“Beauty is a manifestation of secret natural laws, which otherwise would have been hidden from us forever.” -Johann Wolfgang von Goethe
Welcome back to another Messier Monday here on Starts With A Bang! Each Monday, we go through one of the 110 deep-sky wonders of the Messier Catalogue, some of the brightest and most prominent of the night sky wonders. Originally compiled by Charles Messier and his assistant, Pierre Méchain, in the late 18th Century, these telescopic wonders showcase the cosmic beauty and variety easily visible from our vantage point here on Earth.
Some of them are visible year-round, while others are best viewed at specific times of the year. Some are near easily-found celestial landmarks, while others require more nuanced techniques to find. Today, I’d like to introduce you to one of the most difficult Messier objects to find, despite being on prominent display all summer long: Messier 18.
The constellation of Sagittarius, home to the famous “teapot” asterism easily visible above, boasts a whopping 15 Messier objects. This isn’t that surprising when you consider that it also contains the location of the center of our galaxy; there’s a lot more stuff in that constellation than in most, so it’s unsurprising that we’ll have more young star clusters, active, star-forming regions, and interesting-looking nebulae in that direction of the sky.
But Messier 18 is a special challenge, because it’s a relatively dim open cluster (invisible to the naked-eye under the best of conditions), it’s very small and quite distant, it’s located near no bright stars in the sky, and has to compete with the backdrop of the galactic plane on top of that!
The best way (I’ve found) to find this elusive object is to look north of the teapot towards the most prominent star located “atop” the teapot, the blue star μ Sagittarii. If you connect the tip of the teapot’s spout (Alnasl) to μ Sagittarii, and travel about another 4 degrees, you might see a small patch of sky that appears to have a slightly greater density of stars than the galactic backdrop.
Alternatively, if you focus on μ Sagittarii in binoculars or a low-power telescope, you’ll see two prominent stars (just barely visible to the naked eye) just to the north of it: 15 Sagittarii and 16 Sagittarii. If you follow the rough line connecting these three stars about 4 degrees farther — possibly using the stellar landmarks below for guidance — you’ll have the chance to encounter the elusive Messier 18.
It’s perhaps the most difficult of eleven Messier objects that are located relatively close together on the sky. At least Messier 21 has some nearby landmarks to help you out; Messier 18 is just a blip against the confusing galactic morass.
So, if-and-when you finally manage to capture this Messier object, what will you find yourself looking at?
Just a small collection of stars, denser than the surrounding regions, which was discovered for the first time by Messier himself, back on June 3, 1764. He had this to say about it:
A cluster of small stars… surrounded by slight nebulosity, this cluster is less obvious… with an ordinary telescope of 3.5-foot, this cluster appears like a nebula; but with a good telescope one sees nothing but stars.
Yet a “good” telescope to Messier would be on the low-end of amateur equipment today. Here’s what a higher-quality observation looks like.
A few prominent, bright blue stars are the highlight of this cluster, concentrated narrowly in an area just about 0.2 degrees in diameter. The brightest and bluest of these are B-class stars (and as bright as B3, on a scale from 0-9), along with a few bright yellow/orange giants, which are similar stars that have run out of hydrogen fuel in their core and have become giants.
There’s also a small amount of nebulosity in there — or dust — and this tells us the cluster is young, at only about 32 million years of age.
With an estimated distance of 4,900 light-years, this is one of the most distant Messier star clusters, some 10 times as distant as the Pleiades. Its tiny angular size means that it’s only about 17 light-years in diameter, which makes it very compact for a star cluster.
Because of its distance, its location in the galactic plane, and the lack of really high-quality observations of M18, no one’s really sure as to how many stars are in there. The Sky Catalog compiled in 2000 gives the (embarrassing) figure of twenty stars contained within this cluster, which is clearly a lowball estimate. (Stephen O’Meara’s book ups that to 40, which is still a lowball number.)
A good image, like this one (above) by Jim Thommes, shows that there might be around 20 bright stars in there, but a deeper exposure brings out many, many more that are fainter, redder, and probably, but not definitely, members of this cluster.
I say probably because the clustering is clearly greater than it is elsewhere in space, centered on these dim objects. The dust in the area is also brought out very nicely, suggesting that these are all cluster members. But you need to do a spectroscopic study of these stars to determine what their distance is definitively, and as far as I know, one has not yet been performed.
As it stands now, the best professional image comes in the infrared, courtesy of 2MASS.
Seeing through all the dust, there are clearly hundreds of points of light centered on this cluster, but distinguishing what is a cluster member from what’s not is a task that’s simply beyond the data we’ve taken right now.
But my favorite image available of this hard-to-find cluster isn’t from a professional; it’s from Jim Misti of Misti Mountain Observatory!
This image is much higher resolution (and you should click on it), but to give you a feel for just what lies inside this cluster, I’ve rotated it 90 degrees and taken a high-resolution slice through it. As you can see, there are probably hundreds of stars inside! Whether these are cluster members or not remains an open question, but the beauty of this oft-overlooked region (and this photo, too) is, at least for me, unquestionable!
Close by the star-forming region known as the Omega Nebula (M17), these two objects may be related, but for right now, that’s just speculation! In the meantime, enjoy Messier 18 in its own right.
- M1, The Crab Nebula: October 22, 2012
- M2, Messier’s First Globular Cluster: June 17, 2013
- M5, A Hyper-Smooth Globular Cluster: May 20, 2013
- M7, The Most Southerly Messier Object: July 8, 2013
- M8, The Lagoon Nebula: November 5, 2012
- M13, The Great Globular Cluster in Hercules: December 31, 2012
- M15, An Ancient Globular Cluster: November 12, 2012
- M18, A Well-Hidden, Young Star Cluster: August 5, 2013
- M20, The Youngest Star-Forming Region, The Trifid Nebula: May 6, 2013
- M21, A Baby Open Cluster in the Galactic Plane: June 24, 2013
- M25, A Dusty Open Cluster for Everyone: April 8, 2013
- M29, A Young Open Cluster in the Summer Triangle: June 3, 2013
- M30, A Straggling Globular Cluster: November 26, 2012
- M33, The Triangulum Galaxy: February 25, 2013
- M37, A Rich Open Star Cluster: December 3, 2012
- M38, A Real-Life Pi-in-the-Sky Cluster: April 29, 2013
- M40, Messier’s Greatest Mistake: April 1, 2013
- M41, The Dog Star’s Secret Neighbor: January 7, 2013
- M44, The Beehive Cluster / Praesepe: December 24, 2012
- M45, The Pleiades: October 29, 2012
- M48, A Lost-and-Found Star Cluster: February 11, 2013
- M51, The Whirlpool Galaxy: April 15th, 2013
- M52, A Star Cluster on the Bubble: March 4, 2013
- M53, The Most Northern Galactic Globular: February 18, 2013
- M57, The Ring Nebula: July 1, 2013
- M60, The Gateway Galaxy to Virgo: February 4, 2013
- M65, The First Messier Supernova of 2013: March 25, 2013
- M67, Messier’s Oldest Open Cluster: January 14, 2013
- M71, A Very Unusual Globular Cluster: July 15, 2013
- M72, A Diffuse, Distant Globular at the End-of-the-Marathon: March 18, 2013
- M74, The Phantom Galaxy at the Beginning-of-the-Marathon: March 11, 2013
- M78, A Reflection Nebula: December 10, 2012
- M81, Bode’s Galaxy: November 19, 2012
- M82, The Cigar Galaxy: May 13, 2013
- M83, The Southern Pinwheel Galaxy, January 21, 2013
- M86, The Most Blueshifted Messier Object, June 10, 2013
- M92, The Second Greatest Globular in Hercules, April 22, 2013
- M97, The Owl Nebula, January 28, 2013
- M99, The Great Pinwheel of Virgo, July 29, 2013
- M102, A Great Galactic Controversy: December 17, 2012
- M104, The Sombrero Galaxy: May 27, 2013
- M108, A Galactic Sliver in the Big Dipper: July 22, 2013
Join us next Monday for some spectacular views of yet another one of the deep-sky wonders of Messier’s catalogue, only here, only on Messier Monday!