Messier Objects for September 2020

Original article appears in GuideStar September, 2020.

M17 M18 M23.pngThis is a series of columns primarily revolving around observing the Messier Catalogue.  The intent is to provide the reader a sampling of the Messier objects each month that are most visible in the time frame the column is published.  Hence, these deep sky objects should be easily identifiable in and around the month of September.  Some months may have a special treat in addition to the Messier Objects.  Check the trailer. 


M23: Open Cluster

The Sagittarius Milky Way contains so many notable objects that it is easy to overlook some of the smaller treats.  M23, for example, all but hides in the southwest corner of a dark and inconspicuous valley that slices through the hub of our Galaxy.  Yet this open cluster of 150 or more stars spreads across 15 light years and is deceivingly dynamic.

The loneliness of M24 and its dark surroundings is somewhat intriguing.  The cluster is intermediate in age at around 300 million years old.  Surveys have found 30 variable stars ranging in magnitudes from 10 to 17. Seven of the stars are eclipsing binaries including two apparent W UMa contact binaries and one eclipsing binary.  The remaining 23 variables are likely semi-regular variable stars.

Messier notes: (Observed June 20, 1764) Star cluster between the tip of the bow of Sagittarius and the right foot of Ophiuchus very close to the star Flamsteed 65 Ophiuchi. The stars in this cluster are very close together.  

NGC note: Cluster, bright, very large, pretty rich, little compressed, stars of 10 magnitude or greater.


Messier 23 M23 – NGC 6494
Constellation:  Sagittarius  Magnitude: 5.5
Sky Coords: RA: 17h 56.9m   Dec: -19.01 Dist: 2,000 ly+-


M17: Emission Nebula and Open Cluster

More messier objects (15) are in Sagittarius than any other constellation.  The mythical Archer stands vigil in the direction of the of the center of our Galaxy, the most crowded area containing stars, dust, and gas. No wonder that this parcel of sky yields the greatest variety and concentration of star clusters and nebulae, including M17, which is a combination of both.  With the exception of the Orion Nebula (M42), M17 is the brightest galactic nebula visible to observers in mid-northern latitudes.

The Hubble Space Telescope imaged a swath of the nebula’s interior 3 light years wide, revealing it to be a turbulent sea of nebulous splendor with a crib of newborn stars surrounded by blankets of glowing gas.  Intense ultraviolet radiation fleeing from these hot, young stars not only causes the nebula to fluoresce but also eats away at the cold gas from which these stars were formed.  The intense heat and pressure cause some material to stream away from the surface, thus masking background structures.

Messier notes: (Observed June 3, 1764) Streak of light without stars, five to six minutes long, spindle-shaped, and rather like the belt of Andromeda, but very faint. 

NGC note: A magnificent object, very bright, extremely large, extremely irregular in shape, with a large cluster.

Messier 17

M17,  NGC 6618, Omega

Horseshoe, or Swan Nebula

Constellation: Sagitarius  Magnitude: 6.0
Sky Coords:    RA: 18h 20.08m   Dec: -16.11 Dist: 5,500 ly

M18: Open Cluster (Sounds like a real challenge..jk)

M18 is a sparsely populated open star cluster only 1 degree south of M17, near the extreme northern edge of the small Sagittarius Star Cloud (M24).  M18 lies in the Sagittarius spiral arm about 70 light years from the galactic plane and has an age of 30 million years.   Its brightest main sequence member is of spectral type B.  Trumpler rates the object as a poorly populated detached cluster of bright and faint stars with little central concentration. 

Messier: (Observed March June 3, 1764) Cluster of faint stars slightly below… (M17), surrounded by faint nebulosity…resolved with a good telescope only.

NGC: Cluster, poor, very little compressed.

Messier 18 M18, NGC 6613
Constellation: Sagitarius Magnitude: 6.9
Sky Coordinates: RA: 18h 20.0m Dec: -17.06  Dist: 3,900 ly    

September Bonus: Saturn and Jupiter in a 6-month journey to a conjunction culminating in December.

Our two most observable planets begin a long-term hook-up in the early evenings beginning late June. 

Keep Looking up!!

Jim King

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