In the old days, people thought that the Sun might be a burning briquette of coal. At the Earth, the intensity of the radiation of the Sun is about 1400 W/m2. Assume that the intensity is constant with time. The mass of the Sun is 2.0×10^30 kg, and its distance from the Earth is 1.5×10^11 m. The heat energy contained in coal is approximately 25 MJ/kg. How long would the Sun shine, if it really were a piece of burning coal?