What is the primary process by which nebulae contribute to star formation?

The primary process by which nebulae contribute to star formation is the formation of protostars due to gravitational forces. Nebulae are composed of gas and dust, and regions within these nebulae can become dense due to gravitational attraction. As these regions collapse under their own gravity, they begin to form protostars.

During this process, the gravitational forces cause the material to clump together and compress, leading to an increase in temperature and pressure at the core of the collapsing region. Eventually, when the temperatures and pressures become sufficiently high, nuclear fusion reactions initiate, leading to the birth of a new star. This explains how nebulae are integral in the lifecycle of stars, serving as the initial "nursery" where stars are created.

The other processes listed do not directly relate to the mechanism of star formation. While gases may be released into space or planets may form from nebular material, these are not the primary means by which stars come into existence. Thus, the emphasis on gravitational forces leading to the formation of protostars highlights the critical role of nebulae in the process of star formation.