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This project shows students the brilliant colors of stars, then teaches them why stars come in so many colors. Answering this question takes them through the definition of color in astronomy, the nature of thermal (blackbody) radiation, and the use of colored telescope filters.

For more information on how astronomers use color, and on the physical meaning of color, read the About Astronomy: Stars section of SkyServer, or look in an astronomy or physics textbook. Here are a few possible references:

Halliday, David, Jearl Walker, and Robert Resnick, Fundamentals of Physics, John Wiley and Sons, 2000

Hartmann, William K., Astronomy: the Cosmic Journey, Wadsworth, 1989

Kaler, James, Extreme Stars, Cambridge University Press, 20001

Project Goals

By the end of the project, students should be able to:

  • Recognize the rich variety of colors in the night sky

  • Ask scientific questions about everyday phenomena

  • Know that light is a wave, and explain the relationship between wavelength and observed color

  • Know the order of the visible spectrum, from red to violet, and the total spectrum, from radio waves to gamma rays

  • Explain how telescope filters are used in astronomy

  • Explain that stars is related to the peak wavelength of light they emit.

  • Explain why red, orange, yellow, and blue stars are common, while stars with other colors from the visible spectrum are rare

  • Look up data using several interfaces

  • Compare the astronomical definition of color to visual color

  • Calculate the radiant flux of a star from its magnitude

  • Work with identities among logarithms

  • Understand that hot objects give off more thermal radiation than cool objects

  • Explain how the peak wavelength of thermal radiation changes as objects heat up

  • Calculate peak wavelength of thermal radiation from temperature

  • Explain the limitations of finding stellar temperature from color alone

  • Find peak wavelengths of observed stellar spectra

  • Understand why more filters allow more confidence in determining temperature

  • Qualitatively understand how to find astronomical colors from a thermal radiation curve

  • Make simple x-y graphs

  • Identify cool and hot stars on different color-color diagrams

  • Use data to judge when a scientific analysis is appropriate

  • Infer the existence of unusually red stars (red giants) from a color-color diagram

  • Understand the differences between Population I and Population II stars

  • Predict the thermal signatures of stars, planets, galaxies, quasars, and the universe

  • Prepare, execute, and interpret an independent scientific research project

Background Knowledge

Before beginning this project, students should:

  • know that stars come in different colors

  • know that light is composed of waves with definite wavelengths and frequencies

  • know what stars, galaxies, and the universe are

  • know that hot objects glow, and that heat and light are related

  • have some experience with scientific reasoning

  • have mathematics experience up to and including Algebra I.

  • know how to look up information using a web-based interface, such as a search engine

  • have some familiarity with a spreadsheet program (such as Microsoft Excel)

The section "Color and Amounts of Light," including Exercises 2-4, requires that students multiply and divide using logarithms. If your students are not comfortable using logarithms, you may skip this section.

They should also have basic computer skills. They should know how to look up information using a Web-based interface. It is helpful to know how to use Microsoft Excel or some other graphing program, but not necessary.

Project Structure

The Color project is a long-term project, designed to take about 11 hours to complete. You may wish to assign some parts as homework. The project can be divided into five chapters:

Chapter 1: Introduction and Exploration

1 hour

Chapter 2: Definition of Color in Astronomy

2 hours

Chapter 3: Color and Temperature

3 hours

Chapter 4: Color-Color Diagrams

4 hours

Chapter 5: Other Objects/Research Challenge

1 hour

Chapter 1 begins with the Introduction and ends with the Colors of Stars in the SDSS.
Chapter 2 begins with The Definition of Color in Astronomy and ends with Light from Stars.
Chapter 3 begins with Color and Temperature and ends with Observed Spectra.
Chapter 4 begins with Color-color Diagrams and ends with Colors of Other Objects.
Chapter 5 begins with Colors of Other Objects and ends with Your Results.

If you would like to use a shorter version of the project with your class, you may stop after one of the chapters. For example, if you want your students to learn how astronomers define color, without learning the cause of stellar color, you may stop after Chapter 2.

If your students are not comfortable using logarithms, you may skip the "Color and Amounts of Light" section in Chapter 2. The shorter Chapter 2 should take about 1 hour.

The Research Challenge, on the Colors in Astronomy Research page, should not be done in the classroom for credit. It is designed to be a completely open-ended and independent scientific investigation, and it should take many hours to complete. You may wish to give extra credit for completing it. Invite students to discuss their research questions and approaches with you. When students finish Exercise 13 on their own, encourage them to E-mail their results to us. We will look at all the results we receive, and we will put them best of them up on the project web site.

Questions and Exercises

Questions are designed to get students thinking about the reasoning scientists use in their work. Exercises come in two types: Practice and Explore. Practice exercises let students practice using the concepts introduced in the project. Explore exercises are designed to get students to explore SkyServer data to discover concepts on their own. For answers to all Questions and Practice exercises, and sample responses to all Explore exercises, click here. To see the answers, you must have the Adobe Acrobat PDF viewer installed on your computer. Acrobat is available for free download at Adobe's web site.

Students should be evaluated based on their written answers to the questions and exercises. You may use our ssample scoring rubric or develop your own. If you use our scoring rubric, print out a copy for each student and attach it when you return his or her work.

For specific information on any part of the project, click Next.