ro sc opy , ▁scient ists ▁were ▁puzz led ▁by ▁the ▁spectrum ▁of ▁g ase ous ▁ne b ula e . ▁In ▁ 1 8 6 4 ▁William ▁H ugg ins ▁noticed ▁that ▁many ▁ne b ula e ▁showed ▁only ▁emission ▁lines ▁rather ▁than ▁a ▁full ▁spectrum ▁like ▁stars . ▁From ▁the ▁work ▁of ▁Kirch hoff , ▁he ▁concluded ▁that ▁ne b ula e ▁must ▁contain ▁" en orm ous ▁masses ▁of ▁l umin ous ▁gas ▁or ▁v ap our ." ▁However , ▁there ▁were ▁several ▁emission ▁lines ▁that ▁could ▁not ▁be ▁linked ▁to ▁any ▁terrest rial ▁element , ▁bright est ▁among ▁them ▁lines ▁at ▁ 4 9 5 . 9   nm ▁and ▁ 5 0 0 . 7   nm . ▁These ▁lines ▁were ▁attributed ▁to ▁a ▁new ▁element , ▁ne bul ium , ▁until ▁Ira ▁Bow en ▁determined ▁in ▁ 1 9 2 7 ▁that ▁the ▁emission ▁lines ▁were ▁from ▁highly ▁ion ised ▁o xygen ▁( O + 2 ). ▁These ▁emission ▁lines ▁could ▁not ▁be ▁rep licated ▁in ▁a ▁labor atory ▁because ▁they ▁are ▁forb idden ▁lines ; ▁the ▁low ▁density ▁of ▁a ▁ne b ula ▁( one ▁atom ▁per ▁cub ic ▁cent imet re ) ▁allows ▁for ▁met ast able ▁ ions ▁to ▁decay ▁via ▁forb idden ▁line ▁emission ▁rather ▁than ▁coll isions ▁with ▁other ▁atoms . ▁ ▁Not ▁all ▁emission ▁ne b ula e ▁are ▁found ▁around ▁or ▁near ▁stars ▁where ▁solar ▁he ating ▁causes ▁ion isation . ▁The ▁majority ▁of ▁g ase ous ▁emission ▁ne b