Open star cluster M21
Der offene Sternhaufen M21 is in the constellation Sagittarius (Sgr).
Equatorial coordinates (epoch J2000):
Ra (α):18h04.6m, Dec (δ):-22° 30'
Visual magnitude: 7,0m
Distance from the star clusters M21 to the Sun is 3930 light years
Angular size: 18'
External dimensions the star clusters M21 are about 20 light years.
Number of stars: 106
Age of the object M21 is 4—8x106 years
Discoverer: Charles Messier, 1764 year.
The designation M21 in other catalogs: NGC 6531
M21 in the middle latitudes is located quite low above the horizon, so observation is difficult due to atmospheric distortions.
In the northern latitudes is not visible. Optimum magnification for a telescope with observations of M21, depending on the aperture of the telescope (D), is about 15x-30x. Higher magnifications of course add details, but lead to a strong drop in the brightness of the image, making it difficult to observe.
The best month for observing M21 is July. At this time, the object is at midnight near its highest point in the sky, that is, it is less affected by light from the lanterns and atmospheric distortions.
For observers in the northern latitudes:The nights in July are still quite light, so it makes sense to try to observe M21 also in August.
The first half of August is also not very dark, in addition the object will be at midnight not at its highest point,which provides the least exposure and atmospheric distortion. But, the difference is not so noticeable. And already in the second half of August the night sky becomes quite dark for observations (although in September it will more darker).The farther south the place of your observation is located, the earlier the sky will darken - both in time on the clock and on the calendar.