Number of Atoms in the Universe

How Scientists Determine How Many Atoms There Are in the Universe

The universe is made up of atoms. The estimated number of atoms in the universe is based on the size of the known universe.
The universe is made up of atoms. The estimated number of atoms in the universe is based on the size of the known universe. Panoramic Images, Getty Images

The universe is vast. Have you ever wondered how many atoms are in the universe? Scientists estimate there are 1080 atoms in the universe. Obviously, we can't go out and count each and every particle, so the number of atoms in the universe is an estimate. It is a calculated value and not just some random, made-up number.

Explanation of How the Number of Atoms Is Calculated

The calculation of the number of atoms assumes the universe is finite and has a relatively homogeneous composition.

This is based on our understanding of the universe, which we see as a set of galaxies, each containing stars. If it turns out there are many such sets of galaxies, the number of atoms would be much greater than the current estimate. If the universe is infinite, then it consists of an infinite number of atoms. Hubble sees the edge of the collection of galaxies, with nothing beyond it, so the present concept of the universe is a finite size with known characteristics.

The observable universe consists of approximately 100 billion galaxies. On average, each galaxy contains about one trillion or 1023 stars. Stars come in different sizes, but a typical star, like the Sun, has a mass around 2 x 1030 kilograms. Stars fuse lighter elements into heavier ones, but most of the mass of an active star consists of hydrogen. It is believed 74% of the mass of the Milky Way, for example, is in the form of hydrogen atoms.

The Sun contains approximately 1057 atoms of hydrogen. If you multiple the number of atoms per star (1057) times the estimated number of stars in the universe (1023), you get a value of 1080 atoms in the known universe.

Other Estimates of Atoms in the Universe

Although 1080 atoms is good ballpark value for the number of atoms in the universe, other estimates exist, mainly based on different calculations of the size of the universe.

Another calculation is based on measurements of cosmic microwave background radiation. Overall, the estimates of the number of atoms range from between 1078 to 1082 atoms. Both of these estimates are large numbers, yet they are very different, indicating a significant degree of error. These estimates are based on hard data, so they are correct based on what we know. Revised estimates will be made as we learn more about the universe.

Mass of the Known Universe

A related number is the estimated mass of the universe, which is calculated to be 1053 kg. This is the mass of atoms, ions, and molecules and excludes dark matter and dark energy.

References

"Astronomers size up the Universe". BBC News. 2004-05-28. Retrieved 2015-07-22.
Gott, III, J. R. et al. (May 2005). "A Map of the Universe". The Astrophysical Journal 624 (2): 463–484.