Energy from 10% of stars in Galaxy over their lifetime I heard an assertion that referred to the 'energy from 10% of stars in our Galaxy over their lifetime' on Radio 4 Inside Science and wondered if it is possible to estimate/calculate the energy value.
Furthermore, my colleague misquoted the programme stating '75% of all the stars in the universe, at a single moment'.
How much more/less are the energy values for these two statements?
Please advise, if your time permits.
 A: The way you've phrased the question is a bit strange cause energy at a single moment is usually measured in watts or equivalent (energy per second), but over the lifetime of a star is measured in Joules or equivalent.  Units of energy.   Comparing Joules to Watts is an invitation to all kinds of mathematical errors.
Also, 10% of the stars in our galaxy over the stars lifetime or the Milky-way's lifetime?   Many stars have much shorter lifetimes than the lifetime of the Milky-way.
Also, it should say, "all the stars in the observable universe" not "in the universe" as we can't observe the entire universe so we can't say with any certainty the total number of stars.   It might even be infinite (which, granted, is a bit of a mind-bender), but nobody knows for sure.
An estimate, with adjustments above, can be calculated a number of ways.   The easiest one, I'm going to use Wikipedia.
One Milky-way (current output) - 5x10^36  (1/10th of that, 5x10^35)
One observable universe (I assume, output based on current observation not re-calculated based on passage of time), 2x10^49
Ratio of 1/10th of Milky-way to observable universe, 4x10^13.
Number of seconds in 1 year.  3.1x10^7
Number of years it takes 1/10th of the Milky-way to produce the same energy the observable universe produces in 1 second.   About 1.3 million years.   So, in a nutshell, I'd say, based on what you wrote, that the assertion you heard is not correct.    If a moment is 1 second, then the Milky-way over 13 billion years (it's approximate age), produces about 100,000 times more energy than the observable universe produces in 1 second.   The young Milky-way probably produced significantly more energy than it currently does too, when the massive black hole in the center was feeding, but that can maybe be excluded as "not a star".   
But, 1/10th of the Milky-way over it's lifetime (based on current output) produces about what the observable universe produces in 3 hours, and 3 hours is a long time to be referred to as "a single moment".
Now if you calculate "dark energy" in there too, it might get a lot closer provided you replace "a single moment" with "fraction of a second", but in your question said "stars in the universe" not "energy in the universe".  
Numbers aside, I think Douglas Adams said it best
http://www.brainyquote.com/quotes/quotes/d/douglasada164151.html
