Summary

This lesson on stellar classification introduces students to the concept of classifying objects by the shape of a graph of their spectra (light). Students are given some 24 stellar spectra from SDSS and are told to order them in a sequence, and compare to groups of peers. This is then connected (via lecture) to the discoveries of Williamina Fleming, Cecilia Payne-Gaposchkin, and Annie Jump Cannon.

Suggested Time

45 minutes to 2 hours, depending upon the speed/age of your students

Prerequisites

Spectra, wavelength/frequency, reading graphs

Learning Outcomes

Process/Skills

Upon successful completion of this lesson, students will

• be able to determine criteria for categorizing data, and
• be able to observe trends in data.

Content

Upon successful completion of this lesson, students will

• understand stellar classification, and
• be able to identify continuous / black body, absorption, and emission spectra.

High School Frameworks

Massachusetts

• Earth & Space 1.1, 1.3, 1.8
• HS Physics 4.1, 4.2, 6.2
• SIS 1, 2, 3, 4
• Math skills (various)

NSES A, B, D, F, G

Materials

• 26 spectra (webpage, .zip).  Print around 10 for every 3 students, plus one complete set for teacher.  

Background

The following is background material for the teacher (and students).

Spectra

Since we cannot at this point in time actually visit other stars, all the information that astronomers have about stars comes from their light. Just like we can use a prism or water vapor to split "white" sunlight into its rainbow of component colors, we can use instruments called diffraction gratings and spectrometers to split the light from stars into their component colors. When we look through a spectrometer at different types of light sources, we find that there are actually three main types of spectra - continuum, emission, absorption.

Types of Spectra

As described above, stars have a spectrum most similar to continuum, with some absorption superimposed (and occasionally emission as well). Astronomers typically study spectra via a graph rather than a pretty picture - though the pictures are at first easier for us to understand visually, we get more information out of the graph. A graph of the spectrum will have either wavelength or frequency (color) on the x-axis (horizontal, running along the bottom, independent variable, abscissa), and some measurement of the intensity (brightness) at each color on the y-axis (vertical, running along the left, dependent variable, ordinate).

Stellar Spectra

When astronomers first studied the light from stars, they did not have a system to describe stars compared to each other. Astronomers at the Harvard College Obervatory (now Harvard University and the Havard-Smithsonian Center for Astrophysics or the CfA) worked on this probem around the turn of the 20th century. In 1881 Williamina Fleming observed that some stars had stronger Hydrogen absorption lines than others, and ordered them according to this characteristic, starting with A (strongest) through P (weakest). By 1910 Annie Jump Cannon had reordered the sequence according to the temperatures of the stars, which actually corresponds to the location of the peak in emission and dropped a few letters. This new ordering was OBAFGKM - "Oh Be A Fine Girl/Guy, Kiss Me." In 1925 Cecilia Payne-Gaposchkin received Havard's first PhD for her thesis proving Cannon's method to be valid.

Spectral Types

Colors of Spectral Types

In the above image, note the dark hydrogen absorption lines that appear in the spectra.  Type A stars have the darkest, "widest," absorption lines, as we would expect from Fleming's original scheme.  Also note how the "brightest" color moves from the left (blue) to right (red) as you move from hotter (O) to cooler (M) stars.  Stars that are hotter or an "earlier" spectral type (O or B) are inherently brighter overall and more blue in color, while stars that are cooler or a "later" spectral type (M or K) are inherently dimmer overall and more red in color.  

Lesson

Preparation

Print one copy each of the spectra in the materials section.  Organize these starting with with ones that are high or peaked towards the left, move through the peak shifting right, and end with the peak on the furthest right.  The object labeled "4" is not actually a star, the graph says "Unknown" near the bottom, so it should be left out of the sequence.  Number them in order through the main sequence, starting with the left peaking spectra: O5, O7.5; B0, B2.5, B5, B7.5; ...(continue with AFG) ...; K0, K2.5, K5, K7.5; M0, M2.5.  This will be your "answer key," as well as practice recognizing features in the spectra.  

Intro

Remind students of the distinction between the three types of spectra - continuum, emission, and absorption - and that stellar spectra are a combination of continuum and absorption. Explain to students that stellar spectra are the best way that astronomers have to know anything about stars, and that they will get to work with actual spectra from real stars. Their task will be to pretend they're astronomers from around a hundred years ago, and look at the spectra they got from stars and try to figure out how they're related to each other, and what order they should be put in. Inform them that there's no "right" answer, and in fact astronomers did it wrong the first time they tried. (I've used this lesson for some five years so far, and students always come up with the current Cannon/Payne-Gaposchkin system.)

Activity

Students are broken into groups of 2-3, and each group is given up to ten random spectra to categorize. Instruct students to try to put their spectra in an order. Give them plenty of floor space to work. Students occasionally need prompting on this because they're unused to looking at graphs and drawing conclusions from graphs. If they need help, point out things like which side is highest, if it's curved upwards or down, how many sharp dips down there are, how deep the dips go, if there are any peaks up, and so on. This part should take 15-25 minutes.

After each group has some tentative order, merge into larger groups of 4-6. Encourage students to come to a consensus on where certain spectra go - this is sometimes difficult, so ask them to explain their reasoning to you and to each other. This part should take another 10-20 minutes.

While groups are still working, have a couple students who finish early start to put their spectra in order on the board. If you have a blackboard, this can be done with magnets, otherwise use tape. As other groups finish, call up students to add their contributions to the class set. A lot of teacher direction will be needed for this phase of the lesson. When a consensus has been reached with the class as a whole, reveal your "answers" (from your preparation) - both the letters of the original hydrogen line scheme, and the ordering of the stars according to the present-day scheme.  Point out that your students took one or two class periods to figure out what it took a number of astronomers more than 30 years to do!  (Of course the astronomers also had a lot of math in it...)

If you are splitting the activity over multiple class periods, give students paperclips or envelopes so they can store their pictures as they're sorted.  

Follow-Up

Students can perform research during class time or as take-home assignments.  They can investigate topics including: 

• the astronomers
• types of spectra (continuum, etc.)
• spectral class (OBAFGKM) and sub-class 
• spectral class of main sequence vs. giant stars
• the spectral class of bright stars in a given constellation
• the H-R diagram
• Wien's Law

Students can also come up with their own mnemonic devices to remember the spectral classes.  The traditional one is "Oh Be A Fine Girl/Guy, Kiss Me."  

Assessment

Formative

• During the activity, the teacher can observe how students are working together, and whether they are "getting" the goals.
• During discussion, the teacher can assess student knowledge.

Summative

• Students can prepare written descriptions of their ordering scheme.
• Students can perform research on the follow-up topics listed.  

Accommodations

A significant portion of this lesson relies upon typical visual ability, which may be accommodated multiple ways.

• Students with visual disabilities may be paired with other students who describe the graphs to them.  Descriptions may be given verbally, by guiding the hand of the student with visual disability, by "singing" the pitch of the tones, or in other manners.  
• Graphs may be prepared in a raised-line version by tracing over the graph with craft glue (Elmer's), or mounting graphs on oak tag or cardboard and cutting out in the shape of the graph.

Students with Individualized Education Plans can be grouped together in numbers appropriate to their learning style, and be given a smaller number of graphs to work with. Students with ADD/ADHD can be assigned to help pass out materials.  

Students who finish early can be given additional graphs to sort, can work on sub-categories, can assist other students, or can work on follow-up research. 

Links 

• http://cas.sdss.org/dr4/en/proj/basic/spectraltypes/
• http://www.astro.uiuc.edu/~kaler/sow/spectra.html#sequence
• http://www.classzone.com/books/earth_science/terc/content/investigations/es2803/es2803page01.cfm
• http://en.wikipedia.org/wiki/Stellar_classification