Synopsis: Otter learns that light contains many colors, like a rainbow, and can be thought of as a spectrum.  Crab tells Otter that each element has spectrum that is unique.  Otter learns about Helium, Neon, Hydrogen, Potassium, and Nitrogen and sees that each has a different spectrum.  Crab explains that light from stars may  also be "captured" by gaseous elements in between the star and us, leaving gaps in the stars spectrum.  These gaps tell us what elements were present in the gas.  Otter learns that stars are very hot, far away, and contain primarily hydrogen and helium as their constituent elements.