M21 - Webb’s Cross
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An open cluster is a group of up to several thousand stars loosely bound by gravity, formed from a single molecular cloud. They are found only in spiral and irregular galaxies, where new stars are still forming. Open clusters typically are less than a few hundred million years old. As they orbit the center of their galaxy, open clusters may change shape or lose stars due to close encounters with other clusters or gas clouds.
One such cluster is Messier 21 (also known as NGC 6531 or Webb's Cross), which can be found in the night sky northeast of the constellation Sagittarius, near the Messier objects M20, M22, M23, and M25. It was discovered and cataloged by Charles Messier on June 5, 1764. This cluster is relatively young and densely packed. M21 contains several blue giants, although most of its stars are faint objects.
Observations
June 26, 2024, around midnight – Jaworzno (Poland), garden
urban conditions, high level of light pollution
The southern horizon visible from my observation site is heavily affected by light pollution, so I did not expect perfect results when observing M21, which—being located in Sagittarius—lies relatively low in this part of the sky. However, a filter reducing the effects of urban light pollution was very helpful.
With an apparent magnitude of 6.5m, this cluster is certainly not visible to the naked eye. However, on a dark night, even the smallest binoculars can reveal it. The cluster is located near M20, the Trifid Nebula, but it is not gravitationally bound to it. M21 is part of the Sagittarius OB1 association.
M21 lies at a distance of approximately 2,200 to 4,250 light-years from Earth. Its age is estimated to be around 6.6 million years, and its mass is about 783.4M☉.
It is believed that 40 to 60 of the observed low-mass stars are entering the main sequence, identified by their hydrogen-alpha emission and the presence of lithium in their spectra. The stars in the cluster show no significant age dispersion, suggesting that star formation occurred simultaneously.
Photo 1 Parameters:
- Total exposure time: 8 minutes (stack of 24 RAW frames at 20s each, using an appropriate number of dark, bias, and flat frames)
- ISO: 2500
- Lens: zoom type (used at fmax = 250mm)
- Aperture: f/4 (the smallest possible for the lens used)
- Mount: equatorial mount with tracking, aligned using the drift method and controlled by a custom-built system
Further readings:
- Park B.-G., Sung H., Kang Y. H., The Galactic Open Cluster NGC 6531 (M21), Journal of Korean Astronomical Society, 2001, 34, str. 149-155
- Wu Z.-Y., Zhou X., Ma J., Du C.-H., The orbits of open clusters in the Galaxy, 2009, 399(4), str. 2146-2164
- Kharchenko N. V., Piskunov A. E., Röser S., Schilbach E., Scholz R.-D., Astrophysical parameters of Galactic open clusters, Astronomy & Astrophysics, 2005, 438(3), str. 1163-1173
Marek Ples