Formation of stars and clusters over cosmological time
  The concept that stars form in the modern era began some 60 years ago with
the key observation of expanding OB associations. Now we see that these
associations are an intermediate scale in a cascade of hierarchical structures
that begins on the ambient Jeans length close to a kiloparsec in size and
continues down to the interiors of clusters, perhaps even to binary and
multiple stellar systems. The origin of this structure lies with the dynamical
nature of cloud and star formation, driven by supersonic turbulence and
interstellar gravity. Dynamical star formation is relatively fast compared to
the timescale for cosmic accretion, and then the star formation rate keeps up
with the accretion rate, leading to a sequence of near-equilibrium states
during galaxy formation and evolution. Dynamical star formation also helps to
explain the formation of bound clusters, which require a local efficiency that
exceeds the average by more than an order of magnitude. Efficiency increases
with density in a hierarchically structured gas. Cluster formation should vary
with environment as the relative degree of cloud self-binding varies, and this
depends on the ratio of the interstellar velocity dispersion to the galaxy
rotation speed. As this ratio increases, galaxies become more clumpy, thicker,
and have more tightly bound star-forming regions. The formation of old globular
clusters is understood in this context, with the metal-rich and metal-poor
globulars forming in high-mass and low-mass galaxies, respectively, because of
the galactic mass-metallicity relation. Metal-rich globulars remain in the
disks and bulge regions where they formed, while metal-poor globulars get
captured as parts of satellite galaxies and end up in today's spiral galaxy
halos. Blue globulars in the disk could have formed very early when the whole
Milky Way had a low mass.
