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Objectives This exercise will provide an appreciation of the view from Earth of the local stellar neighborhood. In addition, the results of the experiment will

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Counting the Stars in the Sky (Rev 6/12)


This exercise will provide an appreciation of the view from Earth of the local stellar neighborhood. In addition, the results of the experiment will provide a realistic perspective of the significance of the size of the Milky Way compared to the stars viewed in our night sky.


Most everyone can remember looking up at the dark night sky thinking there must be a million stars in the sky. This is especially true if you have seen a clear night sky away from city lights. Just how many stars are there in the sky at night? Answering that question will be the purpose of this lab.

Depending on the weather, this lab can be done outdoors or with a computer planetarium program. You may be wondering how to do this without actually counting every star in the sky. That would be one way, but there has to be an easier method. There is. A very good approximation can be found by counting the star in small random samples of the sky. For instance, we will be pointing some tubes at the sky, and counting the number of stars seen in randomly selected parts of the sky. From the average of these star counts, a little bit of math will tell us what percentage of the sky we were looking at through our tubes. Knowing the percentage of sky seen and average number of stars found there will reveal an approximate number of stars visible in the night sky. It is important to note that the number of stars visible to us in the sky is only a very small fraction of the total number of stars in the Milky Way, our home galaxy. There are a million stars in the Hercules globular cluster, and 200 billion stars in the Milky Way. After completing this lab exercise, it should be clear to you that we only see an itty bitty piece of the Universe from our small planet Earth.

Equipment Needed

  • one tube

  • ruler


Apparatus Construction and Theory Background

Before you go outside, perform the following tasks. Take your ruler and measure the length and diameter of your tube. [see the diagram below]. Record these numbers on your ‘Results’ sheet.

nly part of the sky can be seen through your tube. In effect, the tube cuts out a small circle from whatever part of the sky you look towards. Using the measurements of the tube, you can calculate the size of the area observed in terms of degrees.

The amount of sky seen through your tube is called the field of view. For example, a 5 degree field of view would be the amount of sky seen inside a circle 5 degrees across.

To determine the width of your tube’s ‘field of view’, use the following equation:

*Equation 1*

Once  has been determined, you can find the percentage of sky visible through your tube. Using the idea of relative surface area, the field of view of your tube can be thought of as an imaginary area on an imaginary sphere that represents the sky.

When you look through your tube, only part of the sky can be seen. If you see only twelve stars through your tube, the sky must contain more than 12 stars. But how many exactly? Consider the figure below. Looking through one of these tubes allows only part of the hemisphere of the sky to be seen. The tube allows a fraction of the “area” of the celestial sphere to be seen.

The area of the small tube divided by the area of the hemisphere will be the fraction of the sky seen through the tube. For reference, area of end of the tube = d/2)2

area of the hemisphere = ½ area of sphere = ½ (4L2) = 2L2

Therefore (Equation 2), the ratio of sky seen = area of tube / area of ½ sky. To convert this to a percentage (P), just multiply the ratio by 100%. Now you are ready to go outside. Take something to write with as well as your tube. Since different parts of the sky may have more stars than others, you will make ten separate counts of stars in your tube using random parts of the sky.

Outdoor Experiment

Point the tube to a random part of the sky, and focus on the stars you see inside your field of view. Make a careful count, and record them on your ‘Results’ sheet under trial #1. Repeat this using a different part of the sky each time. Try to keep your hand steady so that you do not count any extra stars outside your field of view. After completing all trials, you can go back inside to finish your calculations and answer the questions on the Results sheet.

Final Calculations

Take the ten star counts you made and find an average, n.

To translate your average star count into the approximate number of stars in the whole sky, you need to consider the fact that your star count only represents a percentage of the whole sky. Earlier, you found P, the percentage of the sky seen with your tube. The problem can be expressed as a word problem:

Let n = ‘the average number of stars seen in your tube.’

n stars are observed in a small part of the sky, which has been found to represent P % of the entire sky. Find N, the total number of stars in the sky, knowing only the number of stars, n, found in P % of the whole sky.

With the information we have already found, you can easily find approximately how many stars in the night sky. The way the above word problem is worded, (P/100) x N = n . So, the N total number of stars visible in the sky is:

*Equation 3*

Congratulations, you’ve counted the number of stars in the sky. Be sure to follow the directions on the Result sheet, and answer the questions there.

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