Here's why they appear reddish:
* Hydrogen: Emission nebulae are primarily composed of hydrogen, the most abundant element in the universe.
* Ionization: The intense radiation from nearby stars can ionize hydrogen atoms, causing them to emit light.
* Red Light: The most prominent emission line from ionized hydrogen is in the red part of the visible spectrum.
Other possibilities:
* Supernova remnants: The expanding shock waves from a supernova explosion can heat and excite surrounding gas, creating a reddish glow. However, these are often more complex and chaotic in appearance than simple loops.
* Planetary nebulae: These are formed when a star like our Sun sheds its outer layers. While they can appear reddish, their shape is more typically spherical or bipolar, rather than simple loops.
Note: The exact appearance and color of a reddish gas loop will depend on the specific composition of the gas, the age and type of the exciting star, and other factors.
