Technologically advanced planetariums are too expensive for most schools and communities. But many of them have purchased inexpensive portable planetariums. For instance, one model has an inflatable dome that is designed to hold 35 children. The projector has a small light bulb enclosed by a plastic cylinder and a black cylinder with clear spots for projecting the stars. There are other affordable cylinders, which show galaxies, constellations, and even the inside of a living cell.