Nebulae are huge interstellar clouds of gas and dust that appears as faint, misty patches of light scattered all over the sky.
They appear either as bright luminous clouds or as dark patches against a brighter background.
A nebulae depends for its luminosity upon the presence of stars that have either arisen from it or are contained in it.
If the stars are extremely hot the hydrogen in the nebulae is ionized and emits a certain amount of light of its own.
If a star is less hot the nebulae shines only by reflection.
If there are no suitable stars the nebulae does not shine and remains dark and can be detected only because it blots out the light of the stars beyond.
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