An accretion disk is a structure formed by gas, dust, and other materials that spiral around a massive object, such as a black hole or a neutron star. As these materials fall toward the central object, they lose energy and heat up, emitting radiation, often in the form of X-rays. This process can create a bright and energetic environment around the massive object. Accretion disks are important in astrophysics because they help scientists understand how stars and galaxies evolve. They also play a crucial role in the growth of supermassive black holes at the centers of galaxies, as they accumulate mass over time.