 Stars <unk> from regions of space of higher density , yet those regions are less dense than within a vacuum chamber . These regions - known as molecular clouds - consist mostly of hydrogen , with about 23 to 28 percent helium and a few percent heavier elements . One example of such a star @-@ forming region is the Orion Nebula . Most stars form in groups of dozens to hundreds of thousands of stars . Massive stars in these groups may powerfully illuminate those clouds , ionizing the hydrogen , and creating H II regions . Such feedback effects , from star formation , may ultimately disrupt the cloud and prevent further star formation . 
