Now, before you go on wondering as to whether, because of the Universe’s expansion and all, this number can be growing, note that what’s taking place is simply a mere expansion or dispersion of the masses that make up the Universe. That is, no amount of matter that wasn’t there in the beginning is ever added during this expansion.
Another slight complication that you might want to take into consideration is Einstein’s equivalence of mass and energy. Hence, between all the fusions and fissions, you’ll have chunks of atoms converted from particles to energies and back. Still, you might want to know how in the world (or universe) were scientists able to get even just a rough estimate, so sit back and enjoy a few seconds of some simple math and a lot of guesswork.
Let’s start with the biggest lump of matter or atoms in the Universe – the galaxies. While a German supercomputer recently ran a simulation and obtained around 500 billion as its estimate, lets try to be a little conservative and assume that there are just around 300 billion. Now, since the number of stars in a galaxy can run up to 400 billion, then the total number of stars may very well be around 1.2×1023. Let’s just peg that at 1023.
We’ll have to use scientific notation from this point onwards as the numbers we’ll be dealing with are going to be very large.
On the average, each star can weigh about 1035 grams. Thus, the total mass would be about 1058 grams. Since each gram of matter is known to have about 1024 protons or about the same number of hydrogen atoms (since 1 hydrogen atom has only 1 proton), then the total number of hydrogen atoms should be roughly 1082.
Once again, this number is just a rough estimate. When use it to estimate the total mass of the Universe, it falls short of what other estimates predict. This is one of the reasons why scientists believe in the existence of a so-called dark matter. In fact, much of what compose entire galaxies are believed to be dark matter. What we see is just a smaller fraction of the whole.
We’ve got a few articles that are related to the number of atoms in the Universe here in Universe Today. Here are two of them:
NASA also has some more:
Tired eyes? Let your ears help you learn for a change. Here are some episodes from Astronomy Cast that just might suit your taste: