One of the massive stars in W51 is a bright X-ray source that is surrounded by a concentration of much fainter X-ray sources, as shown in a close-up view of the Chandra image. This suggests that massive stars can form nearly in isolation, with just a few lower mass stars rather than the full set of hundreds that are expected in typical star clusters.
Fast Facts for W51:
Scale: Image is about 20 arcmin (100 light years) across
Category: Normal Stars & Star Clusters
Coordinates (J2000): RA 19h 23m 50s | Dec 14° 06´ 00"
Observation Date: 2 pointings in June 2003
Observation Time: 20 hours 10 min
Obs. ID: 2524, 3711
References: Townsley, L. et al., 2014, ApJS, 213, 1; arXiv:1403.2576
Color Code: X-ray (Blue), Infrared (Yellow-Orange)
Distance Estimate: About 17,000 light years