interstellar medium



  • The gas and dust spread through the galaxy, observable by means including its reddening effect on starlight, its infrared emissions and the 21cm absorption and emission lines which it shows in radio frequencies. The interstellar medium is at its most visible where it is gathered into dense nebulae which emit visible light, which can also be the seats of star formation. It consists mainly of hydrogen and helium atoms dating back to the formation of the universe, plus heavier atoms formed inside stars and emitted from them during their normal lifetimes or by nova and supernova explosions.