A scientist who studies bodies such as black holes, moons, planets, stars, nebulae, and galaxies, as well as Gamma-ray bursts and background radiation. A related but distinct subject, cosmology, is concerned with studying the universe as a whole. Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using physical laws. Today, that distinction has mostly disappeared and the terms "astronomer" and "astrophysicist" are interchangeable. Professional astronomers are highly educated individuals who typically have a PhD in physics or astronomy and are employed by research institutions or universities. They spend the majority of their time working on research, although they quite often have other duties such as teaching, building instruments, or aiding in the operation of an observatory.