Solution

The first image, and every second image after that each refer to a star. The first letters in the title are HD, hinting that the Henry Draper (HD) catalogue's classifications for these stars should be found. From there, the other images contain either positive or negative numbers, indicating whether to increase or decrease the HD classification number. If the classification is increased and decreased in order, each step will land at a new star. If a line is drawn between all the stars in the correct order, the word orlop will be written across the night sky.