The Cepheus B molecular cloud, located about 2,400 light-years from Earth. This is a composite image, combining data from the Chandra X-ray Observatory and Spitzer Space Telescope. The Spitzer data, in red, green, and blue shows the molecular cloud (in the bottom part of the image) plus young stars in and around Cepheus B, and the Chandra data in violet shows the young stars in the field. Image and text: NASA/CXC/JPL-Caltech/PSU/CfA.
A molecular cloud is a cold, dense interstellar cloud that contains a high fraction of molecules, of which well over 100 different types have been discovered in space (see interstellar molecules). It is widely believed that the relatively high density of dust particles in these clouds plays an important role in the formation and protection of the molecules. The emission of molecular lines often shows several distinct intensity peaks, each representing individual clumps or clouds of gas and dust in a region that characteristically extends for 50 light-years and is often associated with T Tauri stars – young, pre-main-sequence stars – and also hot massive stars and the ionized gas around them. Two distinct types, of molecular cloud are known, both associated with star formation: giant molecular clouds and dwarf molecular clouds.