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Chemistry

A new kind of white dwarf

Representation of a white dwarf (smaller star on the right): new type has an atmosphere composed basically of oxygen

NASA/ESA/G. Bacon (STScI) Representation of a white dwarf (smaller star on the right): new type has an atmosphere composed basically of oxygenNASA/ESA/G. Bacon (STScI)

The final life stage of 98% of all Milky Way stars is to become a white dwarf – a small, hot and extremely dense object, with almost as much mass as the Sun packed into a ball the size of Earth.  After analyzing the absorption spectra of 32,000 white dwarfs catalogued by the Sloan Digital Sky Survey (SDSS), through which researchers can infer the chemical composition of celestial entities, astrophysicist Kepler de Souza Oliveira Filho from the Federal University of Rio Grande do Sul (UFRGS) and collaborators identified a white dwarf with unheard-of properties (Science, April 1, 2016).  Its atmosphere is almost entirely composed of oxygen, with traces of neon, magnesium and silicon.  “No existing theory explains the formation of this kind of white dwarf,” says Kepler.  “We now know that they are rare, but they exist.”  As a general rule, the heaviest elements in a white dwarf (usually carbon and oxygen) are concentrated at the star’s core, while the lightest (helium and hydrogen) are found in its outer layers (the atmosphere).  In the case of SDSSJ1240+6710, the rather awkward name given to this unusual star, atmospheric helium and hydrogen are nowhere to be found.  Astrophysicists speculate that these elements may have vanished as a result of the white dwarf’s interaction with a yet-unidentified companion star.

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