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Emission nebulae are usually red, because hydrogen, the most common gas in the universe, most commonly emits red light. Reflection nebulae are clouds of dust that simply reflect the light of a nearby star or stars. Reflection nebulae are usually blue, because blue light scatters more easily.Emission and reflection nebulae are often seen together and are sometimes both referred to as diffuse nebulae.In some nebulae, the star formation regions are so dense and thick that light cannot get through.eflection nebulae are usually blue, because blue light scatters more easily. Emission and reflection nebulae are often seen together and are sometimes both referred to as diffuse nebulae. In some nebulae, the star formation regions are so dense and thick that light cannot get through.
Many nebulae or stars form from the gravitational collapse of gas in the interstellar medium or ISM. As the material collapses under its own weight, massive stars may form in the center, and their ultraviolet radiation ionizes the surrounding gas, making it visible at optical wavelengths.he size of these nebulae, known as HII regions, varies depending on the size of the original cloud of gas. New stars are formed in the nebulae. The formed stars are sometimes known as a young, loose cluster. Some nebulae are formed as the result of supernova explosions, the death throes of massive, short-lived stars.