MadSci Network: Physics

Re: How many atoms are there in the 'world', solar system, & universe?

Date: Mon Apr 13 12:17:08 1998
Posted By: John Dreher, Staff Astronomer, SETI Institute
Area of science: Physics
ID: 892448375.Ph

Well, most of the universe (90% by number) is made up
of Hydrogen atoms (the simplest atom, just a proton and
an electron).  So I'll simplify things by working out the
equvalent number of hydrogen atoms for each object.  The
actual number of atoms will always be somewhat less, since
all other atoms are heavier.  For the Sun and Solar System,
using the actual compostion will reduce the number by about 20%.
For the Earth, the actual number will be...mmmm...say 40 times
less, since the earth is mainly made up of fairly heavy atoms.
For the universe as a whole, I have given a LOWER LIMIT for
the number of atoms.  We don't yet really know what the mass
of the universe is, nor just how may atoms are in it.

I have given the results both in scientific notation,
for example 1.2E+57 atoms in the Sun, meaning 1.2
multiplied by 1 with 57 zeros after it, and in "normal" format.
Since in the "normal" forma these numbers get rather long,
I have placed them on their own lines -- still they may "wrap"
around onto more than one line when you view them on the Web.

Object	        Mass (kg)	Number of Atoms (assuming all Hydrogen)	
Hydrogen atom	1.67E-27	1	

Earth	        6.42E+23	3.8E+50 

Sun	            1.99E+30	1.2E+57	
Solar System	1.99E+30	1.2E+57	

Universe	    1.99E+52	1.2E+79	

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