There are a total of 50 pieces of Artwork a.k.a. Alien Artifacts available in the game, ten on each map. These are spinning objects that glow quite strongly with red light. To collect them, all you have to do is get close enough to them. Each such artifact unlocks the next piece of art associated with the map on which it was found. It is always the next piece of art, the order in which you find the artifacts is not important.
Finding all of them will unlock the "Arty Facts" achievement / trophy.
