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Messier Monday: The could-be-better cluster, M26

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Messier himself lamented his own original discovery. If only he knew why he was so disappointed, he might’ve been amazed instead!

“Things need not have happened to be true. Tales and adventures are the shadow truths that will endure when mere facts are dust and ashes and forgotten.” –Neil Gaiman

Forming new stars in the Universe is a dangerous game for an atom. You have to get together with thousands upon thousands of solar masses of other atoms to even get a fighting chance, and even then — even if your clouds collapses to form stars — it’s a long shot that you’ll be in one. Most likely, you’ll be part of the interstellar medium that’s blown off back into the abyss of space by ultraviolet radiation from the hottest, largest, first-formed stars. But not always.

Image credit: Mike Keith, via http://cosmicneighbors.net/PeriodicMessier.htm.

This Messier Monday, we take a look at one of the most common types of deep-sky object in the night sky, an open star cluster, but one that has an exceedingly rare property to it: despite being more than old enough that all its dust should have been blown off, there’s a tremendous amount of light-blocking material dimming the center of it!

Today’s object was one of the original, relatively early discoveries of Messier himself way back in 1764: the open cluster Messier 26. Here’s how to find it.

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

Now that daylight savings time has finally ended, the night sky — particularly at high northern latitudes — tends to get dark out very early. By 6 PM where I am, we’re immersed in the near-total darkness of night. While the summer triangle still shines overhead despite being halfway to winter, the teapot of Sagittarius is only barely still visible over the southwestern horizon, where tonight, the planet Mars just happens to coincide with the “top” of the teapot’s dome: Kaus Borealis.

If you follow a curve of stars from Kaus Borealis up toward Altair, the southernmost vertex of the summer triangle, you’ll be well on your way to Messier 26.

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

A series of naked-eye stars curve up and slightly to the west, then break back towards Altair if you start from the top of the teapot’s dome. The westernmost among them is the relatively bright orange star α Scuti, approximately 40% of the way up towards Altair. Just to the east of α Scuti are two other naked-eye stars: δ Scuti and ε Scuti, with δ Scuti lying slightly south.

Connect α Scuti to δ Scuti and continue on just a fraction of a degree, and Messier 26 will be easy pickings for you.

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

Messier himself couldn’t even find it through his standard telescope, saying:

A cluster near Eta and Omicron in Antinous [now Alpha and Delta Scuti], between which there is another one of more brightness: with a telescope of 3.5-foot [FL] one cannot distinguish them, one needs to employ a good instrument. This cluster contains no nebulosity.

Then again, Messier himself thought this cluster was only two arc-minutes in diameter, when it reality it turns out to be fifteen.

Image credit: John C. Mirtle of http://www.astrofoto.ca/john/m026.htm.

While the image above might be only slightly better than what Messier would’ve seen, let’s take a look at what a modest, modern observer might see with today’s equipment.

Image credit: © 2006 — 2012 by Siegfried Kohlert, via http://www.astroimages.de/en/gallery/M26.html.

There are clearly plenty of stars in this cluster, but also a bizarre “dark spot” towards the center. While we see nebulosity — evidence of light-blocking dust and reflective, nebulous gas — in younger star clusters, particularly from ones in-or-near regions that are still forming stars, it normally takes only ten million to a few tens-of-millions of years for all that neutral gas to be ionized and blown back into not only interstellar, but inter-cluster space.

So what’s the deal here?

Image credit: Fred Espenak of http://astropixels.com/openclusters/M26-01.html.

When we look at the stars of Messier 26, we find quite compellingly that the stars are much older than that: they give us an age of 89 million years, or slightly older than the Pleiades. With even a few B-stars left, of class B8 and B9, it can’t be all that old, but also wouldn’t be any younger than that without having brighter, hotter main sequence stars present, too. Practically all the other star clusters that we know of that are that age — or even considerably younger — have no light-blocking dust left in them.

And yet, light-blocking material is very clearly what we’re looking at here, as a glimpse in the infrared (at 2-micron wavelengths) shows.

Image credit: Messier 26 via the Two Micron All-Sky Survey (2MASS).

What might be happening, then, is not that there’s dust in the cluster itself, but rather in interstellar space between us and this cluster. After all, it is 5,000 light-years away, and situated along the galactic plane. It’s possible, then, that there would be some intervening matter along the line-of-sight.

If we take a look at a wide-field view, it turns out that’s exactly what we see.

Image credit: John C. Mirtle, via http://www.astrofoto.ca/john/m026.htm, with annotation by me.

So it isn’t that there’s anything wrong with this cluster, or that its stellar density drops towards the core, or that it’s held onto its light-blocking dust far longer than a cluster of its size and age ought to. Rather, it just happens to be in an unusual location — caught partially behind a galactic dust lane — that gives it its unique appearance!

Image credit: Hillary Mathis, Vanessa Harvey, REU program / NOAO / AURA / NSF.

There are plenty of red giants present in here, just as you’d expect, evidence that the once-hotter, more massive stars are coming to the end of their lives. Over the next few million years, these stars — the prominent, red ones you see — are expected to blow off their outer layers into planetary nebulae, the fate of all such stars that are red giants today without enough mass to go supernova.

Image credit: © — Copyright 2009 — Fort Lewis College — Department of Physics & Engineering, via http://www.fortlewis.edu/observatory/image_detail.asp?ID=241.

And while there aren’t any spectacular professional images of Messier 26 — sorry, everyone; it’s never been viewed by Hubble — there are some amateur ones that, at the very least, would’ve blown Messier himself away. Including this masterpiece of the cluster’s brightest, most core members.

Image credit: R.Sparenberg, S.Binnewies, V.Robering, via http://www.airglow.de/html/starclusters/m26.html.

250 years ago, this cluster was discovered, and despite the fact that only a few others like it were known at the time, it was a great disappointment. Today, we know of thousands such clusters, and this one has turned out to be exciting and unique, shedding light on not just the stars inside of it, but on how remarkably complex the space between a cluster and ourselves can be!

And with that, we come to the end of one of our final Messier Mondays. With just three objects to go now, take a look back at all your favorites:

And come back next week, as we look to the skies once again for another deep-sky wonder, here on Messier Monday!

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