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The Question

(Submitted November 15, 1997)

Hi, we were wondering how you would determine the mass of the visible universe by finding

a)the number of galaxies in the universe

and

b)the mass of an average galaxy.

This is a real poser for us; nobody seems to have an answer more accurate than 'a lot.' So could you give us the best estimate we have today of the number of galaxies in the universe? and the average mass of a galaxy? or at least give us a clue as to how to find or determine these things?

The Answer

The Hubble Deep Field is 2.7 arcminutes on a side "with nearly 3000" detected galaxies.

Astrophysical Formulae by Kenneth Lang discusses how galaxy mass is measured, and gives a list of 106 galaxies and their masses. These range from 0.25 to 2080 billion solar masses, with most within a factor of a few of ~10 billion. These are the masses determined by looking at how rapidly they turn, so not all of this mass is 'visible mass', it includes intragalactic dark matter.

HOWEVER, the sample of galaxies that have had their masses measured is a VERY biased sample of relatively nearby galaxies, and the Hubble Deep Field looks largely at young galaxies as they were a long time ago, before a lot of evolution (e.g. galaxies gobbling up other galaxies) occurred.

David Palmer and Jeff Silvis
for Ask an Astrophysicist

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