Here's why:
* Hydrogen Emission: Hydrogen is the most abundant element in the universe. When hydrogen atoms in a cloud are excited by nearby stars, they absorb energy and jump to a higher energy level.
* Photon Emission: As these excited hydrogen atoms return to their ground state, they release the absorbed energy in the form of photons (light). The specific wavelength of light emitted depends on the energy difference between the excited and ground states.
* Red Light: For hydrogen, the most common transition releases a photon with a wavelength in the red part of the visible spectrum (specifically the Balmer series). This is why many interstellar clouds appear reddish.
Note: While the dominant emission from hydrogen is red, other elements and molecules in the cloud can contribute to other colors. However, the overall appearance of a hydrogen-rich cloud will be dominated by the red glow.
