Messier 25 - The IC 4725 Open Cluster

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Messier 25 open star cluster. Enjoy!

Back in the 18th century, famed French astronomer Charles Messier noted the presence of several "nebulous objects" in the night sky. Having originally mistaken them for comets, he began compiling a list of these objects so that other astronomers wouldn't make the same mistake. Consisting of 100 objects, the

Messier Catalog

has come to be viewed as a major milestone in the study of Deep Space Objects.

One of these objects is Messier 25, an open star cluster located in the direction of the

Sagittarius Constellation

. At a distance of about 2000 light years from Earth, it is one of the few Messier Objects that is visible to the naked eye (on a clear night when light conditions are favorable).

Description:

This galactic star cluster was originally discovered by Philippe Loys de Cheseaux in 1745 and included in Charles Messier's catalog in 1764. Oddly enough, it was one of those curious objects that didn't get cataloged by Sir John Herschel - therefore it never received a New General Catalog (NGC) number.

This is odd, considering that it was part of the 1777 catalog of Johann Elert Bode, observed by William Herschel in 1783, written about by Admiral Smyth in 1836 and even commented on by the Reverend Thomas William Webb in 1859! It was until J.L.E. Dreyer in 1908 that poor little M25 ended up getting added to the second Index Catalog.

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Atlas Image mosaic of Messier 25,obtained as part of the Two Micron All Sky Survey (2MASS). Credit: Univ. of Mass./IPAC/Caltech/NASA/NSF

[/caption]

Cruising along peacefully about 2,000 light-years away from Earth, this little group of stars spans across about 19 light years of space. Caught inside of its influence are four giant stars - two of spectral type M and two of type G. As we know, it contains the variable star U Sagittarii, a Delta Cephei-type, which lets us know this group of 86 or so stars may have began life together as long ago as 90 million years.

But how many stars are really in there? If you're using a large aperture telescope, you're probably detecting the signature of several just beyond the threshold limits. And so has more recent scientific studies. According to a study by

A.L. Tadross (et al.)

of the National Research Institute of Astronomy and Geophysics:

[caption id="attachment_131713" align="aligncenter" width="579"]

Core region of the Messier 25 open star cluster. Credit: Sergio Eguivar

[/caption]

And how many of those stars are surprises? Let's try a few blue straggler stars. According to a study titled "

Blue Stragglers, Be stars and X-ray binaries in open clusters

", by A. Marco (et al):

History of Observation:

Perhaps we know more about it today than our historic antecedents, but our knowledge of its existence is owed to astronomers like Charles Messier, who took the time to catalog it. As he wrote in his notes:

[caption id="attachment_131714" align="aligncenter" width="580"]

Finder Chart for M25 (also shown M8->M9, M16->M18, M20->M24 and M28). Credit: freestarcharts[/caption]

Perhaps William Herschel understood there was more there to be seen, for he commented in his unpublished notes; "Very large, bright, stars and some small, faint ones; I counted 70, and there are many more within no considerable extent."

Yet, it was Admiral Smyth who really understood what lay beyond. From his observations, he wrote:

Locating Messier 25:

Finding Messier 25 with binoculars is quite easy. Simply start at the teapot "lid" star, Lambda, and aim about a fist width almost due north. Here you will encounter a a Cepheid variable – U Sagittarii. This one is a quick change artist, going from magnitude 6.3 to 7.1 in less than seven days, so although it is a cluster member, it may fade on you from time to time as a marker star!

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Location of Messier 25 and other Deep Sky Objects in proximity to the Sagittarius Constellation. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

[/caption]

M25 will appear a a loose, but bright association of stars in binoculars and as a faint hazy spot in binoculars - but behold incredible resolution in a telescope. You'll love the different magnitudes, so stick to around low to medium magnifications to enjoy it most.

As always, here are the quick facts. Enjoy!

Object Name: Messier 25
Alternative Designations: M25, IC 4725
Object Type: Open Galactic Star Cluster
Constellation: Sagittarius
Right Ascension: 18 : 31.6 (h:m)
Declination: -19 : 15 (deg:m)
Distance: 2.0 (kly)
Visual Brightness: 4.6 (mag)
Apparent Dimension: 32.0 (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

Sources: