<P> While the unusual spectra of red giant stars had been known since the 19th century, it was George Gamow who, in the 1940s, first understood that they were stars of roughly solar mass that had run out of hydrogen in their cores and had resorted to burning the hydrogen in their outer shells . This allowed Martin Schwarzschild to draw the connection between red giants and the finite lifespans of stars . It is now understood that red giants are stars in the last stages of their life cycles . </P> <P> Fred Hoyle noted that, even while the distribution of elements was fairly uniform, different stars had varying amounts of each element . To Hoyle, this indicated that they must have originated within the stars themselves . The abundance of elements peaked around the atomic number for iron, an element that could only have been formed under intense pressures and temperatures . Hoyle concluded that iron must have formed within giant stars . From this, in 1945 and 1946, Hoyle constructed the final stages of a star's life cycle . As the star dies, it collapses under its own weight, leading to a stratified chain of fusion reactions: carbon - 12 fuses with helium to form oxygen - 16; oxygen - 16 fuses with helium to produce neon - 20, and so on up to iron . There was, however, no known method by which carbon - 12 could be produced . Isotopes of beryllium produced via fusion were too unstable to form carbon, and for three helium atoms to form carbon - 12 was so unlikely as to have been impossible over the age of the Universe . However, in 1952 the physicist Ed Salpeter showed that a short enough time existed between the formation and the decay of the beryllium isotope that another helium had a small chance to form carbon, but only if their combined mass / energy amounts were equal to that of carbon - 12 . Hoyle, employing the anthropic principle, showed that it must be so, since he himself was made of carbon, and he existed . When the matter / energy level of carbon - 12 was finally determined, it was found to be within a few percent of Hoyle's prediction . </P> <P> The first white dwarf discovered was in the triple star system of 40 Eridani, which contains the relatively bright main sequence star 40 Eridani A, orbited at a distance by the closer binary system of the white dwarf 40 Eridani B and the main sequence red dwarf 40 Eridani C. The pair 40 Eridani B / C was discovered by William Herschel on January 31, 1783; it was again observed by Friedrich Georg Wilhelm Struve in 1825 and by Otto Wilhelm von Struve in 1851 . In 1910, it was discovered by Henry Norris Russell, Edward Charles Pickering and Williamina Fleming that despite being a dim star, 40 Eridani B was of spectral type A, or white . </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 and 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 than colder ones, a star's surface brightness can be estimated from its effective surface temperature, and hence from its spectrum . If the star's distance is known, its overall luminosity can also be estimated . Comparison of the two figures yields the star's radius . 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 . For example, 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". </P>

What theories account for the origin of the solar system