Re: How high is the temperature of the Sun when it borned out?

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.]