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NGC 6501

A lenticular galaxy in the southeastern parts of Hercules, NGC 6501 forms part of a small galaxy group with five other nearby galaxies. Another memmber of this group, the spiral NGC 6500, lies close to NGC 6501, and the two are commonly listed as a pair. The distance of NGC 6501 from the Milky Way is somewhat uncertain. Most sources place it at more than forty megaparsecs, with a typical value equating to 154 million light years, but others estimate it as being rather closer at about 34 megaparsecs (equating to about 111 million light years).

This uncertainty over its distance means that the the diameter of NGC 6501 is difficult to calculate precisely. The greater distance implies a diameter of appoximately 94,000 light years (just a little smaller than the Milky Way), while the nearer distance estimate suggests that the galaxy is rather smaller, with a diameter of about 67,000 light years.

The lenticular galaxy NGC 6501, with its apparent companion spiral NGC 6500 seen to the southwest. NGC 6500 is slightly asymmetric in form, with a string of stars extending towards NGC 6501, seen here on the eastern (left) side of the galaxy's disc. The bright object in the southeast of this image is a foreground star, seventh-magnitude HD 163640. Imagery provided by Aladin sky atlas

A further complication of this uncertainty over distance is that the nature of its relationship with neighbouring NGC 6500 remains unclear. Based on some estimates, the two galaxies may be no more than three million light years apart, while alternative data suggests a rather greater separation from one another. Given the distances involved, the two seem unlikely to have be interacting with one another at present. However, spiral NGC 6500 shows a notable stream of stars running in the apparent direction of NGC 6501, which may imply some degree of gravitational interaction between the two galaxies, at least at some point in the past.