The Smithsonian Hectospec Lensing Survey ( SHELS ) is a window on the star formation history over the last 4 Gyr .
SHELS is a spectroscopically complete survey for R _ { \mathrm { tot } } < 20.3 over 4 □° .
We use the 10k spectra to select a sample of pure star forming galaxies based on their \ha emission line .
We use the spectroscopy to determine extinction corrections for individual galaxies and to remove active galaxies in order to reduce systematic uncertainties .
We use the large volume of SHELS with the depth of a narrowband survey for \ha galaxies at z \sim 0.24 to make a combined determination of the \ha luminosity function at z \sim 0.24 .
The large area covered by SHELS yields a survey volume big enough to determine the bright end of the \ha luminosity function from redshift 0.100 to 0.377 for an assumed fixed faint-end slope \alpha = -1.20 .
The bright end evolves : the characteristic luminosity L ^ { * } increases by 0.84 dex over this redshift range .
Similarly , the star formation density increases by 0.11 dex .
The fraction of galaxies with a close neighbor increases by a factor of 2 - 5 for L _ { \ha } \gtrsim L ^ { * } in each of the redshift bins .
We conclude that triggered star formation is an important influence for star forming galaxies with \ha emission .