Messier Objects - M18

The northwestern region of Sagittarius is filled with amazing objects to view with a telescope.  The Milky Way goes right through this area filled with many star clusters.  Messier 18 is located here and can be found in the evening during the middle of the Summer.  The Summer is sometimes difficult to observe dim objects in the sky due to the unceasing twilight.  However, by later July and August the sky once again dips into night and deep sky objects are easier to find.

Messier 18 is an open cluster found a degree and a half North of the Sagittarius Star Cloud, M24.  At 9 light years in diameter it is smaller than the nearby M25.  It is also more than twice as far away as its neighbour measuring 4900 light years distant from Earth.  This means it is also quite a lot dimmer with an apparent magnitude of 7.5.  This makes it invisible to the naked eye and thus requiring binoculars or a telescope to resolve.  M18 appears as 9 arcminutes in diameter which is about a third of the size of a Full Moon.

Observation Information

This object was part of my Sagittarius marathon I had during the early Summer weeks of 2018.  At this time Saturn was sitting right in the middle of the Sagittarian Messier heartland.  Messier 24 was located just North of the ringed planet and the bright star Mu Sagittarii.  Using the bright star cloud as a guide I slew the telescope North by one field of view.  I was using a 14" reflector with a 41mm (68° FOV) which created a true field of view of 1.39°.

I saw an open cluster of stars that was sparser than most clusters I have observed.  However, I did recognize a 'Big Dipper' type asterism located within the cluster.  There were 6 relatively bright stars aligned in the formation of a spoon.  I hope to come back to this object in future weeks to see if it is familiar and recognizable.  I measured the cluster to be approximately 20% across the field of view of my 13mm (82° FOV) eyepiece.  This corresponds to an observed value of 7.8 arcminutes which is close to the accepted value of 9 arcminutes.

The weather information at this time is listed in the table below.

Characteristic	Value
Temperature	24°C
Humidity	39%
Wind	N 6 km/h
Barometer	101.91 kPa
Sky	Passing Clouds

The observation record for M18 is found below.

M18 Observation Record

Additional Information

This cluster of stars was first discovered by Charles Messier in 1764 and he reported it as follows.  "In the same night, I have discovered a bit below the nebula reported here above, a cluster of small stars, environed in a thin nebulosity; its extension may be 5 minutes of arc".  The nebula Messier is discussing is M17 which is located just North of M18.  Later observations of M18 in the 19th century indicate no nebulosity.  Admiral William Smyth quipped: "This cluster was discovered by Messier, and registered as a mass of small stars appearing like a nebula in a 3 1/2-foot telescope; which affords another instance that the means of that very zealous observer did not quadrate with his diligence".

Research in 2009 has indicated that M18 is a binary cluster companion to M17.  Both objects are stunning through any telescope and should be sought out on a clear Summer evening.