A photon is the smallest unit of light. Faint deep-sky objects deliver only a handful of photons per pixel per minute, which is why exposures are long and total integration (the sum of all exposures) is measured in hours. Because photons arrive randomly, doubling your photons does not double your noise — signal grows faster than noise, so more integration always helps. Everything downstream (sensor, calibration, stacking) is in service of collecting and preserving photons.