Monday, 18 September 2017

Sh 2-71: of yet unknown origin

Sharpless 2-71 (or in short Sh 2-71) is another one of those objects with an unusual name. That's because it was only discovered in 1946, long after the Messier and NGC catalogues were compiled. Nonetheless it is a beautiful, yet unknown planetary nebula in Aquila (the eagle) that definitely deserves a bit more attention. It's not the brightest of objects and medium to large telescopes are required in order to see it clearly, and preferably also a sufficiently dark sky. But the first thing that you'll undoubtedly notice is its highly irregular shape. Unlike most planetaries that are round or at least symmetrical, this nebula looks like if a fifty-tonne truck has just run over it. 

The reason for its irregular shape is thought to be its fairly bright central star, which is in fact a binary (which I wasn't able to resolve at 190x). A companion of the dying star would undoubtedly distort the nebula's form. However, recent investigations have shed some doubt about this star's parental claim. It doesn't seem to emit enough (high energy) ultraviolet radiation and also its faint companion would not really fit the right profile. 

Possibly a more likely candidate would be the tiny star just below the brighter one. It fits the right sort of brightness which you could expect from the nebula's distance (3.200 light-years), but it is unknown at the moment if this is also a double star.

An even more tempting thought, would be that all three stars are involved. The brighter one does emit a large amount of broad hydrogen-alpha radiation, which also appears in some other planetary nebulae. The nebula's multi-polar structure, with its many lobes that formed at different times, suggests that a very complex formation process which at least requires a binary star to explain. Or... perhaps we've just discovered one of the Universe's threesomes?


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