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Glossary > D - Pg. 231

gloSSary 231 billion years ago. Chondrites are so named because they con- tain chondrules. chondrules: Round, millimeter-size pebbles found in chon- drites. They formed as molten droplets before being incorpo- rated into parent bodies. comet: A small icy body. As a comet approaches the Sun, subli- mation (turning material directly from solid to gas) can produce a tenuous envelope called the coma as well as an extended tail. constellation: A grouping of stars in the night sky that forms a pattern, usually derived from mythology. The stars of a given constellation do not necessarily have any physical relation among them; they are located at vastly different dis- tances from the Earth and from one another. By agreement of the International Astronomical Union, the sky is divided into eighty-eight constellations, or unequal regions.