A detailed analysis of cluster [DB2000] 11 suggests that it (and so Sh 2-21) lies at a distance of 7600 parsecs, just a bit in front of the galactic nucleus region. The hottest star likely has class B0V. [1
Avedisova places Sh 2-21 in star formation region SFR 0.50-0.75
, with 47 components, including 4 masers, the optical star cluster Collinder 351
and the infrared dark clouds [DB2001] IRDN 1
and [DB2001] IRDN 2
This interesting Spitzer infrared image
reveals a roughly spherical compact nebula.
This MSX infrared image of the galactic central region
shows Sh 2-21 as the knot of emission terminating a peculiar fish hook structure descending below the galactic plane at the left.
1. ^ Dutra, C. M., Ortolani, S., Bica, E., et al. (2003). "NTT infrared imaging
of star cluster candidates towards the central parts of the Galaxy",
Astronomy and Astrophysics, Vol. 408, 127-134. [2003A&A...408..127D]