- M 4 (NGC 6121) and NGC 6144
M 4 (on the left side of the image) is the well-known globular cluster near fiery Antares in the constellation Scorpius. It contains at least 100,000 stars, with more than 10,000 of them brighter than magnitude 19. The stars in M 4 are not as densely packed as in some other prominent globular clusters, like M 13 in Hercules. It's actual diameter is only around 57 LY, so its apparent visual size is due to its closeness. At 5640 LY, M 4 is the closest globular cluster to us. M4 has an age of around 12,700 MY and contains numerous white dwarf stars. In contrast, the globular NGC 6144 (on the right side of the image) is a less-than-spectacular object. It's relatively small and sparse, and is much farther away than M 4 (around 29,000 LY). This distance contributes to its dimmer appearance since, located near the galactic plane, its light experiences considerable attenuation and reddening by the intervening amounts of interstellar dust. The reddish traces of nebula visible along the upper right side of the image are part of a large nebular structure centered on Antares that bears the collective identifier IC 4606. The common name for this feature is the "Antares Nebula." It is a vast region of dust and gas illuminated by Antares itself (which would be just outside the upper right corner of the image). The presence of this nebula increases the background brightness, which contributes to the difficulty in resolving NGC 6144.
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