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Most nebulae is widespread and that means that they are very large and do not have well-defined boundaries. In visible light, these nebulae can be divided in emission nebulae and reflection nebulae based on how you create the light we see. The emission nebulae contain ionized gas (mostly ionized hydrogen) that produces spectral lines of emission. They are often called H II regions derived from the language of the professional astronomers referring ionized hydrogen.