Whenever we look up at a clear night sky, we can see a multitude of stars. From here on Earth, if you are in an area without light pollution, you can see over 10,000 individual stars. That may sound like a lot, but it represents less than 1% of all the stars in our galaxy. The Milky Way is home to about 100 billion stars, and the Milky Way is just one of the potential trillions of other galaxies. Is it possible to estimate the number of stars in the universe?
Estimate the number of stars
Hubble image of a distant galaxy containing billions of stars. Image credit: NASA/ESA
If you tried to count the stars individually, it wouldn’t take long to lose track. Even if you tried to count all the stars in the Milky Way, it would take years before you finished. Counting the number of stars in the universe would take millions and millions of years, so counting them individually is simply impossible. Rather, astronomers find ways to estimate the number of stars. Since most stars exist inside galaxies, estimating the number of galaxies in the universe is a good first step to estimating the number of stars in the universe. However, estimates of the number of galaxies vary significantly, ranging from 100 billion to 10 trillion. In 2016, a study of Hubble images provided an estimate of two trillion galaxies, which is generally the number used to estimate the number of stars. Astronomers then assume that the Milky Way is a fairly typical galaxy, and the number of stars it contains is the average number of stars contained within a given galaxy. With 100 billion stars in the Milky Way and two trillion galaxies in the observable universe, we simply multiply 100 billion by two trillion. Thus, the number of stars in the universe is estimated to be 200 billion trillion. To give you an idea of how big that number is, here’s the whole set: 200,000,000,000,000,000,000,000.
Problems with estimates
There are some obvious problems with estimating the number of stars in the universe. First, we can only estimate the number of stars in the observable universe. The universe itself extends far beyond what we can actually see, and beyond our cosmic horizon, there are likely more galaxies and more stars. When we estimate the number of stars in the universe, it only represents the regions of the universe that we can physically see. Also, the universe could be infinite in size, in which case there could be an infinite number of stars in the entire cosmos.
A second problem is in the assumption that the Milky Way is a typical galaxy. Galaxies vary significantly in the number of stars they contain, with some containing many fewer stars than the Milky Way and some containing many more. Also, the number of stars in the Milky Way is estimated from the mass of our galaxy, which is about 100 billion solar masses. This means that the total number of stars in the Milky Way is equivalent to 100 billion solar masses. If our galaxy only contained sun-like stars, it would contain 100 billion stars. However, most stars are low-mass stars called red dwarfs, so the number of stars in our galaxy probably exceeds 100 billion.
A third problem is the fact that the number of galaxies in the observable universe is currently unknown, and the number of two trillion is probably a large count. There are probably trillions more galaxies in the observable universe. When we take all this into account, the actual number of stars probably far exceeds the estimated number of 200 billion trillion.
Aidan Remple July 31, 2022 in Science