Messier 21 (M21) - The NGC 6531 Open Star Cluster

By tammy-plotner - August 1, 2016 at 5:15 PM UTC | Stars
Messier-21-and-the-Trifid-Nebula-e1470083327699.jpg

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Messier 21 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 21 (aka. NGC 6531), an open star cluster located in the

Sagittarius

constellation. A relatively young cluster that is tightly packed, this object is not visible to the naked eye. Hence why it was not discovered until 1764 by Charles Messier himself. It is now one of the over 100 Deep Sky Objects listed in the Messier Catalog.

Description:

At a distance of 4,250 light years from Earth, this group of 57 various magnitude stars all started life together about 4.6 million years ago as part of the Sagittarius OB1 stellar association. What makes this fairly loose collection of stars rather prized is its youth as a cluster, and the variation of age in its stellar members. Main sequence stars are easy enough to distinguish in a group, but low mass stars are a different story when it comes to separating them from older cluster members.

[caption id="attachment_130118" align="aligncenter" width="580"]

Messier 21 (NGC 6531). Atlas Image mosaic obtained as part of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.

Atlas mosaic image of Messier 21 (NGC 6531) obtained as part of the Two Micron All Sky Survey (2MASS). Credit: 2MASS/UofM/IPAC/Catech/NASA/NSF

[/caption]

As Byeong Park of the Korean Astronomy Observatory said in a

2001 study

of the object:

But study it in detail they did, finding 56 main sequence members, 7 pre-main sequence stars and 6 pre-main sequence candidates. But why did this cluster... you know, cluster in the way it did? As Didier Raboud, an astronomer from the Geneva Observatory, explained in his 1998 study "

Mass segregation in very young open clusters

":

[caption id="attachment_130122" align="aligncenter" width="580"]

Credit: earthsky.org

Achernar, the flattest star known, is classified as be star. Credit: earthsky.org

[/caption]

But are there any other surprises hidden inside? You bet! Try Be-stars, a class of rapidly rotating stars that end up becoming flattened at the poles. As Virginia McSwain of Yale University's Department of Astronomy wrote in a 2005 study, "

The Evolutionary Status of Be Stars: Results from a Photometric Study of Southern Open Clusters

":

History of Observation:

Charles Messier discovered this object on June 5th, 1764. As he wrote in his notes on the occassion:

[caption id="attachment_130121" align="aligncenter" width="580"]

Messier 21. Credit: Wikisky

Close up of the Messier 21 star cluster. Credit: Wikisky

[/caption]

While Messier did separate the two star clusters, he assumed the nebulosity of M20 was also involved with M21. In this circumstance, we cannot fault him. After all, his job was to locate comets, and the purpose of his catalog was to identify those objects that were not. In later years, Messier 21 would be revisited again by Admiral Smyth, who would describe it as follows:

Locating Messier 21:

Once you have become familiar with the Sagittarius region, finding Messier 21 is easy. It's located just two and a half degrees northwest of Messier 8 - the "

Lagoon Nebula

" - and about a half a degree northeast of Messier 20 - the "

Trifid Nebula

". If you are just beginning to astronomy, try starting at the teapot's tip star (Lambda) "Al Nasl", and starhopping in the finderscope northwest to the Lagoon.

[caption id="attachment_130119" align="aligncenter" width="471"]

Credit IAU/Sky & Telescope magazineRoger Sinnott & Rick Fienberg

The location of M21 in the Sagittarius constellation. Credit: IAU/Sky & Telescope magazineRoger Sinnott & Rick Fienberg

[/caption]

While the nebulosity might not show in your finder, optical double 7 Sagittari, will. From there you will spot a bright cluster of stars two degrees due north. These are the stars embedded withing the Trifid Nebula, and the small, compressed area of stars to its northeast is the open star cluster M21. It will show well in binoculars under most sky conditions as a small, fairly bright concentration and resolve well for all telescope sizes.

And here are the quick facts, for your convenience:

Object Name

Messier 21

Alternative Designations

M21, NGC 6531

Object Type

Open Star Cluster

Constellation

Sagittarius

Right Ascension

18 : 04.6 (h:m)

Declination

-22 : 30 (deg:m)

Distance

4.25 (kly)

Visual Brightness

6.5 (mag)

Apparent Dimension

13.0 (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: