Polaris is a cepheid variable star; this means that its temperature and apparent luminosity vary drastically over relatively short periods of time, although they vary predictably. Cepheid variable stars were not always cepheid variables, nor will they, necesarily, always be such stars; it would probably be more accurate to say that a star is exhibiting cephoid variability. In any case, Polaris shows at present a great deal of variance in terms of brightness,
and in its position in an H-R diagram.
Polaris is a moderately cool star at around 8,000 K, if 'cool' can even apply in the temperaure ranges that we are dealing with here. Polaris is a bright star, with a luminosity of about 2400 times that of the sun. Or, rather, Polaris A is a bright star, with a luminosity of about 2400 times that of the sun; Polaris B, a smaller star that orbits Polaris A, has a luminosity 8 times that of the sun, and although there is a Polaris C, it is too small and orbits too closely to Polaris A to be effectively measured. So far, I have been talking about Polaris A only; any former references to Polaris apply to Polaris A. Relatively little is known about the smaller two Polarises simply because they are so small and distant that very little light from them reaches us, and what little does reach us is swamped by that from Polaris A. The fact that Polaris is a trinary system containing a Cepheid variable star is likely a major cause for much of the confusion about it; no one seems to be entirely sure on any of the fine details about Polaris, as I mentioned earlier. The fact that it is over 400 (431 ± 27) light years away from Earth doesn't make matters any simpler.
Given my estimated mass for Polaris, 4 solar massses, it should use up all of its hydrogen over a period of roughly one billion years. But, of course, one of the many things that we don't yet know about Polaris is its age. Because it is not clearly still in a nebula, but it is still burning hydrogen as a (Cepheid-variable variant) main sequence star, it is fairly safe to assume that it is about middle-aged at this point. So it has probably burned so far for about five million years, and it will likely continue to burn for another five million.