Sunspots 2: Correlating Sunspots to Active Regions

What You Need


  • Graph paper
Sunspots 2: Correlating Sunspots to Active Regions Photo Credit: NASA


To help students understand how the development of new technology has increased our knowledge of how the sun works.


Ultimately, the sun is the source of all life on earth, providing light and warmth to the organisms that inhabit our planet. As a result, the sun has fascinated humans throughout history: it has been worshipped as a god, observed as it appears to move across our skies, and studied for its composition and behavior. Many cultures have built observatories to monitor the sun and its observable properties. As technology becomes increasingly sophisticated, we have been able to gather more and more information about the sun and use this data to infer things about the star's behavior.

This activity is part of a three-part series of lessons aimed at showing students how our knowledge of the universe must be inferred through the use of scientific tools. Specifically, students study sunspots in these lessons through the use of solar imaging from satellite instruments circling the sun (SOHO and the now-defunct Yohkoh satellites).

In the first lesson of this three-part series, students were introduced to sunspots and the types of technology and solar imaging used to visualize them on the sun's surface. This second lesson allows students to see how sophisticated technology can be used to learn about the sun. In this activity, students apply the use of two types of solar imaging to determine a correlation between sunspots and active regions on the sun. The sunspots are visualized through the use of visible light solar imaging whereas the active regions are visualized utilizing X-ray energy. Students compare the two images and determine the area of sunspots and active regions using a Java applet. The information gathered is used to determine the existence of a correlation between the two types of solar features.

Planning Ahead

In the activity, students compare visible light images to X-ray images of the sun using a Java applet. To successfully launch the applet, students will need to use a Java-enabled browser.


Ask students:

  • What is a correlation?
    • (The degree to which two things are related to one another and change together.)
  • Describe examples where two events are correlated to one another.
    • (Students should brainstorm events that are correlated to one another and describe how they are correlated. For example, risk of lung cancer and smoking are correlated in that the more an individual smokes over time, the greater his/her risk will be for developing lung cancer.)
  • How can we determine whether active regions in the corona and sunspots are correlated to one another?
    • (This is a brainstorm question. Students should be given time to review, if necessary, what sunspots, active regions, and the corona are. Students should also review that sunspots were seen in visible light images of the sun, whereas active regions were visualized through X-ray images of the sun. Students may suggest that to determine a correlation, one should be able to determine if a sunspot is always in the same place as an active region. They may also suggest counting the number of sunspots and active regions and determining a correlation that way.)


Tell students they will compare images taken of the sun using visible light and X rays. They will graphically determine whether sunspots seen on the sun using visible light are related to the active regions seen at the sun's corona using X-ray energy. The images they will use were taken by the Yohkoh satellite in January of 1992. There are 26 pairs of images, one pair (one visible and one X-ray image) for each day of January from the 5th to the 30th of the month.

Because there are 26 pairs of images, it is best if the class is divided into pairs, where each pair does a subset of images. Another option is to split the class and have each half of the class collect data on all 26 pairs of images. This will allow the class to collect two sets of data, which can be graphed, analyzed, and compared to one another. Assign images to students and bring the class to a computer lab.

Tell students that they will use an interactive Java applet to visualize their assigned images. The applet will allow students to circle the area around each sunspot and active region using the cursor. The applet will record the area (in pixels) of the sunspot and active regions outlined by students. These areas should then be recorded in a table made independently by the students. The class will then pool together its data and graph the area of the active regions (X-ray) against the area of sunspots (visible light) to determine whether or not there is a correlation.

Students should use their Correlating Sunspots to Active Regions student esheet to go to the Exploratorium's Sunspots site. Allow students time to read page 2 of the activity to introduce them to the activity and the use of the Java applet. Students also should become familiar with identifying the dark sunspots in visible light images and the bright, intense active region emissions that are visible in the X-ray images.

Encourage students to make their own data tables for recording areas of sunspots and active regions on the Correlating Sunspots to Active Regions student sheet. Their data tables should be long enough to include information on all 26 dates for the month of January. Data tables should include this information:

  • Date
  • Area of sunspots (pixels)
  • Area of active regions (pixels)
  • Ratio of X-ray area/sunspot area (There are no units because pixels cancel each other out.)

Students should collect information for their assigned subset of images using the Java applet. After all students have collected the information, pool together data so that each student has areas of sunspots and active regions for all 26 pairs of images of the sun. If the class was able to gather two sets of data, students should make another data table for the second set of data. They should calculate the ratio of X-ray area to sunspot area for each pair of images and record the results in their data tables.

Ask students:

  • The ratio of X-ray area/sunspot area measures the correlation between the two. Do the ratios indicate a correlation between the two?
    • (This will depend on the students' data. However, students should note that looking at so many numbers makes it difficult to ascertain whether or not there is some correlation. Some students may thus suggest that a visual representation, or graphing, will make any correlations easier to see and determine.)
  • How can we graph the data to determine whether or not there is a correlation between sunspot area and active region area?
    • (Students should suggest making line graphs of the X-ray area and the sunspot area.)

Distribute graph paper to students. Tell students that they should three graphs:

  1. Sunspot area vs. Time
  2. Active region area vs. Time
  3. X-ray area vs. Sunspot area

It is best to have students make these graphs and discuss them in this specific order. Students will have an easier time understanding the area vs. time graphs since those graphs will tell them how sunspot or active region area changed over time. The last graph (X-ray area vs. sunspot area) does not factor in time and simply examines whether or not there is a linear correlation between the two. Correlations do not depend on the passage of time. The absence of a time axis may disorient students who have not done much graphing. Thus, if your students need to approach the goal of the research activity more slowly (the existence of a correlation between sunspot and active region areas), start with plotting the areas measured on the images against time.

Each graph should have these features:

  • Title
  • Clearly labeled axes with units
  • Clearly labeled points

Tell students that by making the first two graphs, they will study the change in sunspot area and active region area over time. For each graph, the area (sunspot or active region) will be on the vertical or y-axis. The date (days) will be on the horizontal or x-axis.

The time graphs will have some shape, not necessarily linear or smoothly curved—they may have sudden jumps as spots and X-ray areas appear and disappear because of the sun's rotation, or as active areas become brighter and dimmer. Both the evolution of the sunspots or active regions and the rotation of the sun (once per 28 days) affect each day's data. For sunspots, the effect of rotation will likely dominate, as the sunspots evolve on a time-scale of several weeks or months. X-ray regions may vary on much shorter time scales.

After students have made their graphs (sunspot area vs. time and active region area vs. time), ask them:

  • How did sunspot area change over time in January 1992?
    • (Students should analyze their graphs.)
  • How did active region area change over time in January 1992?
    • (Students should analyze their graphs.)
  • Does comparison of the two time graphs show any noticeable trends? Describe.
    • (If the two graphs show a similar overall shape, comparing them may help students discover the correlation themselves. The more the shapes are alike, the closer the correlation graph would be to a straight line. Remember that the vertical scales of the two time graphs are different, so that a similarity indicates the X-ray and sunspot areas are proportional, not identical. Encourage students to make guesses about what the X-ray vs. sunspot graph will look like.)

After making the first two graphs (sunspot area vs. time and active region area vs. time), students should make a graph of sunspot area vs. active region area. Sunspot area (pixels) will be on the horizontal or x-axis and active region area (pixels) will on the vertical or y-axis. Students should match each day's sunspot area from the visible light image with the corresponding day's active region area from the X-ray image. The points will not be ordered in time because time is not one of the axes on the graph.

Students can use their student esheet to view sample graphs for sunspot area vs. time and active region area vs. time at the Science Education Gateway site in the Graphing Sheets section. (Students should scroll down the page. The Graphing Sheets section is in the middle at the bottom of the page.)

Students' data should yield a linear relationship between area of sunspots and area of active regions. If the data do yield a linear relationship, students should calculate the slope of the line (ratio of X-ray area/sunspot area). However, if a straight line cannot be achieved, students should reach some consensus about whether there is a correlation between the two quantities. If students were able to collect two sets of data, they can compare the two graphs.

Students should look at the Example of values and a plot from the Background Material: Research Activity page of the Science Education Gateway Sunspots lesson. The plot indicates a roughly linear correlation. Each point in the plot shows visible light sunspot area vs. areas of intense X-ray activity. Although the scatter of the points looks bi- or even tri-modal (three different slopes), the plot has a consistent average slope, with about the same number of points falling above and below a central line. Since the scale of the X-ray axis is an order of magnitude (x10) greater than the sunspot axis, the degree of vertical scatter is not too surprising. The sample data can be shown to students; however, students should not be concerned with their data looking just like the sample. Every researcher's measurements will be a little different.

Students should discuss whether or not they see a correlation between the two values. As they discuss, remind students that even now scientists have not determined the existence of a definite correlation.

If the class was able to generate two sets of data, and therefore, two graphs, students can compare and contrast the two. Ask students to discuss possible reasons why the two graphs differ from each other.

Possible reasons include:

  • Different colors were included in the X-ray areas
  • The visible light sunspot areas have far fewer pixels, and so are much more sensitive to small differences: one pixel more or less may be quite significant.
  • The exact scale of the X-ray image color scheme in counts/pixel is unknown.
  • Compare the depth of the X-ray corona, which extends far out into space, with the small, finite thickness of the photosphere where sunspots are located. The X-ray disturbances seen in the images may have shapes and sizes that vary rapidly, especially near the edge of the solar disk.
  • Pixel size may not be a convenient unit of measurement.

Ask students:

  • How do we know if the differences between the points are due to scatter in the data or error on our parts?
    • (This is a thought question. Students should spend time considering how scientists minimize error in measurement. Students should suggest that there were probably inconsistencies in the way different groups measured area.)
  • Describe some difficulties you encountered when measuring sunspot and active region areas on the images.
    • (Students will give varying answers depending on the types of difficulties they faced. Students will probably note that delineating the exact area of a sunspot and particularly an active region was difficult with the mouse. Also, because a pixel is an exact unit with a definite size, it may have been difficult only covering the areas of the sunspots and active regions.)
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Students should be able to answer these questions using the knowledge and skills learned through this activity as well as the first lesson in this sunspots series.

Ask students:

  • How is technology important in gathering information about the sun?
    • (Students should mention telescopes, satellites, and various imaging techniques that gather information about various aspects of the sun.)
  • How do you think technology has allowed us to better understand the relationship between sunspots and active regions on the sun?
    • (Sunspots are visualized through visible light imaging whereas active regions are visualized through X-ray imaging. The use of these two technologies allows us to capture two different types of images of the sun at the same time. These images can then be analyzed to determine a correlation between sunspots and active regions.)
  • Scientists continue to debate whether or not there is a correlation between sunspots and active regions. What types of technology do you think are needed to confirm whether or not there is a correlation?
    • (Students should brainstorm various ideas. The aim of this question is to have students consider how new technologies must be invented to answer certain questions about the world. For example, the telescope was invented to better visualize solar bodies. Similarly, in studying the sun and determining the type of relationship between active regions and solar bodies, new technologies must be invented which will gather specific information about size, shape, location, and other important details.)


Another lesson in which students measure the area of sunspots can be found at the Yohkoh Public Outreach Project site. The lesson, Using the Computer to Measure Sunspots, has students use image processing software to view and magnify images taken by the Yohkoh satellite. Students download specific images from the website and learn to convert the images from GIF to TIFF format.

SpaceWeather.com has listed history's biggest sunspots in tabular form from 1874 to the present. The data includes the date the sunspot was observed and its area. Students can graph this data and determine in what years the largest sunspots were observed. Graphing the data will also allow students to determine trends in the size of sunspots over time. The data can be found at History's Biggest Sunspots.

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