Luminous stars are celestial bodies that emit light and heat due to nuclear fusion occurring in their cores. This process converts hydrogen into helium, releasing energy that makes them shine brightly. The brightness of a star is measured in terms of its luminosity, which can vary significantly among different types of stars. These stars can be classified into various categories based on their size, temperature, and brightness. For example, main sequence stars like our Sun are stable and burn hydrogen for billions of years. Other types include giant stars and supergiants, which are much larger and more luminous, often ending their life cycles in spectacular explosions called supernovae.