The other type—the type that Zwicky observed—begins life as a hydrogen-rich star like our own Sun. As it ages, the Sun will shed its outer hydrogen layer while its core contracts under gravitational pressure. In the end, only the core will remain—a shrunken skull called a white dwarf, with the mass of the Sun packed into the volume of Earth. If a white dwarf had a companion star (and most stars in our galaxy do), then at this point it might start to siphon gas off the other star. In the 1930s, the Indian mathematician Subrahmanyan Chandrasekhar calculated that when a star of this kind reaches
...more
