Universe Today
Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Messier 18 open star cluster. Enjoy!
In the 18th century, while searching the night sky for comets, French astronomer Charles Messier began noticing a series of "nebulous objects" in the night sky. Hoping to ensure that other astronomers did not make the same mistake, he began compiling a list of these objects,. Known to posterity as the
Messier Catalog
, this list has come to be one of the most important milestones in the research of Deep Sky objects.
One of these objects was Messier 18 (aka. NGC 6613), a relatively dim open star cluster located in the
constellation Sagittarius
. Located in close proximity to
Messier 17
(the Omega Nebula), it is possible that these two clusters formed together.
Description:
Located about 4,900 light years from Earth, and spread over an expanse measuring 17 light-years across, this group of around 20 stars is only about 32 million years old. Its hottest members are spectral type B3, yet you will also see many yellow and orange stars as well. But as already noted, M18 may not be alone in space.
According to research done by
R. and C. R. de la Fuente Marcos
, M18 may very well be a binary cluster, paired with the open cluster - NGC 6618 - which is harbored inside M17:
[caption id="attachment_129828" align="aligncenter" width="471"]
The location of Messier 18 in the Sagittarius constellation. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)
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History of Observation:
M18 was one of Charles Messier's original discoveries, which took place in 1764. As he wrote in his notes upon observing the cluster:
In this circumstance, we must give Messier great credit considering his observations were performd long before the nature of open clusters and stellar movement were understood. While Messier seems to have spotted some nebulosity around the cluster (which may have belonged to M17), he takes a later historic cut from Smyth:
What a shame Smyth wasn't around to later know that M18 could be paired with its nebulous neighbor!
[caption id="attachment_129827" align="aligncenter" width="580"]
The open cluster Messier 18 (NGC 6613), as observed by the Two Micron All Sky Survey (2MASS). Credit: University of Massachusetts/IPAC/Caltech/NASA/NSF
[/caption]
Locating Messier 18:
Because Messier 18 is nothing more than a small collection of stars which are slightly brighter than the background Milky Way stars, it isn't easy to distinguish it using binoculars or a finderscope if you've never seen it before. One of the most sure ways of locating it is to become familiar with Messier 17 and simply aim a couple of degrees (about a field of view) south.
While it won't strike you as a grand object, you will notice that the stars are compressed in this area and that there are several dozen of them which appear brighter than the rest. In a telescope, use your lowest magnification. Since this is a very well spread cluster, it is easily resolved in even modest instruments.
And here are the quick facts on M18 to get you started:
- Object Name
-
Messier 18
- Alternative Designations
-
M18, NGC 6613
- Object Type
-
Open Star Cluster
- Constellation
-
Sagittarius
- Right Ascension
-
18 : 19.9 (h:m)
- Declination
-
-17 : 08 (deg:m)
- Distance
-
4.9 (kly)
- Visual Brightness
-
7.5 (mag)
- Apparent Dimension
-
9.0 (arc min)
We have written many interesting articles about Messier Objects here at Universe Today. Here's Tammy Plotner's
Introduction to the Messier Objects
, ,
M1 – The Crab Nebula
,
M8 – The Lagoon Nebula
, and David Dickison's articles on the
2013
and
2014
Messier Marathons.
Be to sure to check out our complete
Messier Catalog
. And for more information, check out the
SEDS Messier Database
.