Hot Stuff

The Sun has a mass slightly less than 2 e+30 kg, and radiates a bit less than 4 e+26 Watts (Joules/sec). That amounts to about 2/10,000 of a Watt per kilogram. For comparison, a 60 kg human (132 lbs) produces about 100 Watts.

On a Watt per kilogram basis, a human is thus producing about 10000 times as much heat as the Sun. So why is the Sun so much hotter and brighter that you are? One way of looking at it is that the Sun has far less surface area per kilogram than you do - something like a billion times less, so the amount of heat leaving it per unit area is about 100,000 times more.

Another thing to think about is the reaction rate. Conversion of Hydrogen to Helium is quite slow in the Sun - an average Hydrogen nucleus (proton) may bump around for ten billion years or so before it (and three friends) get converted to Helium. A molecule of ATP in a human cell only lasts a couple of seconds.

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