### Re: How many atoms make up the universe?

Date: Fri Sep 11 02:55:50 1998
Posted By: Matthew Champion, Grad student, Biochemistry/Biophysics, TexasA&M University
Area of science: Astronomy
ID: 904950601.As
Message:
```
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
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
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

```

Current Queue | Current Queue for Astronomy | Astronomy archives