|MadSci Network: Astronomy|
A difficult question to answer! If you mean the Sun must have been continuously cooling over the course of its lifetime, the simple answer is that it hasn't been! The sun burns by nuclear fusion and so has a constant power source (or will have until the fuel begins to run out in 4,000 million years time or so). However, the Sun is a variable star and varies slightly in brightness and activity on short time scales. Such variability causes changes in climate here on Earth - the mini-ice age felt, at least in Europe during the 18th century corresponded to a period of low activity on the Sun. Few sunspots were seen (it was known as the Maunder minimium) and when activity picked up again the climate became warmer. As we've only been able to make proper studies of sunspot activity since the discovery of the telescope, however, we don't have any detailed longterm data of these fluctuations so I can't provide the exact detail you're looking for. Hope that helps! [Moderator's note: it's also important to understand that the Sun doesn't have "a" temperature. The core, surface (or photosphere), and corona have very different termperatures, and these don't necessarily change in the same way as a star evolves.]
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