Solar Image courtesy of Yohkoh

Using the Computer to Measure Sunspots

Sunspots on Sun's surface courtesy of Yohkoh

Overview: In this tutorial, students will learn to use computer image processing techniques to measure the size of sunspots and analyze various phenomena visible on in solar system images. These skills can be used to scale and analyze images of our Sun, the planets, or any other electronic image found on the Internet.

Learner Outcomes

By completing this activity, the learner will:

Materials Needed:

NASA scientists use computers to conduct scientific investigations everyday. One procedure frequently used is called IMAGE PROCESSING. In this activity, we will learn how to process NASA Yohkoh images of our Sun on the computer. Be sure you have the following items accessible to your computer before you begin.

  • image processing software (Mac or PC or more information)
  • image format converters (Mac or PC) or more information)
  • saved TIFF images of Sun 1 and 2 on your computer's hard drive.
  • a printed image of our Sun (A library image or one scanned from a book)
  • WWW access, a WWW browser, and a computer
  • copy of the student handout for each student

    Teacher Lesson Plans


    Loading Live Image of SunLet's look at a picture of our Sun. What do you see in this picture? It is very dangerous to look at the Sun without special protective lenses. Sunlight will damage your eyes and it won't even hurt - never look directly at our Sun!!

    Ancient peoples used to think that the surface of our Sun was perfectly smooth and had no spots. Now that we have satellites and telescopes, what do you think - is the Sun smooth and spotless?

    sunspots courtesy of YohkohWhen the Sun is viewed through a solar telescope dark spots can be observed on the surface. These spots appear dark to the eye because they are cooler than the surrounding gas although they are still quite hot! The photosphere has a temperature of about 5500 degrees Celsius and a typical sunspot has a temperature about 3900 degrees Celsius.

    A sunspot's lifetime can be as short as an hour or two or as long as several months. The number of sunspots that can be seen on the surface of the Sun increases and decreases in a regular pattern, known as the solar cycle, with a maximum number of sunspots occurring every 11 years.




    Concept Introduction:

    The concept introduction portion of this lesson is presented as a step-by-step tutorial. It is divided into two portions, each requiring one session using the computer. In the first part of the lesson, students learn how to save images to their computer's hard drive. In the second part of the lesson, students learn how to scale and calibrate images in order to make measurements.

    Part I - Downloading, saving, and converting images from the Internet

    Any image that you find on the Internet's World-Wide-Web (WWW) using Netscape or MS Internet Explorer can be saved on your hard drive for analysis. Even better, the process is almost the same on a Macintosh or a PC. All you have to do is the following:

    1. Start your WWW browser (Netscape or MS Internet Explorer, for example)and enter in a URL that has lots of images. A great place to start is the Yohkoh Public Outreach Site (YPOP).
    2. To save an image to your hard drive, position the mouse over the image and hold down the button for a couple of seconds (use the right button if you have more than one button on your mouse).
    3. A menu will appear and you can select SAVE THIS IMAGE. This will give you an option about what to call the image and where to save it. Note where it goes and what it is called. Choose a different name and location if you like.
    4. The image is now saved on your hard drive and you can open it with your WWW browser or with an image conversion program you might have, like the shareware program PaintshopPro (more information). With these programs, you can alter the image's size, color, or even add text to the image as you wish.
    5. Most images on the Internet are in GIF (pronounced a hard "g" as in gift but without a "t") or JPG (pronounced jay-peg) format. Image processing software do not typically use files in this format so they must be converted to TIFF (rhymes with GIF).
    6. Start your graphics conversion program by double clicking on its icon.
    7. Open your image converter program and from the file menu, select OPEN. Find the image that you downloaded and open it using this converter program.
    8. From the file menu, select SAVE AS. In the format box select "save as TIFF format". Close your graphics converter program and you are ready to conduct image processing.

    Part II - Using Image Processing Software for Scaling and Measurement

    1. If you haven't converted your images to TIFF, you need to go back to part I and convert your images to TIFF format. Image Processing Tool Bar
    2. Start your image processsing software (MAC NIH Image or PC ImagePC or get more information). Using the FILE pull-down menu, select OPEN. Select the TIFF image that you converted in Part I (or save this one).
    3. The first task is to calibrate the image (tell the computer the scale of the image). If you are looking at an image of Sun you must know the diameter (you can look this up in a book or on the Internet as 1.4 million km {or 1.4e6 km). Using the segment tool/select line tool (fifth from the top on the right), draw a line across the diameter of our Sun.
    4. Under the ANALYZE menu, select SET SCALE. Note that the computer has already recorded how many pixels (picture elements) long the line you drew is. First, change the UNITS to the known diameter units (km). Second, enter in the known diameter. For large numbers, like 93 000 000, use 93e6. Click OK and your image is now calibrated and you are ready to measure sunspots!!
    5. Use the magnifying glass in the upper left of the tool bar to focus in on one of the sunspots by clicking on it. The software will remember the scale of the image so you can get as close as you need to make a good measurement. To un-zoom , double click the magnifying icon in the tool box or hold down Pthe CNTRL or APPLE key on the keyboard while clicking on the image.
    6. Use the segment tool (select lines) from the middle right of the tool bar to carefully draw a diameter across the entire sunspot. From the ANALYZE menu, select MEASURE.
    7. From the ANALYZE menu, select SHOW RESULTS. You might have to grab the bottom left hand corner of the new window to expand the window to see your results.
    8. Determine the diameter for several sunspots. How big are they? Are they bigger than your house? Are they bigger than Earth?
    9. You can use the same Segment Line Tool to Measure Perimeter and Area: This image processing program has 4 tools for selecting regions for study.
    10. Determine the area of a sunspot using the freehand (heart-shaped) tool to trace the outside. Select MEASURE from the Analyze pull-down menu, then select SHOW RESULTS from the Analyze pull-down menu to view the measured value for perimeter and area.

    Concept Application:

    Image Credit: P. Charbonneau and O.R. White (HAO)Sunspots have two components - a dark inner penumbra and a lighter outer umbra. Divide up into several NASA imaging teams. Each team needs to analyze a different set of sunspots and create a written technical report, a multimedia presentation, and/or an oral presentation that describes:

    1. which images you analyzed
    2. how the images were analyzed
    3. how many sunspots were present, and
    4. how big are sunspots compared to things in the solar system?


    Possible Imaging Team
    Assignments and Resources 
  • Sun One
  • Sun Two
  • Sun Three
  • Sun Four
  • Sun Five
  • Sun Six
  • Sun Seven
  • Sun Eight
  • Sun Nine
  • Sun Ten
  • Sun Eleven
  • Sun Twelve

  • Extensions

    The above process will work on ANY image that you find on the Internet. Try out the following to demonstrate your skills at image processing.

    Classroom Ready Handouts to Print

     Lesson designed by the YPOP Team

    For questions about this lesson, please contact Tim Slater