruple, and 1 sextuple nebulae of this sort. Among the treble nebulae there is one, namely H V.10 [M20], of which the nebulosity is not yet separated. Three nebulae seem to join faintly together, forming a kind of triangle; the middle of which is less nebulous, or perhaps free of nebulosity; in the middle of the triangle is a double star of the 2nd or 3rd class; more faint nebulosities are following.”
While William went on to catalog four separate areas in his books, it was his son John to whom we owe the famous name that we know it by today: “A most remarkable object. Very large; trifid, three nebulae with a vacuity in the midst, in which is centrally situated the double star Sh 379, the nebula is 7′ in extent. A most remarkable object.” Just remember when you observe that sky conditions are everything and that not even a large telescope can make it appear if the sky isn’t right. Even Admiral Symth has his share of troubles spotting it! “I lowered the telescope a couple of degrees, and gazed for the curious trifid nebula, 41 H. IV [H IV.41]; but though I could make out the delicate triple star in the centre of its opening, the nebulous matter resisted the light of my telescope, so that its presence was only indicated by a peculiar glow. Pretty closely preceding this is No. 20 M., an elegant cruciform group of stars, discovered in 1764, which he considered to be surrounded with nebulosity.”
Good luck and enjoy your observations!
B&W image thanks to Palomar Observatory, courtesy of Caltech, first color image with Hubble inset region courtesy of Mt. Palomar 1.5-m telescope, M20 starbirth region by NASA and the Hubble Heritage Team (Jeff Hester (Arizona State University) and NASA) and color image thanks to Todd Boroson/NOAO/AURA/NSF.