Final answer:
Triggered star formation in molecular clouds is inefficient with the gravitational contraction of a hot gas cloud because the increased temperature prevents compression. Mechanisms like supernova explosions and intense UV radiation from massive stars are much more effective in compressing gas and triggering star birth.
Step-by-step explanation:
The triggering of star formation in molecular clouds can occur through various mechanisms. However, one that is considered to be ineffective and inefficient is gravitational contraction of a hot gas cloud. This process involves the conversion of gravitational potential energy into kinetic energy, which leads to a rise in temperature as the cloud collapses. For star formation, cold molecular clouds are preferred because when the gas is cold, it is easier for gravity to overcome thermal pressure, leading to the formation of stars. In contrast, hotter gas expands and is less likely to undergo sufficient compression to trigger star birth.
Conversely, mechanisms such as supernova explosions and collisions between molecular clouds, which generate shock waves, and radiation pressure from the intense UV radiation from hot stars, are effective at compressing the gas and increasing densities in molecular clouds to prompt star formation. These processes provide a substantial energy input that can cause nearby gas to collapse and form new stars in a chain-reaction pattern. Multiple regions within the Milky Way showcase this phenomenon of triggered star formation, where the life and death of massive stars play a crucial role.