We have matched the astrometric data from Gaia Data Release 2 to the sample of stars with measured rotation periods from Kepler .
Using 30,305 stars with good distance estimates , we select 16,248 as being likely main sequence single stars centered within a 0.5 mag region about a 1 Gyr isochrone , removing many sub-giants and unresolved binary stars from the sample .
The rotation period bimodality , originally discovered by McQuillan et al .
( 26 ) , is clearly recovered for stars out to 525pc , but is not detectable at further distances .
This bimodality correlates with Galactic height as well , dropping strongly for stars above Z > 90 pc .
We also find a significant width in the stellar main sequence of \Delta M _ { G } \sim 0.25 mag , as well as a coherent gradient of increasing rotation periods orthogonal to the main sequence .
We interpret this as a signature of stellar angular momentum loss over time , implying a corresponding diagonal age gradient across the main sequence .
Stellar evolution models predict changes in color and luminosity that are consistent in amplitude , but not in direction , with those required to produce the gradient we have detected .
This rotation gradient suggests that main sequence evolution produces offsets in color–magnitude space that are significantly more orthogonal to the zero-age main sequence than models currently predict , and may provide new tests for both stellar evolution and gyrochronology models .