There are two tansyasters, tansyleaf and hoary. The one below, Tansyleaf Tansyaster (Machaeranthera tanacetifolia) grows in the late Spring. Notice the pinnate leaves. This picture was taken June 14. 
Here it is on June 14 on the Pawnee National Grasslands with Foxtail grass and evening primrose.
Hoary Tansyaster (Machaeranthera canescens) This one blooms in late Summer. This picture was taken September 19. Notice the single, narrow leaf.
Hoary Tansyaster (Machaeranthera canescens). It also grows taller than the Tansyleaf.