Complex sentence: Of the approximately half-million stars in Keplers field of view, around 150,000 stars were selected for observation, and they are observed simultaneously, with the spacecraft measuring variations in their brightness every thirty minutes.
Split into simpler sentences: Of the approximately half-million stars in Keplers field of view, around 150,000 stars were selected for observation. All the selected stars are observed simultaneously, with the spacecraft measuring variations in their brightness every thirty minutes.