Free Astronomy Magazine

STELLAR EVOLUTION their abundances and the physical properties of white dwarfs. As expected, only in hotter atmospheres (the temperature of those sur- veyed ranges from 16,000 to 77,000 kelvin) the accretion of elements from the outside appears to be seriously hindered by the strong stellar radiation. The strongest evi- dence in favour of a planetary origin of the metals observed in the white dwarfs’ spectra consists in the relative abundances of carbon, silicon, phosphorus and sulphur, which are quite comparable to those typically found in rocky planets of our solar system. The solution proposed by Barstow and col- leagues seems to finally resolve a tricky issue that has lasted for decades, as well as offer- ing at the same time interesting points for discussion, such as, for instance, the fact that if about a quarter of white dwarfs appear to be contaminated by planetary remains, it is likely that, on average, at least 1 out of 4 stars below 8-10 solar masses possesses pla- nets. Furthermore, future acquisitions of spectra at much higher resolution than those supplied by FUSE, will allow to precisely de- fine the chemical composition of no longer- existing planets and to understand how they have evolved within their systems. Finally, those white dwarfs are also providing us a preview of how the Sun will appear in 5-6 bil- lion years or more, when in its light a possible future alien civilization will perhaps discover what is left of our beautiful planet. an accretion disk that supplies metals to the atmosphere of degenerate stars; until the supply lasts, that is. The great variety that the inner planets of a planetary system may have in terms of number, mass, distance and composition, would justify the randomness of the abundances of metals in the atmo- spheres of white dwarfs, regardless of the physical properties of these latter and the efficiency of radiative levitation. The team led by Barstow has come to this conclusion by examining the spectra of 89 white dwarfs (the largest sample surveyed to date), taken by the Far Ultraviolet Spectro- scopic Explorer (FUSE), which allowed them to establish that at least 23% of dwarfs of type DA contain metals and to confirm that there are no direct relationships between n W hite dwarfs are stars that although rather common are very difficult to discover and study. In this example on the right, some of them present in a small portion of the globular clus- ter M4 have been highlighted with circles. [H. Bond (STScI), NASA] The video below shows a white dwarf transiting over the disk of a red dwarf. De- spite its size, the ultra-dense white dwarf can signifi- cantly deform the space around it and alter the image of the red dwarf. [NASA]