Star Formation: Following the Evolutionary Sequence from Cloud Assembly through Protostars and Circumstellar Disks

银河系中大部分恒星的形成都发生在巨大的分子云中。简单地说，恒星的形成是局部引力坍缩的结果，由此产生的星盘系统的性质取决于气藏的数量和分布，以及它的运动学和角动量。然而，对分子云的观测揭示了一个具有大规模超音速流动、显著剪切和超音速湍流证据的动态环境。云中密度更大的气体，与正在形成的恒星的位置有关，显示出更有序的结构，开始是一个由细丝组成的网络，里面有密集的团块和核心。这些细丝中的一些可能表示激波前界面，另一些可能直接与云内的重力下降有关。事实上，人们预计分子云内部结构的形成是由于重力和大规模超音速（湍流）运动之间的竞争。磁场中介也可能起重要作用。

The bulk of the star formation in our Galaxy takes place within giant molecular clouds. At its simplest, the formation of a star is the result of localized gravitational collapse and the properties of the resulting star-disk system are dependent on the amount and distribution of the gas reservoir, along with its kinematics and angular momentum. Observations of molecular clouds, however, reveal a dynamic environment with large-scale supersonic flows, significant shear, and evidence for supersonic turbulence. The denser gases within the clouds, which correlate with the locations of on-going star formation, show a more ordered structure, beginning with a network of filaments housing dense clumps and cores. Some of these filaments may denote shock front interfaces and others may relate directly to the gravitational infall within the cloud. Indeed, it is expected that the formation of structure within a molecular cloud is due to the competition between gravity and large-scale supersonic (turbulent) motion. Magnetic field mediation may also play an important role.