Human: I've always been fascinated by the stars. What are the different types of stars and how do they form? Assistant: 
The different types of stars are classified according to their temperature, size, luminosity, and chemical composition. Stars that have a high temperature and large size are called main-sequence stars, while those with lower temperatures and smaller sizes are known as red giants. Luminosity is determined by a star’s mass and surface gravity, which determines how fast it burns its fuel and emits energy. Chemical composition refers to the elements found in the star’s atmosphere, such as hydrogen, helium, and carbon.