Nebulae
> Nebulae are huge interstellar clouds of gas and dust that appear 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 nebula 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 nebula is ionized and emits a certain amount of light of its own.
>If a star is less hot, the nebula shines only by reflection.
> If there are no suitable stars, the nebula 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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