The present participle of combust; currently burning or undergoing the process of combustion.
From 'combust' plus the present participle suffix '-ing' (from Old English '-ende'). This form shows the ongoing nature of the burning process as it happens.
The sun isn't actually 'combusting' in the way a bonfire is—it's fusing hydrogen nuclei. Early scientists used 'combusting' because it was the only burning word they had, which is why we say the sun 'burns' when it actually does something totally different.
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