What Is a Nebula? The Clouds Where Stars Are Born and Die

Interstellar space is not empty, only thin. Between the stars drift atoms of hydrogen and helium left over from the early universe, seasoned with heavier elements and microscopic grains of carbon and silicate dust produced by earlier generations of stars. In most places this material is spread so thinly that a cubic centimetre holds a single atom. A nebula is a region where it has gathered, typically to a few hundred or a few thousand atoms per cubic centimetre, which is still a far better vacuum than any laboratory on Earth can make.

A cloud that thin only becomes visible because it is enormous, often light years across, so that along the line of sight the gas adds up to something that can glow or block light. How it looks depends on what is lighting it and from where.

An emission nebula shines with its own light. A hot young star inside it pours out ultraviolet radiation that strips electrons from the hydrogen atoms, and when the electrons recombine they release light at particular wavelengths, most strongly a deep red. The Orion Nebula, visible to the naked eye as the middle star of Orion's sword, is the nearest large example, about 1,300 light years away.

A reflection nebula does not glow itself. Its dust scatters the light of nearby stars, and because dust scatters blue light more efficiently than red, for the same reason the sky is blue, reflection nebulae tend to look blue. The wisps around the Pleiades cluster are the classic case. A dark nebula is a cloud dense enough to block the light behind it, seen as a silhouette against a brighter background, like the Horsehead Nebula or the Coalsack in the Southern Cross.

The densest, coldest nebulae are molecular clouds, so called because their hydrogen is cold enough to pair into molecules. Gravity works slowly on these clouds. Where a region becomes a little denser than its surroundings, perhaps nudged by the shock wave from a nearby exploding star, it begins to collapse under its own weight, and the collapsing knot heats until fusion begins at its centre. A star has formed, and usually a cluster of them, since a large cloud fragments into many knots at once.

The new stars then light the cloud that made them, which is why star-forming regions are also the brightest emission nebulae. Their radiation and winds eventually blow the remaining gas away, and the cluster emerges from its cocoon. The pillars photographed by the Hubble Space Telescope in the Eagle Nebula are columns of denser gas being eroded from outside by exactly this process, with stars forming inside them as they go.

Stars of moderate size, like the Sun, end by shedding their outer layers in a slow wind while the hot core is exposed. The core's ultraviolet light makes the expanding shell glow, producing a planetary nebula, a name that is a historical accident: through eighteenth-century telescopes the round shells resembled planets. They last only a few tens of thousands of years before dispersing. The Ring Nebula in Lyra and the Helix Nebula are the best known.

Massive stars end differently, in a supernova that blasts most of the star into space at thousands of kilometres a second. The debris is a supernova remnant, a nebula that keeps expanding and heating the gas it ploughs through for thousands of years. The Crab Nebula is the remains of a star seen to explode by Chinese astronomers in 1054, and at its heart is the spinning neutron star that the explosion left behind. The main kinds, by what makes them visible:

• •Emission: gas made to glow by ultraviolet light from hot stars inside it
• •Reflection: dust scattering starlight, usually blue
• •Dark: a cloud dense enough to block the light behind it
• •Planetary: the shed outer layers of a dying Sun-like star
• •Supernova remnant: the expanding wreckage of an exploded massive star

Nebulae are where the chemistry of the universe is recycled. The heavy elements forged inside stars, the carbon, oxygen, iron and everything else, are returned to space in planetary nebulae and supernova remnants, mixed into the interstellar gas, and gathered up again in the next generation of molecular clouds. The Sun and the Earth formed from a cloud that had been enriched this way several times over. Every atom in your body heavier than helium spent time in a nebula between the star that made it and you.

Nobody can visit a nebula, so almost everything known about one comes from splitting its light into a spectrum. In 1864 William Huggins pointed a spectroscope at a nebula and saw not the smooth rainbow of a star but a few bright lines, the signature of glowing gas. That settled that some fuzzy patches were clouds rather than distant crowds of stars. Each element emits its own pattern of lines, so the spectrum works like a list of ingredients, and the relative brightness of the lines reveals the temperature and density of the gas.

One green line puzzled astronomers for decades, because no known element on Earth produced it, and some proposed a new element called nebulium. In 1927 Ira Bowen showed it came from oxygen that had lost two electrons, emitting light in a way that is almost impossible in dense laboratory gas but routine in a near vacuum. The same spectral lines are shifted slightly by motion, which lets astronomers measure how fast a shell is expanding. Infrared telescopes, including the James Webb Space Telescope, add a further view by seeing through dust to the young stars hidden inside.

If you look at the Orion Nebula through a small telescope, you will see a soft grey glow rather than the pinks and blues of famous photographs. The reason lies in your eyes. In dim light the retina relies on rod cells, which are very sensitive but cannot distinguish colour, so faint objects appear colourless. A camera has no such limit. It can collect light for minutes or hours in a single exposure, or stack many exposures, building up detail and colour that no eye can gather in an instant.

Many professional images go a step further and use mapped colour. Filters that pass only the light of one element, such as hydrogen, oxygen or sulphur, record separate images, and each is then assigned a colour so that the different gases can be told apart. The widely reproduced Hubble palette shows sulphur as red, hydrogen as green and oxygen as blue, even though hydrogen glows red to a camera without filters. These are not fakes; they are a way of showing where each element is, much like a map that colours countries differently.

In the 1770s and 1780s the French astronomer Charles Messier, a comet hunter, kept a list of fuzzy objects that did not move across the sky, so that he and others would not mistake them for comets. His catalogue still gives many famous objects their names: the Crab Nebula is M1 and the Orion Nebula is M42. But the list mixed true clouds of gas with star clusters and with what we now know are other galaxies, because through his small telescopes they all looked like faint smudges. In 1845 the Earl of Rosse's giant telescope in Ireland showed that some of them, such as M51, had a spiral shape.

Whether these spiral nebulae were clouds inside the Milky Way or separate star systems far beyond it was argued for decades, and was debated in public by two American astronomers in 1920. The answer came in 1923 and 1924, when Edwin Hubble used the large telescope on Mount Wilson in California to find Cepheid variables, stars whose pattern of brightening reveals their true brightness and so their distance, in the Andromeda nebula. They were far too distant to lie inside our galaxy. The spirals were galaxies in their own right, and the word nebula was gradually left to the genuine clouds.

A nebula is a cloud of interstellar gas and dust, made visible by glowing, reflecting or blocking light. Dense cold ones collapse into new stars, and dying stars throw off new ones, so nebulae are both the cradle and the grave of stars and the reason later stars, and their planets, contain heavier elements.