Spectral line ratios are used by scientists to measure the composition of distant stars or galaxies. It's like a game of mixing colors to get a specific color. For example, if your paint container only has two colors: red and blue, you could mix these colors together to make purple. In a similar way, scientists observe two different colors of light coming from stars or galaxies and then measure how much of each color there is to figure out what elements make up the star. This information helps scientists learn more about far-away objects in space.