Spectral Types Teacher Notes

This project teaches students about spectral types of stars. Students will learn that the spectral classification system is actually a temperature scale for stars. They will also learn what causes the absorption and emission lines in a star’s spectrum, and why stars of different temperatures have different spectral lines. They will also have a brief look at stars that do not match the traditional spectral types.

If you are doing this project with younger students, you may wish to skip the section on energy levels. As long as they realize that the lines are caused by different elements and can identify the lines in spectra, they should be able to classify the stars.

Project Goals

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

  • understand that each star has a unique spectrum, with a series of absorption and emission lines
  • understand that stars emit thermal radiation with a peak wavelength that depends on the star’s temperature
  • explain that stellar absorption lines are caused by electrons in the outer layers of a star’s atmosphere absorbing photons from the star
  • know that the pattern of absorption lines depends on the temperature of the star
  • classify the spectrum of a star from its absorption lines
  • find the temperature of a star from its thermal radiation curve if the peak is visible
  • identify which types of stars are most common, and understand that some types of stars are too faint observe, even with large telescopes

Background Knowledge

Students should have a basic understanding of the nature of light before undertaking this project. They should know that light is a wave and that different wavelengths correspond to different colors. Students also need to be able of read and understand graphs of spectra.

The only mathematical calculation is relatively simple – finding the temperature of a star from its thermal radiation curve.

Project Structure

The first section lets students try to classify stars based on their spectra. The students should start learning to look for patterns in spectra during this section. After developing their own classification system for stars, they will discuss their system with another group and try to strengthen the system. It is a good idea to set a firm time limit, or students will take a long time on this section. Estimated time: 30 – 40 minutes

Next, students will learn a little about the energy levels of atoms, and how electrons jumping between energy levels can absorb and emit light. Students should make the connection between atoms absorbing and emitting light and the peaks and valleys they see in stellar spectra. Estimated time: 20 minutes

Then, the students will learn that different elements show absorption lines at different temperatures. They will learn how to use these lines to classify stars, and how to use the lines to estimate the temperatures of stars. Estimated time: 20 minutes

Students will have an opportunity to practice classifying stars. Finally, they will compile all their data in an attempt to find out which types of stars are most common, and which types of stars are least common. Estimated time: 45 minutes

Questions are designed to get students thinking about the way scientists work. Exercises are designed to get students to explore SkyServer data to solve problems. For answers to all questions, email us at voyages@sdss.org.

Students should be evaluated based on their written answers to the questions and exercises. You may use our sample 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.

Guide to project pages

Student classifications of stars

Students should use the Get Spectra tool to find spectra for each of the 14 stars in the table. Many of the lines are labeled, but a few are not. If a student cannot read a label because the spectrum obscures the label, they can look at another star’s spectrum and know that the lines are in the same order on both spectra. Just a note: the four lines from the Balmer series are always labeled as Ha, Hb, Hg, and Hd.

Exercise 1 has no right or wrong answers. Hopefully, students will put some thought into their classification systems, and will be able to justify them to to others. You may have the students exchange their guidelines and see if someone else can reproduce their classifications. Students will not derive the same classification scheme as astronomers because they are working with such small samples. Question 3 tries to get students to revise and improve their ideas, as professional scientists do in their research.

Energy Levels of Electrons

You can choose how much you want to cover the energy levels. Many chemistry classes talk about energy levels, but don’t go over the mathematics, even of the simple hydrogen molecule. This information may be new to some students.

Absorption and Emission Lines

The wavelength values of the absorption lines in the table are approximate. From these values, students should be able to see the lines on the spectra. Exact numbers are not necessary, because the resolution of the .gif images prevents students from reading down below about the nearest 40 to 50 angstroms.

Questions 5 and 6 should be treated as practice. They are designed simply to show students the type of reasoning they will need to employ to properly classify stars as they go through the lesson. Be sure students realize that it is OK if they do not get the star’s spectral type correct the first time. Classifying spectra is not easy.

On all the spectra, the positions of the absorption lines are marked whether or not there is a line present. The table gives positions of some lines in addition to the ones marked automatically on the SDSS spectra. The titanium oxide lines are quite numerous, but occur in distinct bunches at the listed wavelengths.

Exercises 5, 6, and 7 give the students an opportunity to practice classifying stars by doing an informal survey of the prevalences of different spectral types. You might wish to assign each group of students a different plate to work from to avoid duplication of data. Or, you might assign two groups to each plate to provide a check on each other’s data. Either way, you should have students discuss whether they feel they truly did a random sample of the stars in the sky, or whether their sample is somehow biased toward a certain part of the sky.

Exercises 8, 9, and 10 show different types of stars that do not fall into the OBAFGKM sequence. Try to foster discussion of the fact that these are not the only types of stars, and there are several rare classes not represented in the OBAFGKM sequence.


Project 2061 Benchmarks in Science Education

The Project 2061 Benchmarks in Science Education is a report, originally published in 1993 by the American Association for the Advancement of Science (AAAS), that listed what students should know about scientific literacy. The report listed facts and concepts about science and the scientific process that all students should know at different grade levels.

The report is divided and subdivided into different content areas. Within each subarea, the report lists benchmarks for students completing grade 2, grade 5, grade 8, and grade 12. The table below shows which benchmarks are met by which sections of the Spectral Types project.

This page lists all the Project 2061 Benchmarks met by the Spectral Types project. Content headings are listed as Roman numerals, subheadings as letters, grade levels by numbers, and specific points by numbers after the hyphen. For example, benchmark IA8-2 means the second benchmark for eighth grade students in the first content area, first subarea.

The Spectral Types unit meets the following objectives in the Project 2061 Benchmarks:

IVA2-1, IVA8-1, IVA12-1, IVE12-4, IVE12-5, IVF8-1.

IVA2-1. There are more stars in the sky than anyone can easily count, but they are not scattered evenly, and they are not all the same in brightness or color.

IVA8-1. The sun is a medium-sized star located near the edge of a disk-shaped galaxy of stars, part of which can be seen as a glowing band of light that spans the sky on a very clear night. The universe contains many billions of galaxies, and each galaxy contains many billions of stars. To the naked eye, even the closest of these galaxies is no more than a dim, fuzzy spot.

IVA12-1. The stars differ from each other in size, temperature, and age, but they appear to be made up of the same elements that are found on the earth and to behave according to the same physical principles. Unlike the sun, most stars are in systems of two or more stars orbiting around one another.

IVA12-4. Different energy levels are associated with different configurations of atoms and molecules. Some changes of configuration require an input of energy whereas others release energy.

IVE12-5.When the energy of an isolated atom or molecule changes, it does so in a definite jump from one value to another, with no possible values in between. The change in energy occurs when radiation is absorbed or emitted, so the radiation also has distinct energy values. As a result, the light emitted or absorbed by separate atoms or molecules (as in a gas) can be used to identify what the substance is.

IVF8-1. Light from the sun is made up of a mixture of many different colors of light, even though to the eye the light looks almost white. Other things that give off or reflect light have a different mix of colors.

NCTM Principles and Standards for School Mathematics

Principles and Standards for School Mathematics was released in 2000 by the National Council of Teachers of Mathematics. The standards, a collaboration between education researchers and school mathematics teachers, lists what concepts students should understand, and what skills they should possess, at different stages of their mathematics education.

The report is divided and subdivided into ten different content areas. Within the first six areas, the report lists benchmarks for students completing grade 2, grade 5, grade 8, and grade 12. The table below shows which standards are met by the Spectral Types project.

Content headings are listed as Roman numerals, subheadings as letters, grade levels as numbers, and specific points by numbers after the hyphen. For example, standard IA8-2 means the second benchmark for eighth grade students in the first content area, first subarea. Content areas VI through X, which concern skill processes in mathematics, are not divided into subareas or grade levels. The standards met by the Spectral Types project are:

IA8-1, IB8-1, IC12-2, IIB12-5, IVA8-2, IVB12-4, VI-2, X-3.

Students should be able to:

IA8-1. Work flexibly with fractions, decimals, and percents to solve problems.

IB8-1. Understand the meaning and effects of arithmetic operations with fractions, decimals, and integers.

IC12-2. Judge the reasonableness of numerical computations and their results.

IIB12-5. Judge the meaning, utility, and reasonableness of the results of symbol manipulations, including those carried out by technology.

IVA8-2. Understand relationships among units and convert from one unit to another within the same system.

IVB12-4. Use unit analysis to check measurement computations.

VI-2. Solve problems that arise in mathematics and other contexts.

X-3. Use representations to model and interpret physical, social, and mathematical phenomena.