Messier Monday: The Most Curious Object of All, M24

This star cloud is the only one of its kind: the densest large concentration of stars in the entire sky!

Image credit: Dan Bush of Missouri Skies, via http://www.pbase.com/image/115380497.


“It takes considerable knowledge just to realize the extent of your own ignorance.” -Thomas Sowell

Out of the 110 deep-sky objects in the Messier catalogue, 107 of them fall into five simple categories: stellar remnants, star-forming nebulae, open clusters, globular clusters and galaxies. Of the other three, two are true mistakes: the double star M40 and the quadruple star M73. But the one remaining anomaly — today’s object — is unlike anything else in the sky.

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

Located right in the plane of our galaxy — visible from the northern hemisphere most clearly after sunset on a summer’s night — lies the densest collection of stars the entire sky has to offer. But unlike the other stars that make up objects in the Messier catalogue, this isn’t a cluster we’re looking at, but a glimpse at one of our galaxy’s spiral arms, seen from our vantage point over 10,000 light-years away!

Today’s object is the one-of-a-kind curiosity: Messier 24, the Sagittarius Star Cloud. Here’s how to find it.

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

Once the skies darken after sundown tonight, the recognizable asterism of the teapot (in Sagittarius) should shine prominently above the horizon towards the south. If you look at the star that shines brightest directly “above” the top of the teapot — which you can also arrive at by extending the line between the two stars that connect the “spout” to the “pot,” Kaus Australis to Kaus Media — you’ll be well on your way to Messier 24.

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

Fainter than the brightest stars in the teapot but still prominent to the naked-eye, μ Sagittarii is actually a system of at least six stars located nearly 4,000 light years away, and if you follow the rough “arrow” created by it and its two nearby stars, 15 Sagittarii and 16 Sagittarii, a little farther north, you’ll come to an unmistakeable “cloud” of stars in the sky, richer in stars than any other comparably sized region as seen from Earth.

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

That hazy patch is itself Messier 24, and unlike most other Messier objects, is best viewed at low magnification or even through binoculars, because it’s absolutely huge on the sky, taking up the area-equivalent of about nine full Moons! Messier didn’t know what he was seeing but described it so:

“Cluster on the parallel of the preceding [M23] & near the end of the bow of Sagittarius, in the Milky Way: a large nebulosity in which there are many stars of different magnitudes: the light which is spread throughout this cluster is divided into several parts…”

Here’s a glimpse at what he was looking at.

Image credit: Jim Mazur of Skyledge, via http://www.skyledge.net/Messier24.htm.

This big, apparent cluster of brilliant white-and-blue in the sky really stands out from its surroundings. But the reason isn’t what you might have expected: you see, this isn’t a dense region containing more stars and matter than the surrounding ones.

Instead, this is a region of space that happens to have less light-blocking dust than the rest of the environs around it!

Image credit: Fernando Cabrerizo, via http://apod.nasa.gov/apod/ap130712.html.

There are many other Messier objects around it, and those others are “true” objects: nebulae and star clusters, mostly. But this is a chance window, where there isn’t a substantial amount of light-blocking material located in this narrow region of sky, and we can see all the way through to the Sagittarius arm of the Milky Way, presently the closest spiral arm to the Sun!

If you were to take all the dust of the Milky Way away — or look in infrared wavelengths, where dust is invisible — this patch of sky would be representative of the entire galactic plane!

Image credit: Vanessa Harvey, REU program/NOAO/AURA/NSF, via http://tcaa.us/Astronomy/Messier/Messier.aspx?id=M24.

Instead, this is the one place in our sky that treats us to views this distant: the Sagittarius-arm seen through this gap-in-the-dust extends from about 10,000 to 16,000 light-years distant, and contains not only all sorts of different classes of stars, but also contains some nearby, foreground stars that appear to shine much more brightly than the vast majority of the ones in the distant arm.

Image credit: © 2006 — 2012 by Siegfried Kohlert, via http://www.astroimages.de/en/gallery/M24.html.

Even on a night like tonight, where the Moon is bright and lingering in the sky, the Sagittarius star cloud is a spectacular feast for the eyes, whether your equipment is primitive (like a pair of department-store binoculars) or advanced. Although, with such a huge object, you’ll definitely want to enjoy the wide-field views it has to offer, and if you’re on a larger telescope, you very likely won’t be able to take it all in at once!

Image credit: RoryG from East Texas, at http://eastexastronomy.blogspot.com/2011/03/messier-24-sagittarius-star-cloud-redo.html.

Some people mistakenly identify the small, faint cluster seen above towards the lower left — NGC 6603 — as Messier 24; it’s not, and don’t be fooled! Messier 24 is huge, and much larger than the individual cluster you see there. In fact, many interesting visual features, some of which are clusters and some of which are interstellar dust, abound in a wide-field view. Be sure and — if you have a high-powered telescope — take your time scrolling through the wonders of this region!

Image credit: John C. Mirtle of http://www.astrofoto.ca/john/m024.htm.

The best wide-field view, that really encapsulates just how spectacular this region is, comes via Deep Sky Astrophotography, and really shows off just how brilliant this portion of the sky is, particularly compared with everything else around it!

Image credit: ©2009 FotisRizos, via user FSQ106 — Artemis11002c2, at http://www.astrophotographos.com/apps/photos/photo?photoid=20936751.

But if we want to see this up close, at say, professional resolution, we’d only be able to see a small portion of this region. Thankfully, the NOAO image I showed you earlier has that option available to it, in a whopping 15 Gigapixel image! Here’s just a slice — presented in optimal screen-resolution — for your viewing (and scrolling) pleasure.

Image credit: Vanessa Harvey, REU program/NOAO/AURA/NSF, via http://www.noao.edu/image_gallery/html/im0845.html.

And with that, we’ll come to the close of the most anomalous — but in many regards, the most spectacular — Messier Monday of them all! Including today, we’ve examined the following Messier objects:

Come back tomorrow, where we’ll have more wonders of the Universe on tap, and next Monday for another exciting Messier object that you can find even with a full Moon shining, only here on Starts With A Bang!


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