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Star Color

Goal:  Figure out the colors and temperatures of stars!

Procedure:  Measure and compare star brightness counts in images taken with red green and blue filters.

Materials:

  1. HOU image processing software.
  2. HOU teachers and museum or community center partners may download the images for the mystery star and for M42 in .fts format.

Images Taken with Filters

The HOU images used for these activities were taken in sets of three. For each set, images were taken with a red, green and then a blue filter on the telescope.

Why Use Filters?

The purpose of using filters is to be able to compare the amount of starlight which passes through each filter. By comparing the amount of starlight getting through each color filter, astronomers analyze the color of the light coming from the stars.

Star Color and Temperature

All stars are very hot! However, some stars are cooler or hotter than other stars. Cooler stars shine with more light in the yellow-orange-red areas of the spectrum. Hotter stars shine in the bluish areas of the spectrum.

Activity A: Color of Mystery Star. You will compare three images of a mystery star. Each image was taken with a different color filter. You will compare the brightness of the mystery star in each of the three images. The star will look brightest in the image which was taken with a filter that is most like the color of the light coming from that star. Does the mystery star shine with more red light or more blue light? Is this mystery star a cooler star or a hotter star?

Activity B: Color of Stars of M42. You will compare three images of the stars of the Orion Nebula, M42. Each image was taken with a different color filter. You will compare the brightness of a star in each of the three images. The star will look brightest in the image which was taken with a filter that is most like the color of the light coming from that star. Are the stars of the M42 more reddish or more bluish? Are these cooler stars or hotter stars?

bluestar.jpg (5787 bytes) Activity A: Color of Mystery Star

Mystery Star.... Does This Star Shine with More Red or More Blue Light?

Open the image, mysred.fts [mystery star image taken with a (red) filter]. You will repeat this process for mysgreen.fts (green filter) and mysblue.fts (blue filter).

  1. File, Open. Click the down arrow on the side of the box that is labeled Files of type. Select All Files [*.*].
  2. Select mysred.fts. Double click on M42red.fts or click Open.

Color the image. Even though a color filter was used on the telescope, the image still appears in shades of grey. You can use the color palette to help you remember which filter was used when the image was taken at the telescope.

  1. Find the color word Grey on the toolbar. Click on the down arrow to see your choices.
  2. Choose RED (or GREEN or BLUE depending on the image).

Measure the brightness counts of the mystery star.

  1. The Bull's Eye Icon on the tool bar measures brightness (Auto Aperture).
  2. Click on Auto Aperture. Move the cursor over the mystery star. Click again.
  3. A number pops onto the screen next to the star. This number represents the brightness counts for the star. (If the results box is in your way, click on File, and Hide Window.)
  4. Record the brightness counts of the star in this table.

Image and Filter

Brightness Counts

red filter: mysred.fts

 

green filter: mysgreen.fts

 

blue filter: mysblue.fts

 

Compare the brightness counts in the three filters.

  1. For this mystery star which filter allows most starlight to pass through? _________
  2. Do you think the light from the mystery star is more red or more blue? ______________
The color of stars reveal their temperature.
Cooler stars shine with more red light. Hotter stars shine with more blue light.
  1. Is this mystery star cooler or a hotter star? ______________________________

bluestar.jpg (5787 bytes) Activity B: Color of Stars of M42

Bright Stars in the Orion Nebula.... Are They More Red Or More Blue?

In the constellation, Orion, there is a region called the Orion Nebula. A nebula is a fuzzy patch of sky as seen with your eyes or a telescope. The Orion Nebula is a very huge dust and gas cloud in space with lots of stars inside. In fact, in nebulas like Orion, the dust and gas collapses to make new stars. The Orion Nebula has a lot of new, young stars.

When astronomers look at the Orion Nebula (Messier object number 42 or M42) with telescopes, four bright stars appear in the center of the nebula. These stars are called the Trapezium because there are four.

Open the image, M42red.fts [M42 image taken with a (red) filter]. You will repeat this process for M42green.fts (green filter) and M42blue.fts (blue filter).

  1. File, Open. Click the down arrow on the side of the box that is labeled Files of type. Select All Files [*.*]. Select M42red.fts. Double click on M42red.fts or click Open.

Log scale the color palette in order to see the dimmer parts of the image.

  1. Click in the box in front of the word LOG on the toolbar.

Match the image orientation to the photo.

  1. Manipulation, Flip. Select Vertical. Click OK.

Color the image because even though a color filter was used on the telescope, the image still appears in shades of grey. You can use the color palette to help you remember which filter was used when the image was taken at the telescope.

  1. Find the color word Grey on the toolbar. Click on the down arrow to see your choices.
  2. Choose RED (or GREEN or BLUE depending on the image).

Measure the brightness counts of the brightest star in the Trapezium.

  1. The Bull's Eye icon on the tool bar measures brightness (Auto Aperture).
  2. Click on Auto Aperture. Move the cursor over the star which you think is the brightest star of the four stars in the middle of the image. Click again.
  3. A number pops onto the screen next to the star. This number represents the brightness counts for the star. (If the results box is in your way, click on File, and Hide Window.)
  4. Record the brightness counts of the star in this table.

Image and Filter

Brightness Counts
Brightest Star of the Trapezium

M42red

 

M42green

 

M42blue

 

For the brightest star of the Trapezium, compare the brightness counts in the three filters.

  1.   Which filter allows most starlight to pass through? _______________
  1.   Does this star shine with more red light or more blue light?   _______________

The color of stars reveal their temperature.   Cooler stars shine with more red light. Hotter stars shine with more blue light.

  1.   Is the brightest star in the Trapezium in M42 a cooler or a hotter star? _____________

Download the .fts images of the mystery star and M42.
astroline.jpg (9796 bytes)
01/25/2000

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