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37 STELLAR EVOLUTION by Schatzman. The more the tem- perature rises, the more the metals persist or reappear in the atmo- sphere – especially if of internal ori- gin – since above a certain energy flow threshold (with surface tem- peratures above 50,000 -100,000 kelvin), the foreign material in outer space is not longer able to fall on the dwarf, in that it is evap- orated. (A brief note on the tem- perature scale. At such high heat levels, the kelvin (K) are basically equivalent to degrees Celsius; only at low temperatures the dissimi- larities become significant.) Since white dwarfs give off to the external envi- ronment their stored heat, which in the ab- sence of a thermonuclear process is not re- plenished, they are destined to cool down in line with their initial physical properties. If the origin of the metals was internal (or mostly internal), we should therefore expect that the type and abundance of metals in the atmospheres of white dwarfs vary in ac- cordance with the mass, temperature and age, and that they were consequently those synthesized in the core of the progenitor stars. But once again, theoretical predictions and reality differ: where radiative levitation is more efficient it could happen to not ob- serve traces of metals, while the opposite is sometimes true where no metal should be able to rise to the surface, and this regardless of the aforementioned fundamental factors. This lack of consistency has, at various times, led to suppose that the source of the metals was external (or mostly external), but up un- T he most well- known ex- ample of white dwarf is that of Sirius B, which can be seen top right in a realistic visualization dom- inated by the bril- liant Sirius A. [NASA, ESA and G. Bacon (STScI)] On the side: a real image of that stellar system taken by the Hubble Space Telescope. [NASA, ESA, H. Bond (STScI), and M. Barstow (Univer- sity of Leicester)]