The most appropriate one-word substitution for "an optical device for projecting various celestial images and effects" is planetarium.

• Auditorium is a large hall for public gatherings, such as a concert hall or a theater.
• Amphitheatre is a large outdoor theater with a semicircular seating area.
• Galaxy is a huge group of stars, gas, and dust held together by gravity.

The word planetarium comes from the Latin words "planēta," meaning "wandering star," and "-arium," meaning "place of." A planetarium is a building or room that contains a planetarium projector, which is used to project images of celestial objects onto a domed ceiling.