Gaia DR2 selection function
flipping the stellar coin
Stars only appear in Gaia DR2 if they are detected at least five times. The probability of a star being detected is determined by how bright it is. Due to Gaia's scanning law, stars are seen a different number of times in different parts of the sky, and so some stars have fewer chances of being detected. Taken together, this means that the fraction of all stars at the same brightness that Gaia sees varies strongly across the sky.