The effective temperature of the Sun is around 5,778 K. The nominal value defined by the International Astronomical Union for use as a unit of measure of temperature is 5,772 ± 0.8 K. Stars have a decreasing temperature gradient, going from their central core up to the atmosphere. The "core temperature" of the Sun—the temperature at the centre of the Sun where nuclear reactions take place—is estimated to be 15,000,000 K.