How are stars born?
A. We haven’t been able to observe in detail all of the stages in this process, but from many specific young objects we have studied, it all seems to start with a dense, interstellar cloud many light years across. For reasons that we do not fully understand, small regions within such a cloud perhaps a fraction of a light year across begin to collapse under their own gravity. As the collapse continues, the center of this core region becomes denser and denser, climbing from only 100 atoms per cubic centimeter to millions of atoms per cubic centimeter and higher. As it collapses, whatever very slight rotation it originally had gets amplified so that, like a figure skater on ice, it spins faster and faster. Although the gas falling along the axis of the collapsing cloud feels nothing more than the gravitational force of the central core, along the equator of the object, centrifugal forces due to its spinning become so strong that they impede the collapse along this direction. The cloud col
