Solar Luminosity---huge energy output! The first basic question about the Sun is how bright is it? It puts out A LOT of energy every second. How much? The answer from our measurements is 4 × 10 26 watts. Such a large number is beyond most of our comprehension, so let's put the Sun's total energy output (ie., its luminosity ) in more familiar units. It is equal to 8 × 10 16 of the largest power plants (nuclear or hydroelectric) on the Earth. Our largest power plants now can produce around 5,000 Megawatts of power. Another way to look at this is that the sun puts out every second the same amount of energy as 2.5 × 10 9 of those large power plants would put out every year---that's over two billion! Possible Sources of Energy What could produce that much energy every second? Let's first rule out other likely candidates. How about chemical reactions? The most efficient chemical reaction is combining two hydrogen atoms and one oxygen atom to make a water molecule plus some energy. Such a reaction has a
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