Solar Storms and You!

Activity A: Activity Cycles of Other Stars


The Sun is not the only known star whose activity cycle changes. With the Sun, we can monitor the ebb and flow of sunspot numbers. But there are other things that astronomers can look at to tell how active the Sun is. When seen at other wavelengths, the Sun's activity level can also be monitored by looking at how bright certain types of atomic 'lines' are. These lines, which act as fingerprints for these elements, also act as the proverbial Miner's Canary to signal how active the surface of the Sun is.


In this activity, students will plot actual data obtained by astronomers which indicate how bright the so-called Calcium H and K lines are. These lines are easily detected in many sun-like stars, and their brightness measured over the course of weeks and years traces out activity cycles in distant stars even though we cannot directly count the number of 'sunspots' on their surfaces.
Materials The 'HK Project' has been operating since 1965 and measures the Calcium H and K line brightness for 91 nearby stars similar in temperature to the Sun. Here is a table of the line brightness for 13 stars. The data was taken from Figure 7.3 in Solar and Stellar Activity Cycles by Peter Wilson ( University of Sydney), published by Cambridge University Press, 1994:
Year      A    B   C    D    E    F    G    H    I    J    K    L
1968     14   27  17   16   14   30   30   45   40   25   45   58
1970     14   26  20   15   16   29   35   45   26   28   60   60
1972     15   26  17   16   20   32   45   47   34   35   45   64
1974     14   25  16   15   18   29   35   44   35   30   65   68
1976     14   26  17   15   16   30   38   48   30   26   55   68
1978     14   25  19   16   18   29   34   45   24   24   45   71
1980     14   25  17   15   21   35   30   40   30   30   55   74

The stars are:
     Catalog   Type     Name
A   HD 142373   F9    Chi Herculis 
B   HD 25998    F7     50 Persi
C   HD 81809    G2
D   HD 10700    G8    Tau Ceti
E   HD 103095   G8
F   HD 101501   G8     61 Ursa Majoris
G   HD 165341   K0     70 Ophiuchi
H   HD 22049    K2    Epsilon Eridani
I   HD 160346   K3
J   HD 32147    K5
K   HD 201091   K5      61 Cygni
L   HD 156026   K5
The stars are ordered from A to L in decreasing temperature. For comparison, the Sun is a G2 star with a temperature of 5700 Kelvin. A K5 star has a temperature closer to 3500 K. 'Type' means Spectral Type in the 'OBAFGKM' classification system used by astronomers.


  1. Divide the class into 13 groups, and assign each star to one group.
  2. Each group is to construct a plot with the year on the horizontal axis, and intensity on the vertical axis. Arrange for all groups to use the same type of plot so that they can overlay their results at the end to compare different stars.
  3. Estimate the length of each stellar activity cycle, or whether some stars have no apparent regular, periodic cycle.
  4. From a previous activity, plot the solar sunspot cycle for a 25 year period of time.

Key Terminology

Maximum: A large number of sunspots in a particular cycle.
Minimum: A small number of sunspots in a particular cycle.
Sunspot: A dark spot on the sun indicating a cooler temperature.


Specific questions you might want to cover include:
  1. Which stars have cycles approximately like the Sun?
  2. From this sample, is it common for stars to have sunspot cycles?
  3. What is the maximum and minimum range of cycle activity?
  4. Do activity cycles have anything to do with the temperature of the star?
  5. Is there any evidence for a change in the activity cycles with increasing temperature?
  6. How regular are activity cycles?
Compare your plots and conclusions with the HK Project Activity Cycles Page. Learn more about the stars themselves, what constellations they are in. Learn about the solar Maunder Minimum, and compare this feature of the Sun's activity with what you learned from the other stars above. Are activity cycles constant, or can they come and go? How do we learn about the Sun by studying other stars? Related Web Resources

Return to the Activities Page
This activity was developed by the NASA, IMAGE/POETRY 
Teacher and Student Consortium. 
For more information, and a list of other resources, visit 
the IMAGE/POETRYweb site.