Below is the primary yellow-light sodium emission line, from heated sodium atoms. Above is an absorption spectrum where light from a blackbody-like source (such as a star) has passed through a cloud containing sodium on its way to the observer or imaging instrument. The black line is where the sodium atoms have absorbed light preferentially at that frequency, leaving a dark line at that “color”. Image courtesy of ThinkQuest (www.thinkquest.org), funded by the Oracle Education Foundation