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Challenge #1 – M20, The Trifid Nebula in Sagittarius

TargetTypeR. A.Dec.ConstellationMagnitudeSize
M20 (NGC 6514)Bright Nebula18h 02m 36s-23° 02′ 00″Sagittarius6.329×27 arcmins

Overview

The Trifid Nebula was discovered by Charles Messier in June 1764. Its name literally means ‘three lobes’ and while detailed images like the one below appear to show four lobes, we can see why inferior telescopes of centuries ago might only have revealed three, making the name a fitting one.

You can’t fail to note the two different colors in many images of the Trifid. Below we see them as pink and blue, and that’s important because we’re seeing two different nebulae. The pink element is HII gas being ionized by a star (with dust lanes that lend the Trifid name), while the blue element is a reflection nebula, also based on HII gas. It’s known as a reflection nebula because it’s reflecting the light of nearby stars and is not being directly ionized. 

This is one of the youngest star-forming regions that astronomers know of, at just 300,000 years old.

These gaseous regions span a gigantic 30 light-years of space and are around 5,400 light-years away from us. We know that they are both areas of star formation, with the denser gas filaments being particularly active spots. The largest star of the 3,000 or so in the region has a mass more than 20 times that of our sun.

NASA images, like this one from Hubble, show us detailed jets of gas where star formation is happening.

M20, The Trifid Nebula

Finder Charts

This first sky chart, from SkySafari 6, shows the sky looking south at 9:00 p.m. in mid-September.

M20 is pointed at by the top of the ‘lid’ in the Teapot Asterism (circled in orange), which is a great signpost in the summer night sky.

Stars on this chart are shown to magnitude 5.0 and the moon and planets are not shown.

[M20-1] It’s easiest to find the Teapot and then look where the ‘lid’ points us. Click for full-screen.

In the zoomed-in image below, we can see that the lid star is called Kaus Borealis (or Lambda Sagittarii). This is a very bright star, easily seen in most conditions. If we join a line from the star Phi Sagittarii to Kaus Borealis and extend it in the same direction, it will hit M20.

[M20-2] Use the left hand line of the Teapot’s lid to point to M20. Click for full-screen.

In this final star chart, with stars now shown to magnitude 7, you can see a blue circle representing that 1° field of view. Note how the magnitude five star 11 Sagittarii keeps the line in the right direction and that 4 Sagittarii, also a naked-eye star, is in the same 1° field as M20. When looking without a telescope in this area under dark conditions, you’ll notice a faint glow from the combined light of M20, M8, and M21 (more details below), all of which help us pinpoint their exact location.

[M20-3] Two naked eye stars help us hone in on the right spot. Click for full-screen or download the pdf below.

Now you have this beautiful nebula in your eyepiece, what should you expect to see?

Individual Telescope Views

The following views will help you find M20 in different telescope types by presenting the images as your telescope will show them. The first image is with a black sky and white stars, the second picture is the same image but presented in inverse monochrome. Black stars on a white background is often easier to use at the telescope. Stars are shown to magnitude 10.0 and the larger circle is a 1° field of view, as shown in the third star chart above. You can also see the star 4 Sagittarii in each image.

Each image can be clicked on for a full-screen version.

Upright View – This is what your eyes see unaided and through a reflex or red-dot finderscope

[M20-4]
[M20-5]

Upside-down view – This is what reflectors and magnifying finders show, and refractors / Cassegrains without a star diagonal

[M20-6]
[M20-7]

Mirrored View – Refractors and Cassegrain models with a star diagonal show this view

[M20-8]
[M20-9]

Observation

The Trifid Nebula is located in a rich part of the sky for Messier DSOs. In the same 1.5° field of view as M20, we can also see M8, the Lagoon Nebula, and the open cluster M21. You can see the relationship of each of these, plus 4 Sagittarii, in the chart below.

Small Telescopes

Observing M20 through a small telescope is a treat!

To begin, use your lowest power eyepiece and again take in the view of M20 along with its neighbors. There are not many areas in our night sky that contain such beauty, and it is worth appreciating the whole area before focusing on the target object.  As the saying goes, ‘Do not lose the forest through the trees!’

Slowly increase the magnification of your telescope and begin to look for all three types of nebula – emission, reflection, and dark. The emission nebula is brightest and should be readily apparent. At 80 – 100 power, note that the star nearest the center of the emission nebula is a double star.

As you continue to observe, you should begin to notice the lanes of dark nebula separating the lobes of M20. The reflection nebula is less obvious, yet is prominent north of the central emission nebula.

In terms of the star cluster forming within M20, you should be able to see about a dozen of the 50 or so loosely associated stars in a small telescope.

If you have a nebula or contrast filter, try it out on this object – it may enhance the views depending on your local circumstance and observing experience. Check out this article for more information on these different filters.

Can you see all three types of nebulae?  If so, congratulations, you hit the Trifid Trifecta!

Large Telescopes

While you have appreciated its context and seen the major features of this nebula through a small telescope, observing M20 through a large telescope is an opportunity not to be missed.

In my 12.5-inch Dobsonian, I am drawn to the dark lanes that criss-cross the nebula and enjoy exploring the boundaries between the dark and bright nebula in M20. As you explore these intricate and ethereal details, try and notice the faint 4th lobe of the nebula.

The longer you spend observing, the more subtle detail you will see along the edges of the dark nebular lanes. Instead of a smooth edge, you will begin to notice a more uneven boundary and you may note a general mottling across the emission nebula itself. 

The reflection nebula is nearly the size of the emission – how much can you detect?  Can you note any small dark nebular globules in other areas besides the lanes that create the Trifid’s signature lobes?

Observation guide by Dr. Alan Strauss


PDFs for Printing

Each of the star maps above is reproduced as a pdf below. Each star map has a number in [square brackets] beneath it which corresponds to the file number below. If you want image [M20-1], for example, click the ‘download’ button next to it below and you’ll be able to open it as a printable pdf.

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