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.