Re: How many atoms make up the universe?

Robert:
     That is a really good question.  It is actually extremely complicated 
and lots of astronomers and even mathematicians spend their entire 
careers trying to answer it. I will tell you what we do know, sort of.
     The number of atoms in the Universe is a relatively finite number.  
That is to say, it does not really change by any appreciable amount.  So, 
if there are 100 atoms in the Universe, there will always be 100 atoms in 
the Universe.  
     There is a small problem though.  According to Albert Einstein, 
matter and energy are the same, and under certain circumstances they can 
be made to interchange (this happens in nuclear reactions that take
place in nuclear bombs and at the center of the Sun).  So, it is 
probably more accurate to say that the total energy of the Universe is 
constant, it doesn't change.
     Counting all of these atoms is the difficult part.  In astronomy, we 
use a lot of math to make guesses about how much matter (atoms) we should 
have by looking at how stars and galaxies respond to each other 
(gravity).  Then we look at how much stuff (atoms) we can actually count 
and we always come up short.  This is called the dark matter.
A lot of work is being done to find this dark matter since we 
know it is there but we cannot see it.  For example some of this might be 
locked up in black holes, or in black holes in the middle of galaxies to 
name a few possible places. 
     I apologize for such a long answer, but it was a very difficult 
question you asked.  Just to give you some perspective on how many atoms 
it might be, the number of atoms alone in the graphite in your pencil is 
about 25000000000000000000000 atoms... In just the pencil lead!  WOW!
     But, we should probably give you a number to use, just in case you 
are interested.  This won't include dark matter, brown dwarf stars, dwarf 
galaxies and such, but we will count the atoms in a star and multiply this 
by the number of stars in the Universe, since that is mostly what we
can see when we look out.
  A typical star weighs about 2x10^33 Grams, which is about 1x10^57 atoms of 
hydrogen per star... That is a 1 followed by 57 zeros.  
a typical galaxy has about 400 billion stars so that means each galaxy has
   1x10^57 X 400,000,000,000 = 5x10^68 hydrogen atoms in a galaxy
There are possibly 80 billion galaxies in the Universe, so that means that
there are about:
 5x10^68 X 80,000,000,000 = 4x10^79 hydrogen atoms in the Universe.  But 
this is definately a lower limit calculation, and ignores many possible 
atom sources.  That number Bob, is a 4 followed by 79 zeros.

I hope This helps, for some more astronomy questions, check out:
http://astrosun.tn.cornell.edu/students/lazio/sci.astro.html