I have already elaborated very often on the death of ordinary stars and the evolution of the planetary nebulae that form when these stars eventually collapse. This time I'd like to highlight a very interesting phenomenon that happens when these planetary nebulae expand way beyond the original boundaries of their "solar system" and into intergalactic space.
NGC6772 is a planetary of a certain age in the constellation of Aquila, the eagle. The gas shell that was violently expelled during the star's collapse has smashed through the faint outer shell which had already formed many thousands of years earlier, during the last stage of the star's life. Now, it has reached interstellar space and the giant gas bubble that's blowing up at a rate of 30 km/s crashes into a medium with completely different mechanics. Interstellar dust is moving in other directions than matter within the gravitational influence of the former star, driven by the gravitational pull of our galaxy, and the expanding planetary nebula finds it ever harder to plough through it. Gas at the border of the nebula's building up, as if it were hitting a brick wall, and we observe a significant brightening there. The nebula's not spherical anymore, deformed as it is by areas of less or more interstellar resistance. Its central star's cooling down and losing brightness quickly, up to the point that it isn't visible through amateur telescopes anymore, and only shows itself on long-exposure photographs.
Yet, the nebula will continue to plough through interstellar space, at an ever decreasing rate, until it completely dissolves.
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