M6, The Butterfly Cluster, in Scorpius
| Target | Type | R. A. | Dec. | Constellation | Magnitude | Size |
| M6 (NGC 6405) | Open Cluster | 17h 40m 20s | -32° 15′ 00″ | Scorpius | 4.2 | 20.0 arcmins |
Overview
This cluster gets its name from the fact that its bright stars look like a butterfly.
It was first recorded in 1654 by the astronomer Giovanni Battista Hodierna, but it may well have been bright enough for Ptolemy to see it many centuries earlier. Messier added it to his catalog after observing it in May 1764.
Estimated to be around 94 million years old, we believe this bright cluster is about 1,600 light-years away, meaning that it covers about 12 light-years of space. In that volume are housed some 120 stars, the brightest shining at magnitude six, and the dimmest down at magnitude 15. Altogether, they make for a very bright object: the Butterfly Cluster’s total magnitude is 4.2.
This is a beautiful sight at low power and easily available in any telescope.
Finder Charts
This first sky chart, from SkySafari 6, shows the sky looking south at 9:30 pm in mid-July. M6 is between the ‘Teapot’ asterism of Sagittarius and the stinging tail of Scorpius the scorpion. Both of these features are marked with orange ovals, and M6 itself is shown as a green circle.
Stars on this chart are shown to magnitude 5.0 and the moon and planets are not shown.
This is shown in more detail in our second sky chart, which also shows stars to magnitude 5.0.
The orange line joins the two bright stars Alnasi (at the tip of the Teapot’s spout) amd Shaula, at the end of the scorpion’s stinger. M6 is midway along this line and 2° to its northwest. What really helps nail down the location is finding M7 first, which we’ll see on the third chart, below.
This third chart shows stars to magnitude 7. What’s also now shown is the location of M7, also known as Ptolemy’s Cluster. This large group of stars – covering more sky than the full moon – is easily visible to the naked eye as a smokey region shining at magnitude 3.3. As you can see on the chart, it is also midway between the stars Alnasi and Shaula.
From there, head northwest 3.5° to the smaller ‘puff of smoke’ that is M6. For locating purposes, we’ve also shown two bright but unnamed stars nearby, HR 6628 and HR 6630.
Now you have this beautiful cluster in your eyepiece, what should you expect to see?
Individual Telescope Views
The following views will help you find M6 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. 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
Upside-down view – This is what reflectors and magnifying finders show, and refractors / Cassegrains without a star diagonal
Mirrored View – Refractors and Cassegrain models with a star diagonal show this view
Observation
To me, M6 is best seen in binoculars or a small telescope at low power, as it is such a large object.
At nearly the apparent size of the full moon, the butterfly shape is readily appreciated. Both my 7 x 50 and 10 x 42 binoculars reveal two to three dozen stars fluttering about. As you take in the majesty of M6, you will note that there is a bright orange star in the cluster. This star, BM Scorpii, is quite striking against the pale color of the other stars.
These stars are relatively young, at approximately 100 million years old. Keep this in mind as you observe M6 through a telescope.
Observing M6 through a small telescope is certainly worthwhile, once the shape of the butterfly has been appreciated through binoculars.
Using my 76mm refractor, I try to maintain very low power, in the range of 20x to 30x. From a suburban sky, the number of stars visible in M6 through a small telescope is in the range of 60 to 80, most brighter than magnitude 11.
How many do you see?
Large Telescopes
I personally do not recommend using a large telescope (or high magnification in a small one) to observe M6.
While you can resolve more of the cluster’s 100-plus stars, and some of them are quite faint at magnitude 13-14, the cluster loses its identity.
Through my 12.5” Dobsonian, observing M6 is similar to exploring a dense region of the Milky Way. It is fun to sweep through, yet it becomes a challenge to see the boundaries of this cluster, and I quickly move back to my small telescope.
Now that you have seen M6 through a telescope, consider again that this cluster with over 100 member stars is on the order of 20 light-years in diameter.
While this is plenty of ‘space’ for this group of youthful stellar siblings to mature, the stars are still relatively close together. As you may know, the nearest star to our Sun, Proxima Centauri, is a little over 4 light-years away, and this is typical for the average distance between stars within our galaxy.
Over the next several hundred million years, as M6 orbits the Milky Way, the stars will continue to spread out, and eventually, the cluster will no longer be apparent.
- Observation content from 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 [M6-1], for example, click the ‘download’ button next to it below and you’ll be able to open it as a printable pdf.