What’s Out There

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What’s out there?


The nearest known white dwarf is Sirius B, at 8.6 light years, the smaller component of the Sirius binary star. There are currently thought to be eight white dwarfs among the hundred star systems nearest the Sun.

maximum mass for a non-rotating white dwarf, the Chandrasekhar limit—approximately 1.46 M  –  This was done for Sirius B by 1910,[31] yielding a mass estimate of 0.94 M

White dwarfs have low luminosity and therefore occupy a strip at the bottom of the Hertzsprung–Russell diagram, a graph of stellar luminosity versus color (or temperature). They should not be confused with low-luminosity objects at the low-mass end of the main sequence, such as the hydrogen-fusing red dwarfs, whose cores are supported in part by thermal pressure,[50] or the even lower-temperature brown dwarfs.[

Although white dwarf material is initially plasma—a fluid composed of nuclei and electrons—it was theoretically predicted in the 1960s that at a late stage of cooling, it should crystallize, starting at its center.[69] The crystal structure is thought to be a body-centered cubic lattice.[5][70] In 1995 it was suggested that asteroseismological observations of pulsating white dwarfs yielded a potential test of the crystallization theory,[71] and in 2004, observations were made that suggested approximately 90% of the mass of BPM 37093 had crystallized.[69][72][73] Other work gives a crystallized mass fraction of between 32% and 82%.[74] As a white dwarf core undergoes crystallization into a solid phase, latent heat is released which provides a source of thermal energy that delays its cooling.[75]

Metal-Rich White Dwarfs[edit]

Around 25-33% of white dwarfs have metal lines in their spectra, which is unusual because any heavy elements in a white dwarf should sink into the star’s interior in just a small fraction of the star’s lifetime.[86] The prevailing explanation for metal-rich white dwarfs is that they have recently accreted rocky planetesimals.[86] The bulk composition of the accreted object can be measured from the strengths of the metal lines. For example, a 2015 study of the white dwarf Ton 345 concluded that its metal abundances were consistent with those of a differentiated, rocky planet whose mantle had been eroded by the host star’s wind during its asymptotic giant branch phase

The first magnetic white dwarf to be observed was GJ 742, which was detected to have a magnetic field in 1970 by its emission of circularly polarized light.[91] It is thought to have a surface field of approximately 300 million gauss (30 kT).[78], §8

The magnetic fields in a white dwarf may allow for the existence of a new type of chemical bond, perpendicular paramagnetic bonding, in addition to ionic and covalent bonds, resulting in what has been initially described as “magnetized matter” in research published in 2012.[94]  

A white dwarf can also be cannibalized or evaporated by a companion star, causing the white dwarf to lose so much mass that it becomes a planetary mass object. The resultant object, orbiting the former companion, now host star, could be a helium planet or diamond planet.[120][121]



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