<blockquote><font class="small">In reply to:</font><hr /><p>“In 1978 the astrophysicist Michael Hart performed detailed calculations and reached a stunning conclusion. His work included the well know fact that the sun becomes slightly brighter with time. About 4 billion years ago, the sun was about 30% fainter than at present.” <p><hr /></p></p></blockquote><br /><br />I think it varies more over a short period of time that the gradual brightening over the millenia is not ordinarily significant during one's lifetime.<br /><br />The Sun's output is remarkably steady, but still somewhat variable. Overall, it varies on a period of about eleven years, due to changes in its magnetic fields. (This is similar to the Earth's periodic pole shifting, but on a larger and also much faster scale -- the Sun's poles flip every eleven years!) We are currently moving towards solar minimum, when the Sun has the least number of sunspots. Heat output varies during the solar cycle, with it generally being hottest during solar maximum. However, that's not universal, and the Sun has probably been both hotter and cooler than it is now. Like any star, it is very dynamic. Solar output may in fact be partially responsible for global warming, although it's very difficult to get historical data on solar output going back more than a few decades.<br /><br />Fortunately, our Sun isn't hugely variable. There are some very dramatic variable stars (including some special ones called Cepheids that can be used to measure distances by the regularity of their variation) which couldn't possibly have habitable planets.<br /><br />Note: I'm going from memory, and need more coffee, so look this stuff up and make sure it's right before quoting it anywhere. <img src="/images/icons/wink.gif" /> (And if I'm wrong, please tell me!) <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em> -- The Tenth Doctor, "Blink"</font></p> </div>