Messier Monday: A Cluster That Stands Out From The Galaxy, M23

Amidst the great plane of the Milky Way, a glittering masterpiece awaits.


“[T]his all fades to black, and it’s gone. It’s dust. Choose carefully what you obsess about.” -Meshell Ndegeocello

But like everything in the Universe, before the brightest stars fade away, they have their chance to shine. And nowhere will you find large numbers of intrinsically bright stars in our galaxy than in the open clusters, the collections of young nurseries where infant stars live for the first few hundred million years of their lives. While the most common Messier objects are the distant galaxies (with 40) followed by globular clusters (with 29), there are a respectable 26 open clusters, the vast majority of which can be found in the galactic plane.

Image credit: © Ole Nielsen 1999-2007, via http://www.ngc7000.org/ccd/messier.html.

Today’s object is one of the first open clusters discovered by Messier and is quite typical: Messier 23 lies right in our galactic plane, oriented relatively close to the galactic center. But unlike the galactic center, this object is only 2,150 light-years away, and hence it stands out with its stars looking brighter and bigger against the backdrop of the Milky Way. Even if the (nearly full) Moon rises while you’re searching for it, it’s still an object worth viewing in the skies tonight!

Here’s how to find it.

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

After sunset and the onset of darkness, a number of bright points of light rise in the southern portion of the sky. Bright Saturn flies high a little towards the southwest, the brilliant orange giant Antares will be very close to due south, and towards the southeast the constellation of Sagittarius begins to rise. If you’re at a dark sky site and the Moon has not yet risen, you may be able to make out the Milky Way, extending from Sagittarius up through the Summer Triangle (and past Altair, at the upper left, above), the birthing ground of these open clusters.

But it’s north of Sagittarius and east of Antares that you should look if you’re questing for Messier 23.

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

This region of sky — east and south of Sabik but north of Kaus Borealis — actually houses a tremendous treasury of Messier objects: a full 11 are clearly visible in there to dedicated observers!

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

It’s a little too early in the year to view the more southerly ones easily unless you’re putting in some very late nights, but Messier 23 is still within your reach. The only downside is that there aren’t many bright stars nearby to help guide you! The closest bright star — just below center in the image above — is μ Sagittarii, and if you can identify it, it can lead you to your destination.

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

About 4.3° northwast of μ Sagittarii, there’s one (unrelated) naked-eye star that’s located on the outskirts of Messier 23: HIP 87782. If you can find that, you won’t be able to miss the collection of bright, blue (and a couple of red) stars shining nearby.

Image credit: © 2006 — 2012 by Siegfried Kohlert, via http://www.astroimages.de/en/gallery/M23.html.

Discovered in June of 1764 by Messier himself, it was first described as:

A star cluster, between the end of the bow of Sagittarius & the right foot of Ophiuchus, very near to 65 Ophiuchi, according to Flamsteed. The stars of this cluster are very close to one another.

But it isn’t that the cluster stars are very close together, it’s that they really stand out from the background stars from the plane of the galaxy!

Image credit: © Copyright 1970 — 2014 by Fred Espenak, via http://astropixels.com/openclusters/M23-01.html.

The Milky Way appears bright and diffuse because of the billions of stars that make it up, but are mostly intrinsically fainter than even our own modest Sun and, on average, many thousands of light-years away. In addition, there are dark dust lanes where small grains of interstellar material absorbs and blocks the light from the stars behind them.

But this cluster — like most open clusters visible from Earth — was formed relatively recently and nearby, and consists of a population of young, dense, close but also intrinsically bright stars!

Image credit: Astrophotography by Jim Thommes, via http://www.jthommes.com/Astro/M23.htm.

When star clusters form, they typically come about because a molecular cloud — a cloud of neutral gas coming in at many thousands to a few million times the mass of our Sun — collapses under the immense force of gravitation. They’re more likely to form in the plane of our galaxy than anywhere else, and to form when the density waves of our galactic spiral arms pass over them. When the clouds do collapse, regions of star formation give birth to hundreds if not thousands of stars, ranging from the very dim, low-mass red dwarfs all the way up to ultramassive blue giants!

Image credit: N.A.Sharp, REU program/NOAO/AURA/NSF, via http://www.noao.edu/image_gallery/html/im0634.html.

Over time, the most massive, giant stars die the earliest, as they finish burning through their fuel the most quickly. This cluster consists of some 150 stars that are definitely a part of Messier 23, and contain no O-class stars and only the dimmest and faintest of the B-class stars (B9), placing this cluster at around an age of 220-300 million years. This actually makes it one of the older open star clusters in our galaxy, as gravitation causes these clusters to dissociate on timescales of a few hundred million years (on average).

But how many more stars are there inside? To attempt to answer this, let’s take a look at two images: one from the Digitized Sky Survey and one from the Sloan Digital Sky Survey.

Images credit: DSS (above) and SDSS (bottom), via NASA’s wikisky tool and with stitching/annotation by me.

First off, the red stars in here — the brightest ones — are red giants, or stars that have reached the end of their hydrogen-burning phase and are now burning helium in their core. They’ve expanded, and are brighter but cooler than the other stars in the cluster. The blue ones are the B-class and A-class stars that are so prominent, and those are still main-sequence stars, which means they’re still burning hydrogen. But what of all the others? What of the white ones, the yellow and the orange and faint red ones? Which of those are part of this cluster, and which ones are part of the galactic background?

Sadly, we don’t know! That would require doing spectroscopy on each of the individual stars, and those are observations that have never been made. Which is too bad, because doing that would tell us whether there are merely hundreds of stars in here, or whether there are many thousands, which is — if I were a gambling man — what I’d bet on!

Image credit: Sergio Equivar of Buenos Aires Skies, via http://www.baskies.com.ar/PHOTOS/M23%20LRGB.htm.

After all, with over 100 intrinsically bright stars, there ought to be many dozens of faint ones for each of the ones we can see! This is how populations of stars work and how practically all clusters form; all it would take is one dedicated set of observations with say, Hubble (which has never imaged this object) and then we’d know. Until then, all we have are the observations at our disposal combined with the knowledge of how these things work. It’s speculation to say there are thousands of stars in this cluster, but there’s good reason for that speculation, too!

And that takes us to the end of today’s Messier Monday. Including today’s object, here are the ones we’ve profiled so far:

With the Moon finally out of the skies next week, we’ll take an in-depth look at one of the great galactic giants of the night. Enjoy the sights of the sky, and I’ll see you back here soon for another Messier Monday!