| The HK Project is the longest-running program to monitor stellar activity cycles similar to the 11-year sunspot cycle. Almost 100 stars have been observed continuously since 1966; at present the project is monitoring long-term changes in chromospheric activity for approximately 400 dwarf and giant stars. Here is a sample of the long-term behavior for several stars with measured activity cycles: |
 HD 136202 (F8IV-V) 23 yrs |
 The Sun (G2V) 10.0 yrs |
 HD 103095 (G8VI) 7.3 yrs |
 HD 81809 (K0V?) 8.2 yrs |
 HD 3651 (K2V) 13.8 yrs |
 HD 10476 (K1V) 9.6 yrs |
 HD 166620 (K2V) 15.8 yrs |
 HD 160346 (K3V) 7.0 yrs |
 HD 16160 (K3V) 13.2 yrs |
 HD 4628 (K4V) 8.4 yrs |
 HD 201091 (K5V) 7.3 yrs |
 HD 32147 (K5V) 11.1 yrs |
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The data for the Sun were taken from observations of reflected sunlight from the Moon by Olin Wilson on the 100-inch telescope (green), and two long-term programs which measure the disk-integrated K-line flux at Sacramento Peak Observatory (red) and Kitt Peak National Observatory (blue). Some stars show two simultaneous cycle periods. Other stars either have variable activity, or long trends in activity - longer than our 30-year baseline, or appear to be very inactive: |
 HD 190406 (G1V) 2.6 + 16.9 yrs |
 HD 149661 (K0V) 17.4 + 4.0 yrs |
 HD 114378 (F5V) Long |
 HD 39587 (G0V) Var |
 HD 101501 (G8V) Var |
 HD 9562 (G2V) Flat |
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The Sun is the only star that can be spatially resolved. The size and extent of chromospheric active regions varies dramatically over the course of the activity cycle. |
