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The sun only generates a milliwatt per kilogram(or even less) so is the sum a feeble energy source compared to an ant or any form of life here on Earth? In order to clarify my perspective on how a body like the sun relates to a body like an ant I want to say they are both an assembly of atoms. The fact that energy is emitted in very different ways makes no difference in the fact that the living assembly emits many times more energy per kilogram than the star.

jim meyer
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    Are people or ants a good source of energy? What is your criteria for an energy source? How are you measuring it's usefulness? – JMac Nov 28 '17 at 21:11
  • People or ants don't produce energy at all. We just consume energy. – Chris Nov 29 '17 at 00:32
  • @Chris People do produce energy, as per John Donne's answer below. Of course we consume more energy than we produce, but so does the Sun, and everything else in the Universe. – pela Nov 29 '17 at 07:16
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    @pela Well really the correct way of saying it is that we don't consume or produce energy and neither does anything else. The sun is a "producer of energy" largely in the sense that it converts useless-to-humans energy into useful-to-humans energy. Which is effectively the opposite of what people do ;) The sun could also be said to be an energy producer because there is a constant outflow of power (larger than the inflow) over its lifetime, which is not true of humans. – Chris Nov 29 '17 at 07:54
  • @Chris I think we're entering the realm of semantics here, but of course you're right that no is energy ever consumed not produced, just converted. My point was just that both in the case of the Sun and in the case of folks you do have an outflow of energy ($\propto AT^4$), at the cost of at some point having to put in some energy, be it infalling gas or dark bread with liver paté. – pela Nov 29 '17 at 16:18

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If $R_\odot$ is the radius of the sun, $T_\odot$ its surface temperature, and $M_\odot$ its mass, by the Stefan-Boltzmann Law, the emitted power per unit mass of the sun is: $$\frac{E_\odot}{M_\odot}=\frac{4\pi R_\odot^2 \sigma T_\odot^4}{M_\odot} \approx 2 \cdot 10^{-4} \mathrm{Jkg^{-1}} $$ If on the other hand we consider a human at temperature $T\approx 300K$, mass $M\approx 100kg$ and approximate surface area $S\approx 1m^2$, the same law tells us that the power per unit mass emitted by the human is about: $$\frac{E}{M}=\frac{S \sigma T^4}{M}\approx 5 \mathrm{Jkg^{-1}}$$ This is probably the comparison you're referring to.

However I have to point out that comparing electromagnetic output with mass is not a particularly relevant comparison. Moreover, if the sun was switched off the temperature of the human would quickly drop, resulting in essentially null output per unit mass.

John Donne
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  • Your $E_\odot/M_\odot$ is a factor $10^{15}$ too low. – pela Nov 28 '17 at 22:41
  • So if we integrate the energy output over the lifetime of both objects, since the Sun will last more than 9 orders of magnitude longer than the ant, it's actually the more productive one, isn't it? – stafusa Nov 30 '17 at 08:54
  • @stafusa Yes even restricting to the Sun's main sequence (so we don't worry too much about mass and output variation) over a lifetime the Sun would be more productive – John Donne Nov 30 '17 at 11:17
  • @JohnDonne, you mean over the Sun's lifetime, no the ant's, right? – stafusa Nov 30 '17 at 11:19
  • @stafusa Yes that's what I meant – John Donne Nov 30 '17 at 11:21