The completion of the GALEX All-Sky Survey in the ultraviolet allows activity
measurements to be acquired for many more stars than is possible with the
limited sensitivity of ROSAT or the limited sky coverage of Chandra, XMM, or
spectroscopic surveys for line emission in the optical or ultraviolet. We have
explored the use of GALEX photometry as an activity indicator, using as a
calibration sample stars within 50 pc, representing the field, and in selected
nearby associations, representing the youngest stages of stellar evolution. We
present preliminary relations between UV flux and the optical activity
indicator R'_HK and between UV flux and age. We demonstrate that far-UV (FUV,
1350-1780{\AA}) excess flux is roughly proportional to R'_HK. We also detect a
correlation between near-UV (NUV, 1780-2830{\AA}) flux and activity or age, but
the effect is much more subtle, particularly for stars older than than ~0.5-1
Gyr. Both the FUV and NUV relations show large scatter, ~0.2 mag when
predicting UV flux, ~0.18 dex when predicting R'_HK, and ~0.4 dex when
predicting age. This scatter appears to be evenly split between observational
errors in current state-of-the-art data and long-term activity variability in
the sample stars.Comment: 37 pages, 12 figures. To appear in the Astronomical Journa
