Burnham's Celestial Handbook . A Source Book in Astronomy and Astrophysics . They are thought to be the final evolutionary stage for stars whose masses are less than 1.4 times the sun's mass. Their future is to cool down very slowly until they become black dwarfs, unable to radiate any more energy. Most people look at the properties of a white dwarf and say that it is pretty much unclear as to a definite calculation of its density.
Science is only as accurate as the methods and materials that the scientists of the day are using.
Our present knowledge based upon scientific instruments is far from definite. Many books and references show their calculations to be final. The accuracy of this information is debatable. As we can see in the table above, there is no definite answer on the density of a white dwarf star. Astronomers and physicists can only make approximations on the subject.