How many galaxies are there?

In a previous FAQ, I mentioned that a recent Hubble Space Telescope image revealed that there may be over 50 billion galaxies or more in our visible universe. Most of these are very small and faint and would look like the Magellanic Clouds if we could see them up close. The Hubble Deep Field image shows hundreds of galaxies in a region of the sky only an arcminute across; about 1/30 the diameter of the full moon. If this spot of the sky is typical, you can easily estimate how many galaxies there are:

There are 42,000 square degrees over the full sky, and 60x60 = 3600 spots as big as the Hubble Deep Field per square degree, so the total galaxies is about 3600 x 42,000 x 200 = 3.6 x 4.2 x 2 billion or about 40 billion.

In our Local Group there are about thirty galaxies, of which only five would be visible at the distance probed by the Hubble Deep Field. This means we have to multiply 40 billion by about 6 to account for these smaller galaxies. That gives us an estimate of about 240 billion galaxies in our visible universe.

I think if you asked me this question in a year from now, I would come up with about the same kind of estimate!

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