5.5 White dwarfs

Abstract

In this chapter we discuss the properties of white dwarfs including a more general discussion of equations of state (EOSs) in the form of polytropes and the resulting properties of compact star configurations. Let us start with a brief review of the history of white dwarfs. 5.1 A Brief History of White Dwarfs White dwarfs where the first type of compact stars to be observed. It happened by serendipity that the first white dwarf was seen in a telescope. The story goes that Alvan Clark tested his father's new 18-inch refracting telescope in 1862. He pointed it to the brightest star in the sky, Sirius in the constellation Canis Major, which means 'great dog'. Therefore, Sirius is also called the 'dog star'. However, what seemed to be one star for the naked eye or for a small telescope turned out to be double star system in the new powerful telescope of Alvan Clark's father. Besides the bright main star Sirius A, there is a small companion star close to it, Sirius B, which is now called the Pup. In follow-up observations, the mass has been determined to be close to the mass of the Sun and the radius to be similar to the size of Earth, which leads to an average density orders of magnitude larger than that of ordinary stars. In 1915, the spectrum of Sirius B was measured and found to be 'white', implying a surface temperature similar to the considerably larger companion star Sirius A. The extreme compactness of Sirius B could be confirmed by a measurement of the gravitational redshift of spectral lines originating from the surface of the star in 1925. The modern values for Sirius B are M = 1.018 ± 0.011M , R = 0.00864 ± 0.00012R , and T eff = 25.193 ± 37 K from observations with the Hubble Space Telescope (Barstow et al., 2005; Bond et al., 2017a). As of writing this book, Sirius B is the closest and brightest known white dwarf. The theoretical explanation for the unusual properties of this white dwarf came a year later, when Dirac formulated the Fermi-Dirac statistics, the basis for 92