Answer and Explanation:
The nebulae are compressed due to the accumulation of inert helium atoms. This is because a nebula is composed of low mass stars, which spend many years consuming hydrogen and producing helium, like any other stars. However, low-mass snaps do not have enough temperature to cause helium atoms to melt. This causes these atoms to remain inert and accumulate in the nucleus of stars for years. This directly influences the radiation pressure of the star's nucleus, which becomes insufficient to compensate for the gravitational force that the star has in relation to the mass. The solution to this is to start a nebula compression process.
The result of the compression process is heat that acts on the hydrogen atoms and allows an accelerated fusion between them, on the surface of the star. Consequently, the star expands and diffuses energy over a larger area. All of this causes the temperature on the surface of the star to cool, making it red and creating what we call the red giant.