2. The number of stars in the observable universe totals around 1022, with ~101

Question

2. The number of stars in the observable universe totals around 1022, with ~101 of them sitting in a typical galaxy and a total of ~ 1011 galaxies lying within our cosmic horizon It is claimed that "the number of grains of sand on all the beaches on Earth is comparable to the number of stars in the observable universe". We now introduce a skill called ' order of magnitude estimates' to check this claim To begin with, use your everyday experience to estimate the size of a typical grain and its volume Then you could try any sensible approach you could think of to estimate the total volume of sand on the surface of the Earth. For example, you could estimate the length, depth and width of a typical sandy beach, and the number of such sand beaches around the globe; or you can take a look at the great deserts on Earth... hopefully all your inaccuracies in the end cancel each other e.g., you may put in an estimate for the width that is too low, but an estimate for the depth that is too high, so it doesn't matter in the end - and your final answer is accurate to with a couple magnitudes. Now what do you think about the claim? Using the same technique, can you estimate the total number of cells in your body? Google the answer and see how close you are

Explanation / Answer

If we line up grains then there are approx. 25 grains in 1 cm, which accounts for 15,625 grains per cubic centimeter and thus 1.56 x 1010 grains of sand per cubic meter .

A beach is typically approximately 70 meters wide and 50 meters in depth

Now for length we will see the circumference of earth and it is appoximately 40,212,386 meters (By using formula 2 x phi x radius of earth)

Therfore volume of earth is = 40,212,386 x 70 x 50 = 1.4074 x 1011 cubic meters

Now, we know volume of beach and sand grain per volume so we can calculate total number of grains by multiplying both the values i.e.

1.56 x 1010  x 1.4074 x 1011

2.1990 x 1021

This value is comparable to the number of stars in our universe.

Now let's talk about the human body, the average amount of water displaced by a human is around 70 liters and this can be used to calculate the volume of a human being which is equal to this 70 liters

volume = 70 liters

1m3 contains 1000 liters therefore 70 liters is equal to 0.07 cubic meters 0.07 m3

Volume of a human cell is around 1000 x 10-18 m3

Total number of cells = volume of human being/ volume of a cell

Total number of cells= 0.07 /1000 x 10-18 = 7 x 1013

Now as per various literature there are approx. 5.7±0.8×1013 cells present in human body and our value is quiet comparable with this value.

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