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The Trifid Nebula M20 (at the upper right in the image) is a rare combination of an emission and reflection nebula. Hot young stars in the southern part of the Trifid have excited the hydrogen atoms in the surrounding gas, causing them to emit red light. In the northern part of the Trifid, in contrast, the stars are not hot enough to excite the gas; the blue glow is the result of starlight being reflected by a myriad of dust particles. Some dust lanes which are not illuminated by starlight run in front of the red parts, forming the famous Trifid lines.
The Lagoon Nebula M8, on the other hand, is a straightforward emission nebula. It got its name from the shape of the dark lane crossing through the its center. In M8 a lot of small dark globules, the so-called "Bok globules", which may eventually condense into stars, can be found sprinkled throughout the nebula on photographs. A cluster of young stars which is superimposed on M8 originates from the nebula. The brightest part of M8 is named the hourglass region.
M21, which is a loose star cluster of a few stars, is situated close to the Trifid Nebula and can be found near the top of this image.
Milkyway in Sagittarius, telelens photograph.
M8 - Lagoon Nebula, HαRGB image, which this image is part of.
M20 - Trifid Nebula, an enlargement of this photograph.
M8 and M20 are both objects which culminate only about 20 degrees above the horizon in Central Europe, they are therefore often affected by light pollution or haze near the horizon. Usually bigger telescopes than would be necessary if these objects were set higher in the sky have to be utilized to extract the same amount of detail. You should time your observations to get these objects when they are located at maximum altitude for optimal viewing. The ecliptic takes its path just between M8 and M20 and reaches about there its minimum height above the horizon for northern latitudes during the winter solstice, so the nebulae take exactly the same low arc in the southern sky during summer nights as does the winter sun in December.
M8, the Lagoon Nebula, is located a few degrees to then north of the spout of the "teapot" which forms the constellation Sagittarius, and to the west of the main milky way stream. In a transparent night it is recognizable as big fuzzy spot and you can easily target it in your finderscope. If conditions are not so favorable and only the brightest stars of Sagittarius are visible to the naked eye, you can point the finderscope at Lambda Sagittarii in the handle of the "teapot" and move it 5 degrees west and slightly north. If you can't find it that way just go hunting for it west of the main milky way stream in your finderscope. It should be visible in the finderscope as nebulous patch of light elongated in E-W direction with about 3 stars superimposed in horizontal direction. It should be easily recognizable in the finderscope; if not, then conditions are indeed severe and observations there do not make much sense.
M8 is a fine object even in a small 2.5" refractor if you are fortunate to live far enough to the south to enjoy it higher up in the sky. For mid-northern latitudes, a 4" refractor will do fine, a 6" does show more and anyway M8 is an object which will show more and more detail the bigger your telescope is. In an 18" telescope the Lagoon Nebula can be really striking. Like with all nebulae, UHC or OIII filters do improve contrast, especially if the nebula is located low in the sky. Using one of these filters will render a ghostly green color to the nebula.
With a diameter of over 1/2 degree, a wide-field eyepiece is needed to observe the full extent of this object. The brightest parts of M8 are separated by a dark line running from the northeast to the southwest. This central dusty "channel" or "lagoon" is the source of the nebula's name. In the eastern part of the nebula the loose galactic star cluster NGC 6530 is located, which is about 10' in diameter. The west half of M8 is dominated by two bright stars, just 3' apart. The brighter star of these two is 9 Sagittarii which is already visible to the naked eye. The region immediately surrounding these brightest parts features many nebula extensions, especially to the east, northeast and north. They are observable with direct vision only in bigger telescopes, in small ones you will need averted vision as these nebula patches are extended and of low surface brightness. These parts are as well speckled with dark nebulae which render a cloudy appearance to the bright nebula segments. Within the brightest (western) part of M8, just 3' WSW of 9 Sagittarii there is situated "The Hourglass", a figure "8" shaped knot about 30" in size. This detail will take about 200x power in small telescopes and 100x in bigger ones to see well. It is in most cases more easily observable without filter. If the sky background is bright, lower magnification than that should be sufficient, especially if the hourglass region is the only nebulosity to be seen without filter.
To the south of M8 there are some globular clusters and planetary nebulae which are well observable in bigger telescopes, but you will need a good sky chart like the Uranometria to find them.
M20 is easy to find once you have located M8. Just point your telescope a bit to the west and 1.5 degrees due north of M8 until you find two stars surrounded by diffuse nebulosity, you can do that in your finderscope or directly at the eyepiece of your telescope.
The Trifid got its name from the dark lanes which do split the brighter portion of the nebula into three parts in small telescopes and 4-5 parts in bigger scopes or on photographs. These dark lanes are not easy to observe in telescopes smaller than 6" if the nebula is located at low altitude, better views yields an 8" or bigger aperture. In small telescopes you should use moderate magnification to get a glance of them, you will have to experiment there, zoom in until the nebula just begins to dimish. With bigger telescopes magnifications of about 100x will yield the best views of the Trifid lines. With big apertures the individual nebula patches which are separated by the dark lanes do show contrasty edges and look like they were illuminated "from the background", not unlike colored tissue paper in front of a diffuse light source. Use at least 12" of aperture to perceive this effect.
The Trifid Nebula is small and does have relatively high surface brightness, therefore the use of filters is not compulsory; to the contrary, the diffuse reflection nebula which forms the northern part of M20 will simply disappear if you use any nebula filter. But, when the sky brightness there is high, an UHC helps to improve the visibility of the emission part of M20. The reflection part is easier to observe at low magnification. If you use a telescope with at least 8" of aperture, you should be able to see colors in the nebula, the emission nebula appears pink and the reflection nebula slightly blue, just as on photographs. Whether these colors are apparent or not depends not only on telescope size but also on conditions and on the color perception of the observer. Some observers report green and not pink color in the emission part, their eyes respond better to the green OIII line emission. Do not use too much magnification if you want to see the colors of the nebula, as your eye will need maximum light input.
A bit to the northeast of M20 the loose star cluster M21 is located, this object should show up in the same field of view with M20 when using a low-power eyepiece. It is an easy target even for small telescopes.
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© 2010 Walter Koprolin