They go out and point a telescope at the sky, take a picture, identify what are stars and throw them away, look at the galaxies, estimate how far away they are, and plot them up.
Ellos salen y apuntan el telescopio al cielo, toman una foto, Identifican que son estrellas y las botan, miran a las galaxias, Calculan que tan lejos están, y las diagraman.