Which of these is NOT a source of the shock waves that lead to protostars?

A. radiation from the OB stars in emission nebulae
B. expanding Herbig-Haro objects
C. expanding planetary nebula shells
D. violent supernovae explosions
E. collisions between galaxies


To determine which of the options is NOT a source of shock waves that lead to protostars, we need to understand the sources of shock waves in the context of star formation.

During the formation of a star, shock waves are generated by various processes, which result in the compression and heating of interstellar gas, leading to the collapse of gas and the formation of a protostar. Let's go through each option to see if it fits this description:

A. Radiation from the OB stars in emission nebulae: Massive stars, such as OB stars, emit intense ultraviolet radiation that can ionize surrounding gas and create shock waves. This is a source of shock waves that can contribute to protostar formation.

B. Expanding Herbig-Haro objects: Herbig-Haro objects are formed when material ejected from a young star interacts with surrounding gas, creating shock waves. These shock waves can compress gas and contribute to protostar formation.

C. Expanding planetary nebula shells: Planetary nebulae are formed during the late stages of a star's evolution, when the outer layers of the star are ejected into space. The expanding shells of planetary nebulae can create shock waves and compress interstellar gas, facilitating star formation.

D. Violent supernova explosions: Supernovae occur when massive stars reach the end of their lives and explode. These explosions release an enormous amount of energy, which can trigger shock waves and compress surrounding gas, leading to the formation of new stars.

E. Collisions between galaxies: Interactions between galaxies can induce shock waves due to the gravitational forces involved, but this process is primarily related to the formation of galaxies themselves, rather than the formation of individual protostars.

By analyzing the options, we can see that "E. collisions between galaxies" is the one that is NOT a source of the shock waves that lead to protostars. While galactic collisions can create shock waves, they are not directly associated with the formation of protostars.