We now consider why stars shine with such extreme brightness. Over the long-term (i.e., millions of years), the enormous energy emitted comes from the energy generated (by nuclear fusion) in the stellar core, as discussed further in Chapter 18. But the more immediate reason stars shine is more direct, namely because their surfaces are so very hot. We explore the key physical laws governing such thermal radiation and how it depends on temperature.

Jean Perrin argued in the early twentieth century that the agreement, or “convergence,” of measured values of Avogadro’s number was compelling evidence for the existence of atoms. Max Planck argued, in a similar way, that the convergence of measured values of Planck’s constant was compelling evidence for the quantization of energy. Philosopher John Losee has argued that convergence of the measured value of a new constant of nature is the strongest possible evidence for the correctness of the theory that contains the constant. Milgrom’s theory contains such a constant (the “acceleration scale,” a0, or “Milgrom’s constant”), and this chapter presents the results of observational determinations of the value of that constant. The values are convergent, suggesting, according to Losee’s argument, that Milgrom was justified in postulating a modified acceleration law in place of dark matter.