The normal form of hydrogen gas found on Earth, with two hydrogen atoms bound together by two shared electrons. Hydrogen molecules cannot survive in most regions of space, because they are easily dissociated (split) by ultraviolet light from stars, but they do occur in cold, dense molecular clouds where they are shielded by dust particles. Molecular hydrogen is the main constituent of molecular clouds, but it is difficult to observe directly as it is a symmetrical molecule with no strong radio or millimetre-wave spectral lines.