Star Light! How Bright?

Explore the Magnitude Scale!

Background: All we know about stars is from star light! People have always noticed that some stars seem brighter than others. In the second century B.C., a Greek astronomer and mathematician named Hipparchus produced the first known catalog of star names and positions. He invented a scale with six categories to describe star brightness. He assigned each star a number value from 1 to 6 depending on how bright the star seemed to be in the sky. Star astronomers call these brightness categories, magnitudes.

Activity: Use Hands-On Universe image processing software and an image of the Pleiades Star Cluster to find brightness counts of the bright stars. Compare the brightness counts of the stars to their assigned magnitudes.

Challenge: Discover how astronomers arrange the magnitude scale to describe the brightness of stars!

Look at the stars of the Pleiades.  Which star seems the brightest? ____________
Now let's measure the brightness of each star using Hands-On Universe software.

A. Display the image of Pleiades.

Open Pleiades.fts  or  .  Next, flip the image horizontally, change the color palette, and center the image to include all the bright stars. (See following instructions.)

 Go to the Manipulation menu , select Flip, .  Click on Horizontal, click OK. Go to the box with the word GREY. Place your cursor over the arrow on the side. Click on mouse and drag to IGREY; click mouse again. Repeat the process to choose any color palette. (Purple works well.) Go to the side bars and arrows which frame the image. By clicking on the arrows, or by clicking and dragging on the side bars, you can change the portion of the image that you see. Position the image so the bright stars of Pleiades are in the image frame.

B. Compare Star Brightness ….. with Photometry (light measuring) Tools!

Either   Measure several stars' Brightness Counts at once with Find!

1. Go to Data Tools. Select Find.
2. Delete the number 4 in the center box. Type in 3000. Click OK.
3. Brightness count numbers will appear next to the six brightest stars! (A results box also appears. To remove the results box, click on File and select Hide window).
4. To Clean up marks on your image, click on the icon which looks like a broom and dustpan.

Or   Measure each star's Brightness Counts with Auto aperture .

On the toolbar there is an icon which looks like bull’s eye. This tool is called Auto aperture . It gathers all the a starlight from a region, subtracts background ‘sky light’, and reports the number as brightness counts.

1. Go to the toolbars.
2. Select Auto Aperture . Move the mouse over a star you wish to measure. Click. Repeat for each star you wish to measure.
3. A brightness counts number will appear next to the star! (A results box also appears. To remove the results box, click on File and select Hide window).

C. Record and Analyze.
Fill in the data table with the brightness counts for each star.

 Pleiades Star Names Brightness Counts (The numbers next to the stars.) Magnitudes Alcyone Brightness: 2.86 Atlas Brightness: 3.62 Electra Brightness: 3.70 Maia Brightness: 3.86 Merope Brightness: 4.17 Taygeta Brightness: 4.29

 For the rest of the star data, use the Auto Aperture tool instead of Find. Pleione Brightness: 5.09 variable Celaeno Brightness: 5.44 Asterope I Brightness: 5.64 Asterope II Brightness: 6.41

D. Meet the Challenge!

How is the magnitude scale related to star brightness?

1. Which is the brightest star? ___________________
What is this star’s magnitude? ________
2. How many brightness counts does Electra show?  _________________
What is the magnitude of Electra? ________
3. Which is the dimmest star? ____________________
What is this star’s magnitude? ________

Compare the brightness counts of the stars to their magnitudes. Circle your choice.

1. Stars with more brightness counts have a ( higher or lower ) magnitude.
2. Stars with less brightness counts have a ( higher or lower ) magnitude.

What is the rule?

1. In your own words, explain how the magnitude scale is arranged to describe the brightness of stars.

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Think of yourself as a star astronomer!

1. What other questions do you have about the Pleiades?  What other images would you like to have of the Pleiades or other stars?   How would you design a similar or different investigation?

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Resources

Refer to the following and other references to find diagrams of the Pleiades or to read about the magnitude scale.

Levy, David. Skywatching. A Nature Company Guide published by Time-Life Books. San Francisco: US Weldon Owen Inc., 1994. (1-800-227-1114).

Menzel, Donald H. and Jay M. Pasachoff. A Field Guide to the Stars and Planets, 2nd Edition. Boston, Houghton Mifflin Company, 1983.

Refer to the following and other references to read mythology of the Pleiades.

Krupp, E.C. Beyond the Blue Horizon, Myths and Legends of the Sun, Moon, Stars and Planets. New York: Harper Collins Publ., 1991.

Stall, Julius D.W. The New Patterns in the Sky, Myths and Legends of the Stars. Blacksburg, Virginia: McDonald and Woodward Publ. Co., 1988.

Briggs, John. Yerkes Observatory, Williams Bay, Wisconsin, University of Chicago. October 3, 1996 image of Pleiades (Pleiades.fts) taken with a red filter on the 2 inch refractor, Bruce mounting in the South Building.

Burnham, Robert Jr. Burnham's Celestial Handbook, An Observer's Guide to the Universe. Beyond the Solar System, Vol. III. New York: Dover Publications, Inc., 1978.

Cole, David. "Consultation Regarding Magnitude Values." University of Chicago Astronomy Department. Oct., 1996.

Hands-On Universe Informal Science Education Project, Lawrence Berkeley National Laboratory, Berkeley, California.

Hirshfeld, Alan, Roger W. Sinnott, and Francois Ochsenbein. Sky Catalog 2000.0, Volume I: Stars to Magnitude 8.0, 2nd Edition. Cambridge, MA: Sky Publishing Corporation, 1991. pages 86-88.

Strand, K.A. Basic Astronomical Data.