GRE Reading Comprehension: JiJing 352-GRE阅读机经352篇 - 11FO57O5Q7055YYXK

Large spiral-shaped galaxies have giant clouds of molecular hydrogen that can readily form stars. The spiral-arm pattern of these galaxies is maintained by density waves, which continually form new stars by compressing the molecular clouds that they pass through. In contrast, dwarf galaxies are dormant for billions of years, then erupt in furious short-lived bursts of star formation before becoming dormant again. These galaxies have little molecular hydrogen. They do have abundant atomic hydrogen – that is, hydrogen atoms floating freely rather than bound into two atom molecules. Because clouds of atomic hydrogen are less dense than clouds of molecular hydrogen, they are less likely to lapse gravitationally and produce stars. Furthermore, because dwarf galaxies lack density waves and other organized gas motions that can cause clouds to collapse, they are mostly quiescent.