It was one of the first planetary nebulae discovered: it was first observed in September 1782 by William Herschel.
NGC 7009 is also known as Saturn Nebula. It is not difficult to imagine the reason: it can in fact remember the appearance of the planet with the rings seen edge on.
At the center of the nebula there is a white dwarf of 11th apparent magnitude and a surface temperature above 55,000 degrees Kelvin. It was this star that when it was in the final stages of its life in the red giant phase ejected its surface gases into space to form the nebula.
The nebula is visible as its gases emit after being ionized by the ultraviolet radiation produced by the white dwarf.
The structure of the Saturn Nebula is particularly complex: it is not in fact a simple spherical shell of gas that surrounds the central white dwarf, but consists of a series of substructures and polar jets (the two outer red regions) whose dynamics have not yet been understood by astronomers.
Credit: NASA, ESA, Hubble.
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