Messier 23, also known as NGC 6494, 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 stars in the northwest of the southern
constellation
A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object.
The first constellati ...
of
Sagittarius.
[ It was discovered 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 ...
in 1764.[ It can be found in good conditions with binoculars or a modestly sized ]telescope
A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...
.[ It is in front of "an extensive gas and dust network", which there may be no inter-association.][ It is within 5° the sun's position (namely in mid-December) so can be occulted by the moon.][
The cluster is centered about 2,050 ]light year
A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distance, astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by t ...
s away. Estimates for the number of its members range from 169[ up to 414, with a directly-counted mass of ; by application of the ]virial theorem
In mechanics, the virial theorem provides a general equation that relates the average over time of the total kinetic energy of a stable system of discrete particles, bound by a conservative force (where the work done is independent of path), with ...
.[ The cluster is around 330 million years old with a near-solar ]metallicity
In astronomy, metallicity is the Abundance of the chemical elements, abundance of Chemical element, elements present in an object that are heavier than hydrogen and helium. Most of the normal currently detectable (i.e. non-Dark matter, dark) matt ...
of e/H= −0.04.[ The brightest component (lucida) is of magnitude 9.3.][ Five of the cluster members are candidate ]red giant
A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The stellar atmosphere, outer atmosphere is inflated and tenuous, making the radius large and the surface t ...
s,[ while orange variable VV Sgr in the far south, is a candidate ]asymptotic giant branch
The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) lat ...
star.[
A 6th-magnitude star, shown in the top-right corner, figures in the far north-west as a foreground star – HD 163245 (HR 6679). Its ]parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...
shift is , having taken into account proper motion
Proper motion is the astrometric measure of changes in the apparent places of stars or other celestial objects as they move relative to the center of mass of the Solar System. It is measured relative to the distant stars or a stable referenc ...
, which means it is about away.
Gallery
File:M23map.png, Map showing the location of M23
File:M23 Eguivar.jpg, Open cluster Messier 23 in Sagittarius
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 ...
Footnotes and References
Footnotes
References
External links
Messier 23, SEDS Messier pages
*
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Messier 023
Orion–Cygnus Arm
Messier 023
023
Messier 023
17640620
Discoveries by Charles Messier