Subject: concerning measurements in space and others

I've always wondered how scientist have come up with measurements 
concerning space, such as how many billions or trillions of stars,
galaxies there are.  How do you know there are 10 million (for example)
stars per galaxy instead of 12 million.  Or how do you come up with 
an exact temp. of the sun.  It just seems when I hear measurements 
in the astronomical numbers, that you're so exact about it.  
Also, how do you know that the nuclear clock is keeping the precise
time that it does.  Who's to say whether it is off 1 second per 
thousand years and how do you know that the clock measuring that
is not off.
The same question applies with degrees.  How can you tell whether
something is 400,000 degrees or 1,400,000 degrees?

Thanks for your answers,
Bill

Re: concerning measurements in space and others