A planetary nebula, often abbreviated as PN or plural PNe, is a kind of emission nebula consisting of an expanding glowing shell of ionized gas ejected from old red giant stars late in their lives. The word 'nebula' is Latin for mist or cloud and the term 'planetary nebula' is a misnomer that originated in the 1780s with astronomer William Herschel because when viewed through his telescope, these objects appeared to him to be newly forming planetary systems. Herschel's name for these objects was adopted by astronomers and has not been changed. They are a relatively short-lived phenomenon, lasting a few tens of thousands of years, compared to a typical stellar lifetime of several billion years.
A mechanism for formation of most planetary nebulae is thought to be the following: at the end of the star's life, during the red giant phase, the outer layers of the star are expelled via pulsations and strong stellar winds. The exposed hot, luminous core emits ultraviolet radiation that ionizes the ejected outer layers of the star. This energized shell of nebulous gas reradiates the absorbed ultraviolet energy at visible frequencies and appears as a planetary nebula.
Planetary nebulae may play a crucial role in the chemical evolution of the galaxy, returning material to the interstellar medium from stars where heavy elements, the products of nucleosynthesis (such as carbon, nitrogen, oxygen and calcium), have been created. In more distant galaxies, planetary nebulae may be the only objects that can be resolved and yield useful information about chemical abundances.
NGC 6302, also called the Bug Nebula, Butterfly Nebula, is a bipolar planetary nebula in the constellation Scorpius. The structure in the nebula is among the most complex ever observed in planetary nebulae. The spectrum of NGC 6302 shows that its central star is one of the hottest stars in the galaxy, with a surface temperature in excess of 200,000 K, implying that the star from which it formed must have been very large (cf. PG 1159 star).
The central star, a white dwarf, was only recently discovered (Szyszka et al. 2009), using the upgraded Wide Field Camera 3 on board the Hubble Space Telescope. The star has a current mass of around 0.64 solar masses. It is surrounded by a particularly dense equatorial disc composed of gas and dust. This dense disc is postulated to have caused the star's outflows to form a bipolar structure (Gurzadyan 1997) similar to an hour-glass. This bipolar structure shows many interesting features seen in planetary nebulae such as ionization walls, knots and sharp edges to the lobes.