 When stars form in the present Milky Way galaxy they are composed of about 71 % hydrogen and 27 % helium , as measured by mass , with a small fraction of heavier elements . Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere , as iron is a common element and its absorption lines are relatively easy to measure . The portion of heavier elements may be an indicator of the likelihood that the star has a planetary system . 
 When stars form in the present Milky Way galaxy they are composed of about 71 % hydrogen and 27 % helium , as measured by mass , with a small fraction of heavier elements . Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere , as iron <a href='drop_aux' title='1'><mark>is</mark></a>a common element and its absorption lines <a href='drop_aux' title='2'><mark>are</mark></a>relatively easy to measure . The portion of heavier elements may be an indicator of the likelihood that the star <a href='got' title='3'><mark>has</mark></a> a planetary system . 
When stars form in da present Milky Way galaxy they r composed of about 71 % hydrogen and 27 % helium , as measured by mass , wid a small fraction of heavier elements . Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere , as iron a common element and its absorption lines relatively easy to measure . The portion of heavier elements may be an indicator of the likelihood that the star got a planetary system .