A group of elongated clouds of gas and dust with a fine thread-like structure as seen from Earth. Many filamentary structures may actually be sheets or parts of shells seen edge-on, rather than threads. The best-known filamentary nebulae, such as the Veil Nebula, are supernova remnants. Although these filaments have temperatures of 10 000 K they are actually the coolest parts of the remnant, other parts of which remain at over 1 million K.