The sessions are short and informal, though total time depends on how clear the sky is that night and how many students show up. The purpose of these sessions is to allow the students to actually see how the telescopes and observatories work. There is a schedule put out every semester listing about 15 to 20 sessions to choose from, but please note that they can be cancelled due to weather, so call the information number that is listed to make sure the session is still planned for that evening. Also note that the open house sessions at HLCO and ASC each month are held rain or shine. We can tell by looking at the spectra of the star from the star's emission of light.
All stars radiate energy, particularly in the forms of heat and light.
Think of a prism hanging in a window. When light comes through the prism, the prism casts tiny rainbows around the room. These rainbows are tiny spectra of the light. Just like a microscope magnifies tiny objects so we can see finer detail, spectroscopes and spectrographs allow us to study the spectra of different objects in much greater detail. They do so by using prisms or gratings that spread out the light.