The universe is vast. Scientists estimate there are 1080 atoms in the universe. Since we can't go out and count each particle, the number of atoms in the universe is an estimate. It is a calculated value and not just some random, made-up number.
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 of 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 multiply 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 a 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.
- Whitehouse, David. "Astronomers size up the Universe." BBC News, May 28, 2004.
- Gott III, J.Richard, et al. “A Map of the Universe.” The Astrophysical Journal, vol. 624, no. 2, IOP Publishing, May 2005, pp. 463-84.