Spiral arms , for example , are the location of star formation .
Arms contain numerous giant molecular clouds and a high density of stars that can perturb a star 's Oort cloud , sending avalanches of comets and asteroids toward any planets further in .
Spiral arms , for example , are the location of star formation , but they contain numerous giant molecular clouds and a high density of stars that can perturb a star 's Oort cloud , sending avalanches of comets and asteroids toward any planets further in .