# Estimating the number of galaxies

by jimjohnson
Tags: estimating, galaxies, number
 P: 84 The Hubble deep sky image is 3 x3 arc minutes and shows an estimated 10,000 galaxies. What percent is 3 x 3 arc minutes of the total area? If this ratio is known, one could estimate the number of total galaxies. Thanks
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Thanks
P: 26,148
hi jim!
 Quote by jimjohnson What percent is 3 x 3 arc minutes of the total area?
the total area is 4π,

and 3 arc minutes is π divided by … ?
 P: 84 I know the area of a sphere is 4 x π x r2 . But what percent is the area of 3 x 3 arc minutes? When you look up at a dome, a square is not a square except at the horizon. There may be a simple answer but it is not obvious to me. Thanks
 P: 887 Estimating the number of galaxies For small angles, you can approximate the sky sphere as flat. Just find the area of the square. Should be close enough.
 Sci Advisor PF Gold P: 9,452 Compared to the celestial sphere, 3x3 arc minutes only comprises about one ten millionth of the surface area of the entire sky. All you need to do is calculate the surface area of the celestial sphere and compare it to 9 square arc minutes.
 P: 84 Thanks, I was having a mental block. The result is 5.2 x E10 galaxies which is similar to other estimates assuming a finite universe.
P: 4,804
 Quote by jimjohnson Thanks, I was having a mental block. The result is 5.2 x E10 galaxies which is similar to other estimates assuming a finite universe.
Not assuming a finite universe. These are just measures of the visible galaxies. The total number, including the parts of the universe we can't observe, is almost certainly vastly larger. How much larger? We don't know.
 P: 84 Good point, I should have said the "observable universe". Also, in the original calculations I erroneously inserted the diameter of the Moon rather than the radius. Thus, the correct number of galaxies extrapolated from the Hubble ultra deep field image is 1.56 x E11. My calculations are probably unorthodox ( a simpler way may exist). But, I used the Moon's radius to compute the Moon's area. The Moon's distance from the Earth was used to compute the surface area of a sphere. Dividing the larger area by the smaller area defined the number of Moons required to fill the entire sphere, 2 x E5. Addressing the ultra image, 3 x 3 arc minutes (9 square arc minutes) is 78 times less than the area of the Moon with a 15 arc minute radius (706 square arc minutes).The factor used to extrapolate from the 10,000 galaxies in the image was 1.56 x E7 (2 x E5 x 78) which gives 1.56 x E11 galaxies. I did the same calculation for the Hubble eXtreme deep field which is 2.3 x 2 arc minutes and has an estimated 5,500 galaxies. The answer was 1.67 x E11 galaxies - consistent results. The Hubble images support an estimate of 155-165 billion galaxies in the observable universe. Comments?