What Is a Nebula? The Clouds Where Stars Are Born and Die
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The word comes from the Latin for mist, and for centuries a nebula was simply any fuzzy patch in the sky that was not a star. Better telescopes sorted the fuzzy patches into very different things. Some turned out to be whole galaxies. The ones that kept the name are clouds of gas and dust inside our own galaxy, and they mark the two ends of a star's life: the places where stars are made, and the debris left when they die.
What the clouds are made of
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.
Glowing, reflecting and dark
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.
Where stars are born
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.
Where stars die
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
Why they matter
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.
The takeaway
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.