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 up 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 an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
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 Ameri ...
). It was discovered and catalogued by
Charles Messier 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. It ...
(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 a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed 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
A B-type main-sequence star (B V) is a main-sequence (hydrogen-burning) star of spectral type B and luminosity class V. These stars have from 2 to 16 times the mass of the Sun and surface temperatures between 10,000 and 30,000 K. B-type stars ...
s.[ An estimated 40–60 of the observed low-mass members are expected to be pre-main-sequence stars,][ with 26 candidates identified based upon hydrogen alpha emission and the presence of lithium in the ]spectrum
A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...
.[ 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 (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...
.
Gallery
File:M21map.png, Map showing the location of M21
See also
* List of Messier objects
References
External links
*
*
{{Ngc70
Messier 021
Carina–Sagittarius Arm
Messier 021
021
021 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