In the chromosphere of the Sun and cool stars, the areas surrounding magnetic features (sunspots or starspots) show emission at the center of the singly ionized calcium (Ca II) H and K spectral lines. Presumably, when an active region is present on the surface of the star, the emission at the center of the line increases. For the Sun, where we can resolve the disk, we can measure the emission across the disk.
Profiles of the Ca II K and H lines are shown in flux normalized to the continuum (y axis) versus wavelength (in Angstroms, Â). The H and K absorption lines are very broad: the large V-shaped feature across the whole plot is only the central part of the photospheric absorption line! The shaded area shows the central 1Â where the emission exists. This central core is what the HK Project measures in order to measure chromospheric activity.
In this false-color image of the Sun, taken with the emission features of the Calcium H and K lines, the brighter features (gold and white) are regions of stronger chromospheric activity. The blue background also shows slight variations in the chromospheric network. (Also visible are several prominences around the limb of the Sun in red.)
If we place the slit of a spectrograph across the surface of the Sun, we can trace the change in the emission of the calcium K line. The image to the left does just this: wavelength is across, and the location along the slit follows from to bottom. Next to the images, several profiles of the Ca II K line are plotted corresponding to their positions in the image. There is quite a bit of change in the amount of emission in these profiles as the slit passes over magnetically active and quiet areas on the solar surface.
At present we cannot resolve the disks of stars similar to the Sun, even those close to the Sun. Therefore, we have to use variations in the disk-integrated light to infer information about the stars such as their rotation periods and cycle periods. Nearly all information about the distribution of active regions on the surface is lost. However, important advances in techniques like adaptive optics and optical interferometry are happening, and by the end of the century, many of the results of the HK Project may be confirmed by direct measurements.