Messier Monday: The Butterfly Cluster, M6

The farther north you are, the harder this wonder is to see. But oh, the rewarding sights for those who find it!

Image credit: Emi; Ivamov, via http://www.emilivanov.com/CCD%20Images/M06_LRGB.htm.


“Happiness is a butterfly, which when pursued, is always just beyond your grasp, but which, if you will sit down quietly, may alight upon you.”
-
Nathaniel Hawthorne

Back in the 1700s, comet-hunters faced a daunting challenge: how to search for potential new discoveries without getting distracted by the fixed objects that appeared in their telescopes. To that end, Charles Messier set out to catalogue the various clusters and nebulae that were easily visible up in the night sky, creating the first comprehensive catalogue of bright, deep-sky objects. The catalogue’s 110 objects stands as a monument to his efforts, and today they represent some of the best sights the night sky has to offer!

Image credit: SEDS Messier Catalogue, from http://messier.seds.org/.

Each Monday, we’ve been taking an in-depth look at once of these objects, seeing both how it appears through a variety of views and telling the scientific story of what’s going on inside of it. Many of these objects are actually located well south of the celestial equator, and the current season — from July through September — offer the best (and in some cases, the only) views of these deep-sky wonders.

Today’s treat is one of the most southerly star clusters in the entire catalogue: Messier 6, the Butterfly Cluster. Here’s how to find it.

Image credit: me, using the free software Stellarium, available at http://stellarium.org/.

Once the sky darkens after sunset, look towards the south, where the constellations of Scorpius and Sagittarius reign supreme. Scorpio is home to the brilliant red giant, Antares, while Sagittarius contains the very bright teapot asterism, both clearly identifiable in the early part of the night.

Image credit: me, using the free software Stellarium, available at http://stellarium.org/.

Just beyond the “nose” of the teapot (the star Alnasl), and just north of the second-brightest star in Scorpius, Shaula, you can find two Messier star clusters close together: Messier 7 (the most southerly of all Messier objects), larger and a little farther southeast, and today’s object, Messier 6, a little higher in the sky and farther to the northwest.

There aren’t any brilliant stars to guide you there beyond the ones you see above, but if you follow the top of the “spout” of the teapot, from Kaus Media to Alnasl and beyond, you’ll run into a clearly-visible naked eye star, HIP 87220.

Continue onwards about another 1.5°, and you won’t be able to miss Messier 6, heralded by a bright orange giant (often visible to the naked eye) against a backdrop of fainter, bluer stars.

Through even a small, low-powered telescope (or a pair of binoculars), this cluster puts on a brilliant show.

Image credit: Ezequiel Bellocchio, via http://www.surastronomico.com/galeria/Ezequiel_Bellocchio/Ezequiel_Bellocchio_23.htm.

Discovered all the way back in the 1650s (and possibly even as early as Ptolemy), Messier added this to his catalogue as one of the first objects back in 1764, writing:

“Cluster of small stars between the bow of Sagittarius & the tail of Scorpius. At simple view [to the naked eye], this cluster seems to form a nebula without stars; but even with the smallest instrument one employs for investigating one sees a cluster of small [faint] stars.”

The orange one may be the brightest, but even a small amount of magnification shows the others shining brilliantly alongside it.

Image credit: © Bryan Sayler, via http://www.bryansayler.net/p432420682/h291466B5#h291466b5.

Messier often described stars as “small,” meaning faint, indicating that they appeared small and low-in-brightness in the optics of his own telescope. But these stars are only faint as seen from Earth; in reality, they’re huge and brilliant, even compared to our own Sun!

What you’re looking at here is a very young cluster of stars that’s relatively nearby, located in the plane of our galaxy towards the very center. The brilliant orange-tinted stars are giants; having burned through their core’s hydrogen, they’ve moved on to helium, destined to end their lives in short order in a planetary nebula, with their core contracting down to a white dwarf. On the other hand, the blue stars (as well as all the other cluster members not a part of the galactic background) are quite young, at slightly under 100 million years, and thought to be somewhere between two-and-eight times as massive as our own Sun.

Image credit: © 2010 — 2014 Dean Jacobsen, via http://www.astrophoto.net/nebula_page.php?photo_id=19.

We can tell what age a star cluster is by looking at the brightnesses and colors of the stars that are there, and inferring what stars must have burned out by now. In the case of Messier 6 — known as the Butterfly Cluster because of it’s very rough resemblence to the eponymous insect — there are no O-class stars remaining, and no B-class stars brighter than class B4 (where 0 is the brightest/bluest and 9 is the dimmest/reddest), telling us that those stars have all burned through their fuel and died by now.

Since those stars are the shortest-lived, we can infer an age of 51-to-95 million years from the ones remaining!

Image credit: N.A.Sharp, Mark Hanna, REU program/NOAO/AURA/NSF, via http://www.noao.edu/image_gallery/html/im0379.html.

There’s a small amount of dust (or ‘nebulosity’) left here, which should continue to burn away as the hot, ultraviolet radiation exerts enough pressure to drive this material back out into the interstellar medium. It’s even conceivable that what we’re seeing is not dust intrinsic to the cluster itself, but simply dust that’s part of outer space that happens to be passing through the cluster.

At a distance of only 1,600 light-years from Earth, this is one of the brightest star clusters visible from our world!

Image credit: © 2006 — 2012 by Siegfried Kohlert, via http://www.astroimages.de/en/gallery/M6.html.

To those of you further south, the cluster rises higher in the sky, and will appear even more brilliant and spectacular, as there’s less atmosphere to contend with. At just six light-years in radius, at such a close distance, there were actually eighty individual members of this cluster identified as long ago as the 1950s.

Of course, using modern telescope and CCD (charged-coupled-device) camera technology, dedicated amateurs can identify literally hundreds of stars in this region of space. Have a look at this masterpiece by Jim Misti,

Image credit: Jim Misti of Misti Mountain Observatory, http://www.mistisoftware.com/astronomy/Clusters_m6.htm.

which clearly shows that for every bright, easily-identifiable “blue” star in this cluster — and remember, the blue stars are the hottest, brightest and least numerous stars — there are many others that are white, yellow or red, and much dimmer! And because this cluster is so close to us, even the amazing image above only encapsulates about a quarter of the cluster. Even though it has never been observed by Hubble, it’s hard to ask for a better photo than the one you see above!

And with that, we’ll come to the end of another Messier Monday! Including this one, we’ve taken a look at the following Messier objects:

With only 14 objects to go, including open clusters, globular clusters, star-forming nebulae, with a planetary nebula and a galaxy left, which one will be next? Come back next week to find out, only here on Messier Monday!


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