Messier 21 or M21, also designated NGC 6531 or Webb's Cross, is an
open cluster
An open cluster is a type of star cluster made of tens to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, and ...
of
star
A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...
s located to the north-east of
Sagittarius in the night sky, close to the Messier objects
M20 to
M25 (except
M24 M24, M.24 or M-24 may refer to:
Military
* HMS ''M24'', a Royal Navy M15 class monitor
* HMS ''M24'', Royal Swedish Navy ship, (see List of mine warfare vessels of the Royal Swedish Navy)
* M24 Chaffee, an American light tank
* M24 mine, an America ...
). It was discovered and catalogued by
Charles Messier
Charles Messier (; 26 June 1730 – 12 April 1817) was a French astronomer. He published an astronomical catalogue consisting of 110 nebulae and star clusters, which came to be known as the ''Messier objects'', referred to with th ...
on June 5, 1764.
[ This cluster is relatively young and tightly packed. A few blue giant stars have been identified in the cluster, but Messier 21 is composed mainly of small dim stars. With a magnitude of 6.5, M21 is not visible to the naked eye; however, with the smallest binoculars it can be easily spotted on a dark night. The cluster is positioned near the ]Trifid nebula
The Trifid Nebula (catalogued as Messier 20 or M20 and as NGC 6514) is an H II region in the north-west of Sagittarius in a star-forming region in the Milky Way's Scutum–Centaurus Arm.
It was discovered by Charles Messier on June 5, 1764. I ...
(NGC 6514), but is not associated with that nebulosity.[ It forms part of the Sagittarius OB1 association.][
This cluster is located ][ away from Earth with an ]extinction
Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...
of 0.87.[ Messier 21 is around 6.6 million years old with a mass of .][ It has a tidal radius of 11.7 pc,][ with a nucleus radius of and a coronal radius of . There are at least members within the coronal radius down to visual magnitude 15.5,][ including many early ]B-type star
In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...
s.[ An estimated 40–60 of the observed low-mass members are expected to be ]pre-main-sequence star
A pre-main-sequence star (also known as a PMS star and PMS object) is a star in the stage when it has not yet reached the main sequence. Earlier in its life, the object is a protostar that grows by acquiring mass from its surrounding envelope o ...
s,[ with 26 candidates identified based upon ]hydrogen alpha
Hydrogen-alpha, typically shortened to H-alpha or Hα, is a deep-red visible spectral line of the hydrogen atom with a wavelength of 656.28 nm in air and 656.46 nm in vacuum. It is the first spectral line in the Balmer series and is emit ...
emission and the presence of lithium in the spectrum
A spectrum (: spectra or spectrums) is a set of related ideas, objects, or properties whose features overlap such that they blend to form a continuum. The word ''spectrum'' was first used scientifically in optics to describe the rainbow of co ...
.[ The stars in the cluster do not show a significant spread in ages, suggesting that the star formation was triggered all at once.][
As of January 2022, Messier 21 is one of the few remaining objects within the Messier Catalog to not have been photographed by the ]Hubble Space Telescope
The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...
.
Gallery
File:Messier 21.jpg, Open cluster Messier 21 in Sagittarius imaged by John Saunders
File:M21map.png, Map showing the location of M21
See also
* List of Messier objects
The Messier objects are a set of 110 astronomical objects catalogued by the French astronomer Charles Messier in his ' (''Catalogue of Nebulae and Star Clusters''). Because Messier was interested only in finding comets, he created a list of th ...
References
External links
Messier 21, SEDS Messier pages
*
*
{{Ngc70
Messier 021
Carina–Sagittarius Arm
Messier 021
021
069 is:
* in Brazil, the telephone area code for the city of Rio de Janeiro and surrounding cities (Greater Rio de Janeiro)
* in China, the telephone area code for the city of Shanghai.
* in Indonesia, the area code for the city of Jakarta and su ...
Messier 021
17640605
Discoveries by Charles Messier