Question

The Hubble Ultra Deep Field led to the discovery of 10,000 galaxies in a small area of the sky covering 1.6×10−3 deg2 . It is estimated that there are 7.7 times more galaxies in that region of the sky which are too faint for the Hubble to see. Assume this area of the sky is representative of the full observable Universe from Earth.

Clearly stating your assumptions, estimate:

(i) The number of galaxies detectable with Hubble over the entire sky and the total number of galaxies in the observable Universe.

(ii) The total number of stars in the observable Universe.

(iii) The total number of planets in the observable Universe.

[Hint: the full sky contains 3602/π deg2 .]

Answer 1

(i) The density of observable stars by Hubble is,

Therefore the total number of observable stars over full sky is,

(ii) Total number of stars is 7.7 times of observable stars by Hubble, Therefore we can calculate the total number of stars as,

(iii) The average number of planets in one galaxy is equal to, 150 billion therefore we can estimate the total number of planets as,