OPEN CLUSTER IN SAGITTARIUS
(Image Centred at ra 18h:04.06m / dec -22:29:34)
September 2014 - Home Backyard in Martinez, Buenos Aires, Argentina
TYPE: Open Cluster
Apparent magnitude: 5.9
Apparent dimension: 16 arc minutes
Distance: 4.250 light years
SCOPE: 8" Orion Optics UK w/TeleVue ParaCorr working at f5.75 and 1150 mm fl
CAMERA: QSI 583 WS. Resolution 0.97
FILTERS: Baader LRGB, 2" IDAS LPS for all channels
SKY CONDITIONS: urban skies 1/4 Moon
EXPOSURES: LRGB (30,30,30,30) all bin 1x1
OBJECT DESCRIPTION AND IMAGE SESSION
Messier 21 is an open cluster which shows quite a strong star concentration toward its center. It is located in the galactic plane near Sagittarius active star forming region. The cluster is close to the well known Trifid Nebula, however the proximity happens only if seen from Earth perspective. Studies indicate that both objects are in fact not related. Messier 21 is a relative young cluster with an estimated age of 4.6 Million years.
Charles Messier discovered this object on June the 5th of 1764. It is not an naked eye target but with binoculars it can be spotted from a dark site. Just north of the center there is a nice two minutes diameter ring consisting of several magnitude 9.5 blue / white stars. Because its relative 40 arc minutes separation, this open cluster is often framed with the magnificent Messier 20. In this case I just wanted to give Messier 21 a well deserved protagonist role.