Nebulae

Point a telescope at the sword hanging from Orion's belt and the middle "star" dissolves into a glowing cloud, bigger than the full Moon, with a knot of brilliant young stars at its heart. That cloud is the Orion Nebula, and it is one of the most beautiful objects in the sky. The word nebula is simply Latin for cloud, and for centuries astronomers used it for anything fuzzy. Today it means a cloud of gas and dust in space, and such clouds come in several very different kinds.

Emptier than empty

The first surprise about nebulae is how thin they are. A nebula that looks like a dense bank of fog contains, on average, a few hundred to a few thousand atoms per cubic centimetre. The air you are breathing contains about 25 billion billion molecules in the same volume. The best vacuum chambers on Earth are still denser than most nebulae. They only appear solid because they are so enormous, often tens of light-years across, so that the thin gas adds up along the line of sight.

Most of that gas is hydrogen, the universe's commonest element, with helium and traces of everything else. The dust is tiny grains of carbon and silicate, like fine soot and sand, which block starlight and make parts of a nebula appear as dark silhouettes.

Clouds that glow

Nebulae shine for three reasons. Emission nebulae glow with their own light. Hot young stars inside them pour out ultraviolet radiation that strips electrons from the surrounding hydrogen atoms; when the electrons recombine, the atoms emit light at specific colours. Hydrogen's signature is a deep red. Oxygen gives blue-green, sulphur a different red. The vivid colours in nebula photographs are real, though often made more intense by the camera, and each colour is a chemical fingerprint.

Reflection nebulae do not glow themselves. Their dust simply scatters the light of nearby stars, and because dust scatters blue light more efficiently than red, they tend to look blue, for the same reason the daytime sky does.

Dark nebulae are clouds so thick with dust that they block the light behind them entirely. The Horsehead Nebula is a dark cloud standing in front of a glowing red one.

Star nurseries

The great emission nebulae are the places where stars are being born. Deep inside the Orion Nebula, infrared telescopes see hundreds of newborn stars still wrapped in the cocoons of gas and dust that fed them. Some are surrounded by flat discs of material, protoplanetary discs, in which planets are presumably forming right now. Our own Sun and its planets were born in a cloud like this 4.6 billion years ago.

The brightest stars in such a nebula are also its destroyers. Their fierce radiation and stellar winds carve the cloud into pillars and bubbles, and within a few million years they blow it apart completely, leaving behind a cluster of stars and no nebula at all. The Pleiades, the little cluster of blue stars visible on winter nights, is a nursery whose cloud has already been dispersed.

Clouds made by dying stars

Not all nebulae are cradles. When a star like the Sun reaches the end of its life, it puffs its outer layers into space over a few thousand years. The hot exposed core lights the drifting gas, producing a planetary nebula, often a delicate ring or hourglass shape. The name is a historical accident; the eighteenth-century astronomer William Herschel thought their round discs resembled the newly discovered planet Uranus. Planetary nebulae are brief by cosmic standards, fading away within about ten thousand years.

Massive stars leave behind something more violent: a supernova remnant. The Crab Nebula is the shredded remains of a star that Chinese astronomers watched explode in the year 1054, so bright it was visible in daytime for three weeks. Its filaments are still racing outward at 1,500 kilometres per second, and at its centre a neutron star spins thirty times a second.

Why it matters

Nebulae are the recycling plants of the galaxy. Dying stars return their gas, now enriched with carbon, oxygen, iron and heavier elements, to the clouds between the stars. Those clouds collapse to form new stars and planets that inherit the enrichment. The Sun is a third-generation star at least, built from material processed through earlier nebulae. The oxygen you are breathing was once part of a glowing cloud like Orion.