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SOLUTION TO EXERCISE 3
SOLUTION TO EXERCISE 3 As we've just computed in Exercise 1, there are about 100 billion galaxies in the observable universe. We've also just computed in Exercise 2 that our own galaxy has about 100 billion stars. If we make the reasonable assumption that the Milky Way is a typical galaxy in the universe, then we can estimate that there are (100 billion * 100 billion) = 10,000 billion billion stars in the observable universe. If I count fast, I can count about 5 stars a second. If I am a typical counter among the people on Earth, then together we all can count 5 * (6 billion) = 30 billion stars per second. That means it would take us all about 10,000 billion billion stars ------------------------------ = 333 billion seconds to count all the stars. 30 billion stars/second There are 60 seconds/minute, 60 minutes/hour, 24 hours/day, and about 365 days/year. So there are about 60 * 60 * 24 * 365 = 31,500,000 seconds per year. So it would take 333,000,000,000 seconds ------------------------- = 10,571 years. 31,500,000 seconds/yr Once again, since I've made a number of ballpark assumptions in this computation, it would be incorrect of me to count individual years, decades or even centuries in my answer - I'd just be adding false precision. So I will take the nearest round number and say that it would take about 10,000 years for every human on Earth together to count all the stars in the observable universe. |