| The HK Project began in the mid-1960s. Olin
Wilson had been interested in searching for activity cycles of other
stars for many years, but had to wait until technology caught up to the
point where instrumentation could be developed that was sensitive enough
to monitor the minute changes in the stellar spectra related to activity
variations.
His goal was to start a project that would last one solar cycle, 11 years, and look at a broad sample of cool, dwarf stars such as the Sun. Close binaries were avoided, and the sample was spread across the sky. For this project he was awarded the nights near the Full Moon each month on the 100-inch telescope. This was also advantageous, because the reflected light of the Moon's surface could be used as a proxy measurement of the Sun's activity using the same instrument! |
 HK Project founder, Dr. Olin Wilson |
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After a few years, noticeable trends appeared in the records of many stars, while other stars showed little or no variation. Wilson continued his observations until 1978; he then published a landmark paper showing the records of 91 stars, many with cyclic variations. |
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 Dr. Arthur Vaughan modernized the HK Project instrumentation and made the transition from an occasional project on the 100-inch to a permanent project on the 60-inch telescope. |
Since Dr. Wilson had retired in 1974. Dr. Arthur Vaughan continued the
project, and built a second-generation ``HK spectrometer'', incorporating
more modern technology. Since time on the 100-inch telescope was in demand,
these improvements permitted the project to be moved to the 60-inch telescope.
At this time a survey of the local solar neighborhood - the stars nearest
to our solar system - was begun. Measurements of the activity of several
hundred stars were made.
Wilson had noted in his paper that the level of ``noise'' in the observations was greater than expected from the instruement used. This suggested that either there was a source of error that had been unaccounted for all these years, or that the stars themselves had variations occurring on time scales shorter than the activity cycle. Wilson suggested rotational modulation as the cause. Vaughan and collaborators obtained permission for a 100-day run on the 60-inch telescope to closely monitor the activity of approximately 40 stars in order to search for rotation. |
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The results were striking. Rotational modulation was clearly present in many of the stars' records. This was a giant leap for stellar astrophysics - until then, the only way to measure stellar rotation was to look at the Doppler broadening of spectral lines. However, this has two shortcomings. First, line broadening is affected by the angle of the spin axis of the star to our line of sight. It is typically impossible to know the inclination of the rotation axis in a single star, so that ambiguity of the projected rotational velocity cannot be resolved.. Second, for slowly-rotating stars, the broadening becomes too small to distinguish from the other processes in the stellar atmosphere that also broaden the spectral lines. Therefore, knowledge of stellar rotation was limited to rapidly-rotating stars. But with the HK Project, as long as there were inhomogeneities on the surface that could show detectable rotational modulation (unless the star were nearly pole-on) it is possible to measure rotation, no matter how slowly the star rotates. In the early 1980's several improvements were made. A standard lamp was added to monitor long-term changes in instrument sensitivity. A second, broader spectrographic slit was added so that evolved stars could be added to the monitoring program. And, several additional stars were added to the observing list, frequently coming from outside investigators. Since 1980, the HK Project has used many of the nights on the telescope. |
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| Over the years, the continued long-term recording of activity has produced unforseen breakthroughs. Long-term monitoring of rotational modulation showed that for some stars, the rotation period seems to change in step with the star's activity cycle. Therefore, surface differential rotation was first seen for other stars. Coarse measurements of the time scales of active region evolution for several stars have been made. The continuum passbands used to form the activity index have been found to be a useful indicator of a star's metallicity (so in effect, the metal abundance of over 2,000 stars were determined overnight). Rotation and long-term activity have now been recorded for giant stars. Larger surveys of the solar neighborhood have been done (almost 2,500 stars to date). And, activity variations on time scales longer than the activity cycle are beginning to be noticed. |
 Current HK Project Principle Investigator, Dr. Sallie Baliunas |
| If these variations are similar to the Sun's behavior in
the 17th and 18th centuries, the ``Little Ice Age'' or ``Maunder Minimum'',
then continued observations will have impact on understand not only the
solar-stellar connection, but also the solar-terrestrial connection.
The HK Project currently uses the 100-inch telescope for its observations. After 30 years over 350,000 observations have been made. |
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