Messier 21

Article Updated: 24 Dec , 2015


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)


Locating Messier 21: Once you have become familiar with the Sagittarius region, finding Messier 21 is an easy two and a half degrees northwest of Messier 8, the “Lagoon” Nebula and about a half a degree northeast of Messier 20. 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. 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 and the small, compressed area of stars to its northeast is 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.

What You Are Looking At: 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 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. “In the case of a young open cluster, low-mass stars are still in the contraction phase and their positions in the photometric diagrams are usually crowded with foreground red stars and reddened background stars.” says Byeong Park, “The young open cluster NGC 6531 (M21) is located in the Galactic disk near the Sagittarius star forming region. The cluster is near to the nebula NGC 6514 (the Trifid nebula), but it is known that it is not associated with any nebulosity and the interstellar reddening is low and homogeneous. Although the cluster is relatively near, and has many early B-type stars, it has not been studied in detail.” But study it in detail they did… Finding 56 main sequence members, 7 pre-main sequence stars and 6 pre-main sequence candidates.

So why did this cluster… cluster? Ask Didier Raboud of the Geneva Observatory. “The study of the very young open cluster NGC 6231 clearly shows the presence of a mass segregation for the most massive stars. These observations, combined with those concerning other young objects and very recent numerical simulations, strongly support the hypothesis of an initial origin for the mass segregation of the most massive stars. These results led to the conclusion that massive stars form near the center of clusters. They are strong constraints for scenarii of star and stellar cluster formation.” say Raboud, “In the context of massive star formation in the center of clusters, it is worth noting that we observe numerous examples of multiple systems of O-stars in the center of very young OCs. In the case of NGC 6231, 8 stars among the 10 brightest are spectroscopic binaries with periods shorter than 6 days.”

But are there any other surprises hidden inside? You bet. Try Be-stars. “Be stars are a class of rapidly rotating B stars with circumstellar disks that cause Balmer and other line emission. There are three possible reasons for the rapid rotation of Be stars: they may have been born as rapid rotators, spun up by binary mass transfer, or spun up during the main-sequence (MS) evolution of B stars.” says Virginia McSwain of Yale University’s Department of Astronomy, “To test the various formation scenarios, we have conducted a photometric survey of 55 open clusters in the southern sky. We use our results to examine the age and evolutionary dependence of the Be phenomenon. We find an overall increase in the fraction of Be stars with age until 100 Myr, and Be stars are most common among the brightest, most massive B-type stars above the zero-age main sequence (ZAMS). We show that a spin-up phase at the terminal-age main sequence (TAMS) cannot produce the observed distribution of Be stars, but up to 73% of the Be stars detected may have been spun-up by binary mass transfer. Most of the remaining Be stars were likely rapid rotators at birth. Previous studies have suggested that low metallicity and high cluster density may also favor Be star formation.”

m21aHistory: Charles Messier discovered this object on June 5, 1764 and from his notes he states: “In the same night I have determined the position of two clusters of stars which are close to each other, a bit above the Ecliptic, between the bow of Sagittarius and the right foot of Ophiuchus: the known star closest to these two clusters is the 11th of the constellation Sagittarius, of seventh magnitude, after the catalog of Flamsteed: the stars of these clusters are, from the eighth to the ninth magnitude, environed with nebulosities. I have determined their positions. The right ascension of the first cluster, 267d 4′ 5″, its declination 22d 59′ 10″ south. The right ascension of the second, 267d 31′ 35″; its declination, 22d 31′ 25″ south.”

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, for his job was to locate comets and the purpose of his catalog was to merely not objects that were not. In later years, Messier 21 would be revisited again by Admiral Smyth who would describe it well as: “A coarse cluster of telescopic stars, in a rich gathering galaxy region, near the upper part of the Archer’s bow; and about the middle is the conspicuous pair above registered, – A being 9, yellowish, and B 10, ash coloured. This was discovered by Messier in 1764, who seems to have included some bright outliers in his description, and what he mentions as nebulosity, must have been the grouping of the minute stars in view. Though this was in the power of the meridian instruments, its mean apparent place was obtained by differentiation from Mu Sagittarii, the bright star about 2 deg 1/4 to the north-east of it.”

B&W image thanks to Palomar Observatory, courtesy of Caltech, and color image thanks to REU program/NOAO/AURA/NSF.

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