Our Earth feels like all there is, but we know that it’s just a tiny planet in a vast Solar System. And our Solar System is just one member of a vast Milky Way galaxy with 200 to 400 billion stars. But how many galaxies are there in the entire Universe?
This is a difficult number to know for certain, since we can only see a fraction of the Universe, even with our most powerful instruments. The most current estimates guess that there are 100 to 200 billion galaxies in the Universe, each of which has hundreds of billions of stars. A recent German supercomputer simulation put that number even higher: 500 billion. In other words, there could be a galaxy out there for every star in the Milky Way.
As I mentioned, these numbers are considered rough estimates. In order to create these estimates, astronomers use a powerful telescope, like the Hubble Space Telescope, to deeply study a region of the sky. By gathering light for hundreds of hours, Hubble is able to see more deeply than any Earth-based telescope could ever hope to look. Astronomers count up the number of galaxies in the cone of space that makes up the deep image, and then use this as an average for the rest of the sky. Even though they’re really only observed a tiny fraction of the sky at that depth, they can estimate the rest.
Most of the galaxies in the Universe are probably tiny dwarf galaxies. For example, in our Local Group of galaxies there are only 3 large spiral galaxies: the Milky Way, Andromeda, and the Triangulum Galaxy. The rest are dwarf and irregular galaxies.
We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.