How Many Galaxies Are There?

The realization that the universe is home to many billions of galaxies is a fairly recent discovery. Prior to the 1920s, the prevailing wisdom was that the Milky Way was the only galaxy, with the rest of the universe being static and eternal. In 1924, astronomer Edwin Hubble discovered that the universe contains an abundance of galaxies beyond the Milky Way. When we look at images of the universe, there appears to be countless galaxies. Counting the number of galaxies in the universe is no easy task, and astronomers rely on estimates since counting each galaxy individually would take an unbelievable amount of time. How many galaxies are in the universe?

Estimating The Number Of Galaxies

It is impossible for any telescope to take a full picture of the universe; space is simply too big. In order to estimate the number of galaxies, astronomers must take wide images of different sections of space and count the number of galaxies. Interestingly, the universe is fairly uniform, and so the number of galaxies in one section of space will be similar to other regions of space. Using this assumption, astronomers can roughly estimate the number of galaxies in the observable universe. 

The Hubble Deep Field

Hubble deep field
The Hubble Deep Field. This image contains an estimated ten million galaxies. NASA

To estimate the number of galaxies in the universe, astronomers used the Hubble Space Telescope to capture a wide image of deep space. The region of interest appeared completely empty at first, yet over the course of ten years, Hubble slowly captured more and more light, revealing millions of individual galaxies in this seemingly empty region of space. Interestingly, the region of space Hubble looked at was smaller than a full moon. The region was so small that no stars in the Milky Way were caught in the image, which just goes to show how empty the region appeared at first. Over time, more and more galaxies came into view, revealing that even the seemingly empty regions of space are full of galaxies. The image became known as the Hubble Deep Field, and it contains roughly ten million galaxies. Most of the galaxies in the image are also the farthest known galaxies in the universe at over 12 billion light years away. By knowing the number of galaxies in the Hubble Deep Field, astronomers can estimate how many galaxies there are in the entire universe. Most estimates place the total number of galaxies between 100 billion and 200 billion. 

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