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White Dwarfs Exposed: Planetary Nebulae The ‘Ring’ Nebula ‘Planetary’ Nebulae discovered by Herschel Planetary Nebulae are Shells that Look Like Rings despite the name, they have nothing to do with planets Soap Bubble Shells Can Also Look Like Rings! note the very prominent edges Some Examples [thousands are known] The Shell is ‘Puffed Off’Gently In total, a considerable fraction of the star’s mass comes off gently, at speeds of perhaps 30 km/sec. Only part of the original stellar mass is left behind. Note that the material in the nebula is raw star stuff – not the products of the nuclear reactions deep within the core! It is almost pure H + He. The escaping material eventually merges into the interstellar medium, and is available to be used in other stars that may form later. This Process Leaves Behind Incandescently Hot Dense Cores The Cores… These intensely hot, luminous cores are new white dwarf stars that will never succumb to gravity. Thanks to their extreme heat, they emit an enormous amount of ultraviolet light. That energetic radiation causes the gas in the planetary nebula to glow by fluorescence, like a celestial neon lamp. The nebula disappears after some tens of thousands of years as (i) the gas dissipates and (ii) the central star cools down and becomes fainter. Some Real Examples Found Everywhere Cold carbon clinkers – ‘diamonds in the sky’ [although not like any diamond you ever encountered! – a million times the density of water, held up by electron degeneracy – but nevertheless full of Carbon nuclei] There are literally tens of millions of such stars in our galaxy. “Like a Diamond…” (sort of!) White Dwarfs in Plenty! Here, in the core of the globular cluster M4 The ‘Cooling Track’ The prominent sequence to the right is the main sequence. All the brighter hotter stars (to the upper left) have evolved away; the bright red giants lie to the upper right of this figure, off the plot. The star shown in yellow has been there for billions of years, essentially unchanging. The red spot to the left is a recently-formed white dwarf – still relatively hot and bright. It will cool off and get fainter, moving down along the cooling track. Many other white dwarfs have already done so. The Sun’s Life In Summary