• The branch of astronomy which examines astronomical phenomena in terms of the laws and principles of physics. One of the most fruitful branches of the science – and a science in its own right – astrophysics is especially powerful in areas like the properties of stars, interstellar gas clouds, and other large-scale objects, and in the study of the areas where modern nuclear and particle physics can be used to interpret astronomical objects, especially phenomena like pulsars and black holes where relativistic effects are important.