Why are stars sometimes referred to as automatic furnaces?

Stars are often referred to as automatic furnaces because they generate immense heat and energy through the process of nuclear fusion in their cores. In these reactions, hydrogen atoms fuse to form helium, releasing vast amounts of energy in the form of light and heat, similar to a furnace burning fuel. This self-sustaining process allows stars to maintain their brightness and temperature over billions of years, akin to an automated system continuously producing energy.