Question: How many galaxy's are ther

Sarah Appleby answered on 22 Jun 2020:

Great question! Trying to answer this question is a whole area of astronomy by itself. From what I can tell from reading online, there are an estimated 125 billion galaxies in the universe, although it’s really hard to be sure for a few reasons.

Ideally we would just be able to count up all the galaxies, but there are limits to what we can see with our telescopes because small galaxies and very distant galaxies are extremely faint i.e. not emitting many photons that we can detect with our telescopes. It’s also really challenging and time consuming to count everything across the whole sky.

In general the strategy is to look at small areas of the sky in great detail and try to get the best estimates possible for these smaller areas. Then we have to assume that that area of sky is fairly typical and therefore we can get the total number of galaxies in the universe by just multiplying by the total area of the sky. Astronomers did this with the famous Hubble Deep Field in 1999 and again more recently with the Hubble eXtreme Deep Field, which is where the 125 billion number comes from.

Another complication is that the number evolves with time, because small galaxies are constantly being pulled together under gravity to form more massive galaxies. Galaxies like our Milky Way have small satellite galaxies orbiting it that eventually will become part of the Milky Way itself. We can expect that over time, the number of galaxies should actually decrease, so that’s another reason we need to be careful with making these estimates.