There is no such thing as an "Ultra massive" white dwarf. A white dwarf can never be more than 1.44 solar masses, otherwise electron degeneracy pressure will not be sufficient to stop gravitational collapse to a neutron star. So a 1.2 solar mass white dwarf is within the normal range of masses.
Hubble just observed something ultra-rare — an ultra-massive white dwarf star formed from a white dwarf merging with another star, instead of the usual process of white dwarf formation. Findings from this study raise questions about the origin of the currently known white dwarfs.
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