Stars are luminous balls of gas, mainly hydrogen and helium, that shine due to nuclear fusion reactions in their cores. They range in size from red dwarfs, which are smaller than Earth, to supergiants, which can be thousands of times larger than the Sun. Stars vary in color, temperature, and brightness, and they evolve over time, going through different stages until they eventually die. Stars are the basic building blocks of galaxies and are essential for life as we know it, as they provide light, heat, and energy to planets that orbit them.