Nebulae and star birth

The space between the stars is not quite empty. Here and there it holds huge clouds of gas and dust. A cloud like this is called a nebula, which is the Latin word for cloud.

Stars are born in these clouds. The gas is very thin, but gravity slowly pulls it together into clumps. As a clump grows, its gravity gets stronger and it pulls in more. In the end it falls in on itself. The middle is squeezed and heats up, and that hot ball is the start of a new star.

You can see one of these places without a telescope. On a dark night in January, find the three stars in a row that make Orion’s belt. Close by, just below them as seen from the northern half of the world, is a faint, fuzzy patch. That is the Orion Nebula, and thousands of young stars are inside it.

Other nebulae mark the end of a star, not the start. When a star dies, it throws its outer layers back into space as a glowing cloud.

In time that gas drifts into new clouds, and new stars are made from it.

A nebula is a cloud of gas, mostly hydrogen and helium, mixed with fine dust. The word covers two very different things: clouds where stars are forming, and clouds thrown out by stars that are dying.

How thin the gas is. The gas between the stars can be as thin as one atom in every ten cubic centimetres. The air you breathe at sea level holds about 25 million trillion molecules in a single cubic centimetre. A nebula is a place where that gas has gathered thickly enough to make a cloud.

Five kinds. Nebulae are sorted by how they are seen.

Kind	Why it can be seen	Example
Emission nebula	Hot young stars make the gas itself glow	The Orion Nebula
Reflection nebula	Dust scatters the light of nearby stars, often bluish	The cloud round the Pleiades
Dark nebula	Dust blocks the light of whatever lies behind	“A hole in the sky”, as William Herschel called such a patch
Planetary nebula	A dying star like the Sun has blown off its outer layers	The Helix Nebula
Supernova remnant	A star has exploded	The Crab Nebula

In the first three, the cloud is lit by stars or hides them. The last two are clouds that stars threw out as they died.

The clouds stars come from. Stars form in cold clouds called molecular clouds. One cloud holds from 1,000 to 10 million times the mass of the Sun and can be hundreds of light years across. Being cold matters. Cold gas clumps, and where a clump grows dense enough its own gravity makes it fall inwards.

From clump to star. The falling gas heats up and becomes a protostar. At first its glow comes only from the heat of the collapse. Over millions of years the middle is squeezed and heated until hydrogen begins to fuse into helium. The energy from fusion holds the star up against its own weight, the shrinking stops, and a star has begun its long, steady life. Gas and dust left over settle into a flat disc round the young star, and planets may form in it.

The Orion Nebula. This is the nearest large place where stars are forming, about 1,350 light years away. It is just bright enough to see by eye, close to Orion’s belt. At its heart are four heavy stars called the Trapezium, each 15 to 30 times the mass of the Sun and only a few hundred thousand years old. Their ultraviolet light makes the whole nebula glow and is hollowing out a space in the cloud. Thousands of other young stars lie in the nebula with them. The Hubble telescope has photographed dark discs of dust round some of them. The Solar System probably formed from a disc like these 4.6 billion years ago.

Stars shape their own birthplace. Light and winds from the heaviest new stars push the gas about, carving hollows and pillars. That squeezing can set off more star birth in one place, and in another strip the gas a smaller star was still gathering. The Pillars of Creation, in the Eagle Nebula, are towers of gas and dust in a cloud where stars are forming now.

Seeing inside. Dust hides new stars from ordinary telescopes. Infrared light gets through dust far better than visible light does, which is one reason the Webb telescope works in infrared. Its pictures show parts of these clouds that could not be seen before.

Clouds from dying stars. A star like the Sun ends by blowing its outer layers away. The bare core left in the middle, a white dwarf, lights up the drifting gas with ultraviolet. The result is called a planetary nebula. The name is an accident of history. In early telescopes some of these clouds looked like the round discs of planets. They have nothing to do with planets. The Helix Nebula is one, about 650 light years away. The Sun is expected to make one of its own.

A heavy star ends in a supernova, which throws its material out at 15,000 to 40,000 km a second. The Crab Nebula is the wreck of one that was seen in the year 1054, bright enough to show in daylight for weeks.

Round again. The gas thrown out by dying stars mixes back into the clouds between the stars, carrying the new elements those stars made. Later stars form from it.

What the 3D view shows. The view of the night sky draws stars, not clouds, so the Orion Nebula itself is not there. What you can find is where it lies: in Orion, among the stars that hang below the three of the belt.