H-K Project

Activity Cycles

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

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

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.

Image of the solar cycle as seen in the chromosphere