Astrophysics Since the Middle of the Twentieth Century

Abstract

The Universe that we could still discover as children in the 1950s, by reading or listening to our elders, comprised the planets of the Solar System, the Sun, stars, and galaxies. Our own star, the Sun, is just one of 100 billion others making up the Milky Way, the galaxy it belongs to. The planets gravitate around the Sun and a few moons, or natural satellites, orbit these planets, just as our own Moon revolves around the Earth. We knew the mass, the distance from the Sun, and the size of the planets and their moons, but little else apart from this handful of numbers. It was known at the time that the stars were made of gas and that nuclear reactions in their core transform hydrogen into helium and thereby release the energy that allows them to shine. The main lines of the evolution of these stars were also known. We knew that some galaxies closer than others are bound together into immense clusters, but without understanding why these systems should have remained distinct from their surroundings since the beginning of time, without dissolving into the rest of the cosmos. The expansion of the Universe had been clearly established. It seemed likely that the Universe must cool as it expands, whence it must have been much hotter in the past than it is now.