Yellow Stars, known scientifically as G-Type Stars, are a type of main sequence star that appears yellow. Earth's star, known as Sol is yellow. The energy produced by yellow stars can be processed by Kryptonians and Daxamites, via the Photonucleic Effect. Red Stars, such as Krypton's star of Rao, produce more red light, which prevent Kryptonians and Daxamites from gaining powers.
Solar energy from the yellow sun is the source of energy for Kryptonian powers. When Superman is exposed to large amounts of sunlight, his powers can be greatly magnified. If a Kryptonian is powerless, absorbing a large amount of sunlight radiation can regenerate their powers.