NGC 3918

NGC 3918.

NGC 3918 is the brightest planetary nebula in the southern constellation of Centaurus, nicknamed the “Blue Planetary” or “The Southerner”. It was discovered by Sir John Herschel in March 1834, and around 4900 light-years from Earth. The slightly oval diameter is telescopically between 8 and 10 arcsec, though deep images extends this to about 19 or 20 arcsec. More surprising is the beautiful rich blue colour that looks much like the coloured images of Neptune taken by Voyager 2 in 1989.

The central star is 14.6 visible light magnitude, and remains invisible to optical observers, as it is obscured by the sheer brightness of the surrounding nebula. In the center of the cloud of gas, and completely dwarfed by the nebula, are the dying remnants of a red giant. NGC 3918’s distinctive eye-like shape, with a bright inner shell of gas and a more diffuse outer shell that extends far from the nebula looks as if it could be the result of two separate ejections of gas. But this is in fact not the case: studies of the object suggest that they were formed at the same time, but are being blown from the star at different speeds. The powerful jets of gas emerging from the ends of the large structure are estimated to be shooting away from the star at speeds of up to 350,000 kilometers per hour. By the standards of astronomical phenomena, planetary nebulae like NGC 3918 are very short-lived, with a lifespan of just a few tens of thousands of years.

The chemical content of the planetary nebula NGC 3918 is investigated through deep, high-resolution UVES at VLT spectrophotometric data. More than 750 emission line have so far been identified and measured, making it one of the deepest spectra ever taken for a planetary nebula. The high ionization of NGC 3918 was detected, which explained the large presence of recombination lines of multiple ionization stages of C, N, O and Ne. The abundances obtained for these elements by using recently-determined state-of-the-art Ionization Correction Factor schemes or simply adding ionic abundances are in very good agreement, demonstrating the quality of the recent ICF scheme for high ionization planetary nebulae.

These extraordinary and colourful planetary nebulae are among the most dramatic sights in the night sky, and often have strange and irregular shapes, which are not yet fully explained. Studying images of proto-planetary nebulae is important to understanding the process of star death. A star begins to die when it has exhausted its thermonuclear fuel – hydrogen and helium. The star then becomes bright and cool (red giant phase) and swells to several tens of times its normal size. It begins puffing thin shells of gas off into space. These shells become the star’s cocoon. The intense ultraviolet radiation from the tiny remnant star then causes the surrounding gas to glow like a fluorescent sign. In the Hubble images, the shells are the concentric rings seen around each nebula.