The teacher will place two small balls on the extremities of the rod and insert the loop in the middle and make the system spin. After, the teacher will substitute consecutively a small ball with larger ones, as can be guessed, the outcome will be that he will have to move the location where he places the center of the loop towards the heavier balls. He should also place the large light ball to show the location of the center of mass does not depend on the volume but on the mass. Using a heavy and light ball pupils understand why sometimes the center of mass lies inside the more massive object. This demonstration is also useful to explain eclipsing binaries.
Pupils can see how one star can eclipse another total or partially.
These candles can be later employed to show how another method of distance determination is measuring interstellar absorption. The instructor can place tinted viewgraphs amongst the candles to show how light intensity decreases by absorption and dispersion and consequently it is possible to estimate distances to stars assuming extinction is proportional to distance. The teacher should mention the difficulty of calculating distances to extended objects such as planetary nebulae whose envelope varies in size. He can explain how, by comparing the extinction of stars of known distance, that are in its vicinity to that of the planetary nebulae, its distance can be estimate. Some students do not understand that the main purpose of a telescope is to gather light.