Astrophysics is the science of physical processes in the cosmos. It uses data gathered by astronomers using telescopes on Earth and in space – combined with the laws and theories of physics – in order to interpret the universe around us. If astronomy asks what and where, astrophysics asks how and why. A sister science – planetary science – studies the planets in our solar system and distant solar systems in our Milky Way galaxy. Another sister science – cosmology – studies external galaxies and voids, and the large-scale structure and history of the universe.For example, an astronomer might spend nights at the telescope gathering data on a star. Putting on his or her astrophysicist hat – and depending on which instruments were used in conjunction with the telescope (photometers or spectrometers, for example) – that scientist would then turn to the laws of physics to understand how that star produces its energy, whether it has a companion (or perhaps planets, or perhaps an encircling disk), and how the star moves through space.