<P> Bessel roughly estimated the period of the companion of Sirius to be about half a century; C.A.F. Peters computed an orbit for it in 1851 . It was not until 31 January 1862 that Alvan Graham Clark observed a previously unseen star close to Sirius, later identified as the predicted companion . Walter Adams announced in 1915 that he had found the spectrum of Sirius B to be similar to that of Sirius . </P> <P> In 1917, Adriaan van Maanen discovered Van Maanen's Star, an isolated white dwarf . These three white dwarfs, the first discovered, are the so - called classical white dwarfs . Eventually, many faint white stars were found which had high proper motion, indicating that they could be suspected to be low - luminosity stars close to the Earth, and hence white dwarfs . Willem Luyten appears to have been the first to use the term white dwarf when he examined this class of stars in 1922; the term was later popularized by Arthur Stanley Eddington . Despite these suspicions, the first non-classical white dwarf was not definitely identified until the 1930s . 18 white dwarfs had been discovered by 1939 . Luyten and others continued to search for white dwarfs in the 1940s . By 1950, over a hundred were known, and by 1999, over 2,000 were known . Since then the Sloan Digital Sky Survey has found over 9,000 white dwarfs, mostly new . </P> <P> Although white dwarfs are known with estimated masses as low as 0.17 M and as high as 1.33 M, the mass distribution is strongly peaked at 0.6 M, and the majority lie between 0.5 and 0.7 M . The estimated radii of observed white dwarfs are typically 0.8--2% the radius of the Sun; this is comparable to the Earth's radius of approximately 0.9% solar radius . A white dwarf, then, packs mass comparable to the Sun's into a volume that is typically a million times smaller than the Sun's; the average density of matter in a white dwarf must therefore be, very roughly, 1,000,000 times greater than the average density of the Sun, or approximately 10 g / cm, or 1 tonne per cubic centimetre . A typical white dwarf has a density of between 10 and 10 g / cm . White dwarfs are composed of one of the densest forms of matter known, surpassed only by other compact stars such as neutron stars, black holes and, hypothetically, quark stars . </P> <P> White dwarfs were found to be extremely dense soon after their discovery . If a star is in a binary system, as is the case for Sirius B or 40 Eridani B, it is possible to estimate its mass from observations of the binary orbit . This was done for Sirius B by 1910, yielding a mass estimate of 0.94 M . (A more modern estimate is 1.00 M .) Since hotter bodies radiate more energy than colder ones, a star's surface brightness can be estimated from its effective surface temperature, and that from its spectrum . If the star's distance is known, its absolute (overall) luminosity can also be estimated . From the absolute luminosity and distance, the star's surface area and its radius can be calculated . Reasoning of this sort led to the realization, puzzling to astronomers at the time, that Sirius B and 40 Eridani B must be very dense . When Ernst Öpik estimated the density of a number of visual binary stars in 1916, he found that 40 Eridani B had a density of over 25,000 times the Sun's, which was so high that he called it "impossible". As Arthur Stanley Eddington put it later in 1927: </P>

The range of masses for white dwarf stars
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