Saturday, August 26, 2006

How to count the stars

Over at NASA's Star Count, students from all around the world can learn how to count the stars and participate in this global project to find out if people see different numbers of stars. The results will then be displayed using Google Maps. Cool. From Star Count:
It is impossible to count every star in the night sky. The method you will use will allow you to get very close to the actual number. This method is like those used in surveys. Imagine that you want to find out how many of the students in your school would support a longer school day. It would be very difficult to ask every student in a large school. Your friends' opinions probably wouldn't represent all of the students accurately. If you stood at the door of your cafeteria and asked every 20th student, you could probably get an accurate survey. At a very big school, you might choose every 30th student. Each student was chosen at random (by chance) to help your survey represent everyone.

To count the stars you will choose 10 parts of the sky at random (by chance) and count the stars. Then you will find the average number of stars counted in each observation. (For example, if you observed zero stars in one count and 100 stars in another count, the average is 50 stars per count.)

But, the average number of the stars per count isn't the same as the total number of stars in the sky! Imagine that the average number of stars per count is 50. What fraction of the sky did you observe each time? If you observed 1/10th of the sky each time, then there are about 500 stars (10 times 50). If you observed 1/100th of the sky in each observation, there are about 5,000 stars (100 times 50).

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