Time Scales in Starbursts

Starbursts are associated with distinct time scales spanning at least three orders of magnitude from 10 to 10 yr. The triggering of nuclear starbursts occurs over 10 to 10 yr, when gas is flowing to the galaxy center due to angular momentum loss, thereby raising the nuclear gas density. Star formation then increases as a result of the increased gas pressure. In addition, spontaneous star formation can occur in nuclear and off-nuclear regions if gas clouds are compressed by strong shocks. The star formation time scales in individual starburst regions are short: typically 10 to 10 yr. The subsequent evolution of a newly formed starburst is determined both by dynamical effects and by the influence of the massive stars on the surrounding interstellar medium over an evolutionary time scale (about 10 yr). Eventually starbursts fade since the ambient interstellar gas is removed from the birth sites due to winds and supernova explosion over times scales of order 10 to 10 yr.