Star formation differs widely in different galaxy types (e.g.,
Kennicutt 1998),
ranging from slow, low-efficiency
events that may be long-lasting to intense, short-duration starbursts.
Along the Hubble sequence, typical global present-day star formation
rates range from ~ 0
M
yr-1 in giant ellipticals
to ~ 20 M
yr-1 in gas-rich, late-type spirals.
Starburst galaxies show star formation rates of up to ~ 100
M
yr-1, and ultra-luminous infrared galaxies (ULIRGs)
appear to form up to ~ 1000
M
yr-1 in stars. Star
formation may be localized or encompass a large fraction of the
baryonic gas mass of a galaxy. It may be continuous, declining or
increasing in intensity, or episodic. It may be triggered by internal
processes within a galaxy or by interactions with other galaxies.
Typical sites of present-day star formation in galaxies are located in
the extended disks of spirals and irregulars, in the dense gas disks
in galaxy centers (circumnuclear star formation), and in regions of
compressed gas in starbursts, interacting galaxies, or tidal tails.
The star formation histories of galaxies vary with galaxy type, mass,
gas content, and environment as well as with time, and are tightly
coupled with their chemical evolution.