NGC 6164 and NGC 6165

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This image shows way more than strictly NGC 6164 and 6165. NGC 6164 is the bright bar located in the lower (North) part of the central red nebulosity. NGC 6165 is the bright bar located in the upper (South) part of it. This nebula is the result of the mass ejection by an unfrequent star type, O6.5f (only five such stars are known, three in our own galaxy and two in the Small Magellanic Cloud).

This nebulosity is the result of a two-step fierce mass ejection from the central star. HD 148937 is a massive, very hot star, with a mass about 40 times that of our Sun. This kind of stars live fast and last short. Its age has been calculated in about four million years and still has about two to three million years to go. At its end, it will most probably explode as supernova. This kind of stars (O6.5f) burn hydrogen through the CNO cycle. This cycle is responsible for the notably higher than usual concentration of nitrogen in the nebula. According to one theory, the complex structure that is seen in the inner nebula can be due to the star’s rotation in addition to strong magnetic fields that can be sculpting the whole gas nebula. Several so called “cometary knots” can be spotted in the brightest bars and are probably due to the impact of the stellar wind on slower, denser shells of gas.

The external bluish nebulosity is much fainter and very rich in OIII. This bubble-like nebulosity is the result of an earlier mass ejection by the same star. It already interacts with the surrounding hydrogen-rich gas in its region.

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