Astronomy Answers
From the Astronomical Dictionary

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The description of the word you requested from the astronomical dictionary is given below.

the filament ⇒ ⇒

'fi‧la‧ment, plural filaments; filum = [Latin] thread

In general, a filament is something that looks like a thin thread.

In astronomy, filament is used as a name for (amongst other things)

• a thread-like structure in the Universe produced by the distribution of galaxies.
• a thread-like cloud of gas above the surface of the Sun, which looks dark when seen against the bright solar disk, especially in the light of the Hα spectral line. If such a cloud is seen beyond the limb of the Sun then it appears bright and is called a prominence.
• a dark or bright thread-like structure in the penumbra of a sunspot.
• a thread-like structure in a cloud of gas such as a supernova remnant.