If astronomers detect an unseen companion orbiting a star, and that unseen companion's mass is calculated to be 1.3 solar masses, that unseen companion is most likely a white dwarf
White dwarfs are stars that have burned up all of the hydrogen they once used as nuclear fuel. Because of this Fusion in a star's core produces heat and there occur an outward pressure, which is being balanced by the inward push of gravity generated by a star's mass.
The heaviest observed white dwarf has a mass of around 1.2 solar masses, while the lightest weighs only about 0.15 solar masses.
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