Astronomers can measure the temperature, the size, and the lifetime of stars. They know why stars shine and how they generate energy. They understand how stars are born, how they live, and how they die. Have you ever wondered how astronomers know all these things about stars? In this simple introduction to spectroscopy, you and your students can learn how astronomers do it. In this exercise, you will determine what stars are made of by Breaking the Secret Code of Starlight.

A Lesson Plan by:
Dr. Joan T. Schmelz
University of Memphis
Department of Physics
Memphis, TN 38152
Site Design by:
Matthew T. Scott
University of Memphis
Department of Electrical Engineering
Memphis, TN 38152

Table of Contents:
  Objectives:
Grade Level:
Prerequisites:
Time Requirements:
Introduction:
Materials:
Procedure:
  Part I:
  Part II:
  Part III:
Student Activities:
Assessment:
Closure:
 

Objectives:
  Students will be able to identify how Astronomers know what stars are made of.
Students will understand how starlight is spread into a spectrum, and what this spectrum signifies.
 
Grade Level:
  This lesson was specifically designed for 5th grade students But it has been adapted for use by middle school and high school students and even teacher groups
 
Prerequisites:
  This lesson requires nothing more than:
an interest in learning about stars
an elementary understanding of elements and atoms
 
Time Requirements:
  If a brief introduction to stars is included, this lesson requires approximately 1 hour to complete.
 
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Introduction:
 

A night sky full of stars inspires awe and wonder, not just today, but ever since humans have existed. We now know that those stars are similar to our Sun, but that they are so far away that their enormous size is reduced to a point of twinkling light. How can astronomers know what stars are make of when no scientist has ever been to a star, and no astronaut has ever brought back a sample of stellar material that we could study in our laboratories or in our classrooms? Despite these limitations, astronomers know with certainty what stars are made of. They do this by studying the one thing that the star sends us: its light. Astronomers must do two things in order to study starlight:

Spread the light out into its component colors, i.e., its spectrum, like a flamenco dancer opens a fan;
Understand what the light is trying to tell us, like a code breaker reading an encrypted message.


The students will use a simple diffraction grating to spread out the light from different gasses (Neon, Krypton, Mercury, and Hydrogen). They will draw the patterns they see on a worksheet that becomes their code breaker. Then they will compare these patterns with the actual spectrum of a star and determine which of their code-breaker patterns best resembles the starlight pattern.

This elementary introduction to the science of Spectroscopy will help the students understand how astronomers know so much about stars. This works because the light from each element emits a unique spectrum. It doesn't matter if the light is emitted in the classroom by a spectrum tube, in the chemistry laboratory by a sample under investigation, or by a star many light-years from the solar system. The pattern of colored lines is the same.

Spectroscopy is not only used in astronomy, but also in forensic science as seen in the hit television show, CSI: Crime Scene Investigation, in cosmetic science to identify the oils in a perfume, and by nature every time we see a rainbow in the sky.

 
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Materials:
  Worksheets for Each Student/Group of Students
Markers or crayons in the colors of the rainbow (red, orange, yellow, green, blue, violet).
Prism(s) (if desired, to illustrate splitting light into it's spectrum)
Diffraction gratings *
Spectrum Tube Power Supply *
Spectrum Tubes (Neon, Krypton, Mercury, and Hydrogen) *

* Available from Edmund Scientific, www.scientificsonline.com, 1-800-728-6999
 
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Procedure:
Part I
  Ask the students to name the different kinds of things astronomers study. (List of examples)
An introduction to stars.
Briefly discuss how astronomers know so much about stars, commenting that:
  We cannot reach a star, even one as close as our own Sun
  We cannot bring back a sample of a star to study, like astronauts brought back moon rocks
Ask the students to name the only thing that the stars send us (Answer: Starlight).
There is a message hidden in the starlight, but it is Compressed: we need to spread it out; show example of an actual star spectrum.
Encrypted: we need to decode it; in this exercise, the students will build the code breaker
Print out the example of a 'secret message' and fold it up like a fan.
Show the fan to the students; if we want to understand the message, first we must spread it out.
Let the students read the coded message. The words written in pink are in regular English, but the words written in blue are in code; if we want to understand the message, second, we must decode it.
Print out the Code Breaker and show it to the students; they should now be able to read
the secret message: If you don't know how it works, it's magic. If you do know how it works, it's science.
Discuss how Astronomers must do the same things with starlight:
  They spread the starlight out using the diffraction grating
  They compare the pattern of spectral lines with a code breaker
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Part II
  Hand out the diffraction gratings; these look like slides you would put into the slide projector. The least-expensive ones have cardboard edges. Ask the students to hold them by the cardboard edges and not to put their fingers on the plastic.
Have the students hold the diffraction gratings in front of their eye, not right up against it and not at arms length, but just comfortably in between these two extremes.
Have the students observe a standard (low watt) incandescent bulb through their diffraction gratings to see the rainbow emitted by this 'white' light. Stress that rainbows are NOT what they want to record on their worksheets (they want discrete lines of color, not blurred smudges).
Put the neon Spectrum Tube into the power supply and turn it on. The tube should emit an orange-red glow.
Make the classroom as dark as possible (this is vital) - the darker the better.
Instruct the students to look directly at the glowing neon tube (it's not dangerous - it's the same light that they would see from a glowing neon sign on a store or restaurant), and then turn their heads and bodies (and diffraction gratings) slowly to the left, but at the same horizontal level of the neon tube.
They should see something that resembles a colored bar code. This usually solicits Oohs and Ahhs. If they don't see it:
  The room might not be dark enough
  They may be looking at the wrong angle: the angle varies depending on where they are in the room
  Rotate the diffraction grating 1/4 turn (90 degrees) and try again. The grating has finely etched lines that separate the light and are too close together to see. They have to be oriented properly. Some of the gratings we use have Edmund Scientific (the distributor) stamped on them. If you hold the grating so you can read the label, it is oriented properly. If the gratings are blank, the easiest thing to do is to test them out before hand and write 'up' or draw an arrow so it is easy for the students to orient them.
Ask the students to count the number of red lines. Depending on the quality of the grating, the darkness of the room, and their position in the classroom, they usually get something between 10 and 20. The good news is that the exact answer doesn't matter for the final outcome. I usually take a vote; majority rules.
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Part III
  Hand out the worksheets. I've found that things work best if I draw an example and have the students follow along. I've printed a copy of the worksheet onto a transparency and have a set of markers in the rainbow colors (red, orange, yellow, green, blue, and violet).
Write 'Neon' in the blank space under Element #1 and draw the winning number of red lines in the box under the label 'red'; it's okay if they spread out to 'orange' or even 'yellow.' There are a lot of red lines. The students should do the same with their worksheets and crayons.
Do the same with the orange, yellow, and green lines. They may see blue and violet lines (most don't) depending on the grating quality, the darkness level, and their position. The pattern of lines is unique: no other element shows this pattern.
Then replace the 'Neon' tube with the 'Krypton' tube. They like it because it sounds like kryptonite, the rock that kills superman, but it is just another gas. Turn on the power supply. The color of the glowing gas is different, and the spectrum is different (and much simpler).
Write 'Krypton' in the blank space under Element #2 and draw the spectrum. Students may argue about the color of the 'orange-yellow' line. I usually draw one line - a dashed orange line and fill in the spaces with yellow - in between the 'orange' and 'yellow' labels.
Repeat for 'Mercury' and 'Hydrogen' - You have just completed constructing the 'Code Breaker.'
Make a transparency of the actual star spectrum and draw only the three brightest lines in the box. Compare it to the patterns of the elements.Which pattern is the closest match to the star's spectrum (Answer: Hydrogen)
From the data you have gathered, what do you think stars are made of? (Answer: Hydrogen)
There are other faint lines in the spectrum, primarily a pair of yellow lines.

Ask the students to speculate on where these other lines come from. Someone usually comes up with the right answer: stars are not PURE Hydrogen, but only about 90% Hydrogen. The pair of yellow lines for example, comes from Calcium.

 
Disclaimer: The real analysis is more complex than this. The spectra of different stars contain lines from different elements even though they are all composed primarily of hydrogen. This is because the spectrum depends on properties other than the star's composition, mainly temperature. The experiment has been simplified so 5th graders can find a reasonable result.
 
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Student Activities:
  Students observe and decode the 'coded message' to learn about the need to 'decode' the information contained in starlight.
Students observe and record the spectral lines from the four Spectrum Tubes on their worksheets
Students compare these spectra with the starlight spectrum to identify the major element present in the star.
 
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Assessment:
  Ask the students the following questions, or use them as a quiz:
How do Astronomers know so much about what stars are made of? (Answer: They study the Spectrum of the starlight)
What is a Spectrum? (Answer: A spread-out image of what a certain light is made of)
  So how do we use a spectrum to find out about stars? (Answer: We compare it with the spectrum of other elements to find a match, which tells us what that spectrum came from)
  Why can't we just go get a piece of a star and test it? (Answer: They're too far away, we have no way of containing the material)
  So what are stars made of, mostly? (Answer: Hydrogen)
 
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Closure:
  Ask the students to 'be a star' and create their own short message (using the same Decoder Key) including their name, and to fold and put those messages in a box. Then have the students (as scientists) come up and draw out a message to decode and then return that message to the original student.
 
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