Stars emit light (electromagnetic radiation) and heat. If you take the light from a star and send it through an instrument called a spectrograph, you can essentially break up the light into its component colors (wavelengths). You’ve seen one form of a spectrum in nature: it’s called a rainbow and it was created by light being broken up through a prism of raindrops.
The image above is a graph spectrum showing us the chemical elements that exist in a star called NGC1333-IRAS 4B. The infrared light was analyzed by an instrument aboard the Spitzer Space Telescope (which is sensitive to infrared wavelengths). The scientists compared it to a model of a water spectrum, and found water vapor in the region surrounding the star. What they think is happening is that ice particles in the surrounding environment are falling toward the star. When they hit the disk of gas and dust around the star, they heat up and melt, forming water vapor.
These details are in the spectrum, which tells us about the motion of the ice particles surrounding the star.
Spectra are a part of astrophysical research that can look pretty boring or confusing to people who don’t see them every day. Yet, if you know how to read them and what to look for, they can reveal details of an astronomical object that you just can’t see with the naked eye or in an image. Here’s another one, from a recent Gemini Observatory press release, that shows the evidence for water and ammonia ices on Pluto’s companion world, Charon. It is centered on infrared light radiating at 2.2 microns. The solid line is a model of a surface with ices called ammonia hydrates, along with water ices. Other dots are the data from the surface of Charon that represent ammonia hydrate ices. (You can read more about this one here.)
Now I don’t normally “do” spectra in my planetarium shows, mostly because they require more explanation than we often have time for. But, spectra ARE treasure troves of information, hidden right before our eyes.