NASA - National Aeronautics and Space Administration

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Count the Stars

Now that you've completed the instructions at Prepare for Your Star Count, you are ready to go out and perform the Star Count activity. The Star Count activity should be done on a clear night. It is best to have no clouds. A few scattered clouds are all right if you use only clear parts of the sky for the Star Count.
Important: Allow your eyes to become "dark-adapted." Go outside to the darkest spot you can find and wait for at least 20 minutes before you begin this activity. This waiting time allows your eyes to be at their most sensitive to the faint light of stars.

Safety Warning: Do this activity with an adult. Don't go into a dark spot unless you know it is safe. Use a flashlight when walking in the dark.

Once your preparations are complete, you are ready to start counting stars! Image Credit: NASA

Start Counting

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).

The Star Count Data Sheet has a box where you can calculate the total number of stars. The equation uses the average count per observation and the fraction of the sky you can see through your tube. This calculation uses the length and radius of your viewing tube. (Why does the fraction of the sky you can see depend on the length and radius of the tube? Can you think of an experiment to prove the amount of sky you can see does depend on length and radius of the tube?)

1. Point the Star Count viewing tube at a random (chance) point in the sky. Close one eye. Hold the end of the tube up to your open eye. Count the number of stars that you can see through the tube. If you see no stars through the tube, you must count zero. Don't move the tube to look for stars. (Of course, if you are looking at a house, trees, a hill, cloud or anything but clear sky, you can point the tube in a different direction.)

2. Record the number of stars you counted on your printed copy of the Star Count Data Sheet. Use the line after "Star Count 1" in the Record Observations section.

3. Repeat procedures 1 and 2 nine more times, for a total of 10. Point your viewing tube at a different location in the sky. Make sure that you choose the direction at random (by chance) again. Do not move the tube to see more stars or to select brighter stars. Do not count when looking at anything but clear sky. Record the number of stars you count each time on your data sheet. Use the lines after "Star Count 2" through "Star Count 10."

4. Find the total number of stars you saw. Record this number in the TOTAL blank on the Star Count Data Sheet.

5. Find the average number of stars. Record this number in the AVERAGE blank on the Star Count Data Sheet.

6. Measure the distance, in meters, from the nearest security light, street light or other bright light (if less than 50 meters). Record the distance on the Star Count Data Sheet.

Be sure to double-check your math for errors! Image Credit: NASA

Calculate Star Count
1. The Star Count Data Sheet has a box where you may calculate the total number of stars you can see in the sky.

2. The first line has an equation. This equation allows you to turn the average of your 10 random counts into a number that is a very good approximation of the total number of stars you can see.

3. The third line repeats the equation with empty lines for l, r and AVERAGE. Write in the values of the viewing tube length (l), tube radius (r), and the AVERAGE of all 10 Star Count observations into the correct boxes of the equation.

4. Solve the equation with your values of l, r and AVERAGE. This number is the Star Count number for your Star Count site on the date you performed your observations. Be sure to double-check your math for errors.

Star Count investigators from around the globe can use this data. Image Credit: NASA

Enter Data Online

Now you can enter your data online for students everywhere to use. Check to see that you have entered data into all boxes for which you have data.

When all of the data are entered, click the Add Data button at the bottom of the Star Count Data page. You have just entered your Star Count Site Data into a database that can be viewed worldwide.
Enter Data Online  →

Investigation: Do people everywhere see the same number of stars in the night sky? Why or why not?

Doing your own Star Count is only the beginning. Research other students' observations to answer the questions above. Information about the location of observations may be helpful. Use all available information. Why might other students report a different number of stars?
View Other Student Data   →