Stars ~ Stars form in the densest regions of the interstellar medium, or ISM, called molecular clouds. The ISM is the name given to the gas and dust that exists between the stars within a galaxy. It is 99% gas and 1% dust, by mass. Molecular clouds are perfect star-forming regions because the combination of these atoms into molecules is much more likely in very dense regions.
Monday, Dec. 8, 2014: Stars form in textbook fashion inside emission nebula NGC 2174, where molecular clouds condense into star formation regions. New stars next slowly blow unused material back into the interstellar medium. After the lengthy process has almost concluded, the stars have broken out into the open.
The Physics of the interstellar medium / J.E. Dyson, D.A. Williams #novetatsfiq
Our Galaxy is filled with gas. Most of this gas is hydrogen, some is helium, but there is a trace amount of relatively heavy molecules, including carbon monoxide (CO) - a component of smog. The above wide-angle radio CO image shows the incredibly diverse structures that the molecular interstellar medium forms. Dense clouds show where stars may be forming and open voids may indicate the action of strong winds from massive, recently formed stars. http://apod.nasa.gov/apod/ap010115.html
Beautiful... Stars form within nebula NGC 2170, which lies in the constellation of Monoceros (The Unicorn). A dark nebula, such as this one, provides raw material for the star formation going on inside them. The newly formed, massive blue stars seen here continue to push away traces of the dust that previously hid them from view. The material that remains will eventually disperse in the interstellar medium.
A supernova explosion enriches the surrounding interstellar medium with heavy elements. The outer rings are caused by previous ejecta, long before the final explosion. Image credit: ESO / L. Calçada, of the remnant of SN 1987a