emission nebula

Some of the most beautiful sights in the sky are great glowing clouds of pink and red — the famous nebulae photographed in poster after poster. An emission nebula is a cloud of interstellar gas that shines with its own light, not merely reflecting starlight but actually producing its own glow, powered by hot young stars buried within or beside it.

Here is how it works. A very hot, massive star pours out ultraviolet light, which is energetic enough to tear electrons off the surrounding hydrogen atoms — ionizing them. The freed electrons then wander until they are recaptured by protons, and as an electron cascades back down through the atom's energy levels it emits light at specific, sharp wavelengths. The strongest of these is the deep-red hydrogen-alpha line at 656 nanometers, which gives most emission nebulae their characteristic rosy color. The ionized region around a hot star is called an HII region (said 'H-two,' meaning ionized hydrogen). A nearby example, the Orion Nebula, is about 1,300 light-years away and 24 light-years across.

Emission nebulae are signposts of recent star birth — wherever you see one, massive young stars have lit up the leftover gas of the cloud they formed in. They are living laboratories for studying how hot stars sculpt their surroundings, and their light, split into a spectrum, reveals the chemical makeup of the gas. A common mix-up is to think the nebula glows because it is reflecting the star's light; in truth it is emitting its own light at distinct wavelengths, which is why its spectrum shows bright emission lines rather than a smooth rainbow.