Field guide · Deep sky

Emission nebula

A cloud of hydrogen lit from inside: hot young stars flood it with ultraviolet light, and the gas glows like a neon sign.

Made of

Mostly hydrogen, lit by stars inside it

Typical size

Tens to hundreds of light-years

Lifetime

A few million years

Best seen with

Dark skies, low power, a nebula filter

Most of the space between stars is almost empty, but not quite. Where the gas gathers into a cloud and a hot, young star switches on inside it, that star’s ultraviolet light strips the electrons off the hydrogen atoms. When the electrons fall back, each atom gives the energy back as light of one exact colour — mostly a deep red called hydrogen-alpha. That is literally how a neon tube works.

These clouds are where stars are born. The Orion Nebula is making stars right now, and the four bright stars at its heart, the Trapezium, are the ones lighting it up. In a few million years their winds will have blown the cloud away and left an open cluster behind.

To the eye the red barely registers — our night vision is almost blind to it — so a nebula looks grey-green through a telescope. The famous pinks and reds are what a camera sees. A nebula filter, which blocks everything except the few colours the gas emits, makes a startling difference from a light-polluted garden.

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