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marcus

Science Advisor

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Some say the sun will go red giant when it has consumed 12 percent of its hydrogen.

The sun is now 75 percent hydrogen. We know its mass so we can tell how much hydrogen is supposed to be consumed before red giant stage.

I like trying out natural units (c = G = hbar = 1) which seem

surprisingly serviceable in a wide range of contexts, so I calculated the lifetime of the sun in those units as a check for ant284. It turned out we both got the same estimate of the lifetime!

The key thing one must know is the power or "wattage" of the sun. This determines how fast the hydrogen is being consumed,

and allows one to estimate the life.

What do you think the wattage of the sun is? If you like using metric units then of course you will wish to express the sun's power as a certain number of watts. What number?

Ant284 posted a number of watts for the sun, down in Homework section. But why not guess first? It seems that the sun is important to us so shouldn't we have some idea of its power?

Compared to the natural unit of power c

^{5}/G, the sun's power is 1.07E-26. That is just what fraction (about E-26) of the natural power unit it happens to be.

And in terms of the natural unit of energy, conversion of that much hydrogen (12 percent of 75 percent of the mass of the sun) will release 6.0E34 units of energy.

So essentially one just divides 6E34 by E-26 to get the life.

Or more precisely 6E34 by 1.07E-26.

But you may prefer to make the corresponding calculation in metric...