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Star Census




This activity is about students making data observations, analyzing the data locally and then sharing their results with one another.

OBJECTIVE:
To encourage students to observe the quality of the night sky and to determine the number of stars that can be seen from their local area.

ENGAGE:
Ask students how many stars there are outside at night. Accept all estimates and record them on the chalkboard. Ask how they could go beyond guesses and estimates. Tell students that they are going to devise a way to count the stars in the sky. If you have access, tell them their data will become part of a national, on-line collaboration.

EXPLORE/EXPLAIN:
Ask students to explain the phrase "Twinkle, Twinkle, Little Star". Ask them what "twinkling" means. Explain to students that only stars twinkle--the moon and planets do not. As a group, make the predictions as suggested on the activity sheet below. Pick a time for students to make night-time observations of stars.

MATERIALS

EXPAND
Plan a time for students to take a "Star Census". Review with students how to do the counting. If possible, it would be interesting to have students make these observations in different locations (near a city or out in the country) and at different times (when there's a bright moon and when there's no moon). For younger students, you can use fewer observations. Just remember that each observation represents 1/144th of the sky. If students make only 6 observations, they would multiply the total number of stars observed by 24 (which is 144 divided by 6).

PROCEDURE
Have your students try this experiment (at night at the agreed upon time) to measure the number of stars you can see.

  1. Make an "Observing Device" from a bathroom tissue or paper towel tube. Measure the diameter of your tube. Cut its length to be three times its diameter. Through the tube, you will see only a small portion of the sky. It would take 144 tubes to cover the whole sky.

    One by one, face in each of the 4 compass directions (North, South, East and West).

    Hold the tube 3/4 of the way up from the horizon in each direction and count the number of stars seen through the tube. Hold the tube half-way up from the horizon and repeat the count. Repeat the procedure again with the tube pointed a third of the way up. Repeat observations for the other directions.
    (To determine 3/4, 1/2 and 1/3, students can either use a protractor or they can simply estimate the angle)

  2. Add up the number of stars for all 12 sightings. If it takes 144 tubes to cover the sky, then you have observed 1/12th of the sky. Multiply your sub-total by 12 to estimate the total number of stars in the sky. Estimated total number of stars: (includes the stars above and below the horizon)

  3. Add up and compare the three measurements in each direction. Why do you see more stars in certain directions?

Remind students that they need to take RANDOM samples. That means that they need to use the samples where they see no stars at all, not drop that sample in favor of one where they see stars. In urban situations, kids are likely to have "blank" samples. That's what light pollution does to our skies. If urban kids sample UNTIL they have 12 samples with stars, then they are going to have false high readings.

One related resource which you may find interesting is Dave Nash's discussion about light pollution.



ACTIVITIES SHEET: "TWINKLE, TWINKLE, LITTLE STAR"

Have you ever wondered what makes a star twinkle? On the next clear night look at a bright star.

A star is a point of light. It is so far away that even the largest telescope cannot show the star's disk. The atmosphere changing between the star and your eye causes starlight to twinkle.

Make these predictions about twinkling:

Hint: Count star blink rates to answer the first question.

Share your data with students in other locations to answer the remaining questions.

"Seeing" is the term astronomers use to describe the steadiness of images. "Seeing" is best when the twinkling is least. When the seeing is good, astronomers can collect better data about the brightness and color of distant stars and galaxies.


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