<P> A spiral galaxy like the Milky Way contains stars , stellar remnants , and a diffuse interstellar medium ( ISM ) of gas and dust . The interstellar medium consists of 10 to 10 particles per cm and is typically composed of roughly 70 % hydrogen by mass , with most of the remaining gas consisting of helium . This medium has been chemically enriched by trace amounts of heavier elements that were ejected from stars as they passed beyond the end of their main sequence lifetime . Higher density regions of the interstellar medium form clouds , or diffuse nebulae , where star formation takes place . In contrast to spirals , an elliptical galaxy loses the cold component of its interstellar medium within roughly a billion years , which hinders the galaxy from forming diffuse nebulae except through mergers with other galaxies . </P>
where do most star formation occur in the milky way today
Higher density regions of the interstellar medium form clouds , or diffuse nebulae