How Do Stars Form?

Star formation begins in giant molecular clouds, regions of gas and dust that are colder and denser than the space around them. Cold matters, because warm gas has enough internal pressure to resist collapsing.

Something usually has to trigger the collapse. A nearby supernova, a collision between clouds, or passage through a spiral arm can compress part of the cloud enough for gravity to take over locally. Once that happens, the region begins to fall inwards on itself.

As material falls inwards it speeds up, and collisions convert that motion into heat. The centre of the collapsing region becomes a protostar, glowing from the energy of collapse rather than from any nuclear reaction.

Because the original cloud had a slight rotation, the collapsing material spins faster as it shrinks, in the same way a skater speeds up by pulling in their arms. This flattens the surroundings into a disc, which is where planets later form.

The protostar keeps contracting and heating until the core reaches roughly ten million degrees. At that point hydrogen nuclei begin fusing into helium, releasing enough energy to push outwards and halt the collapse.

This is the moment a star is born. The balance between inward gravity and outward pressure from fusion is called hydrostatic equilibrium, and a star spends most of its life in this stable state. Our Sun has been doing this for about 4.6 billion years.

Almost everything about a star's life follows from how much mass it gathered:

• •Below about 8 percent of the Sun's mass, sustained hydrogen fusion never starts and the object becomes a brown dwarf
• •Low mass stars burn slowly and can last for hundreds of billions of years
• •A star like the Sun lasts around ten billion years and ends as a white dwarf
• •Massive stars burn through their fuel in a few million years and explode as supernovae

Star formation is happening today in regions like the Orion Nebula, close enough to study in detail. Infrared telescopes can see through the dust to the protostars inside, which visible light cannot penetrate.

These regions also show the process is messy. Young stars blast out jets and winds that disrupt the surrounding cloud, and a single collapsing region usually forms a whole cluster of stars rather than a lone one.

Stars almost never form alone. A collapsing giant molecular cloud fragments into many separate cores, each producing its own star, so a single event typically creates a cluster of hundreds or thousands.

Our own Sun almost certainly formed this way and its siblings have long since dispersed around the galaxy. Evidence for this comes from short-lived radioactive isotopes found in ancient meteorites, which imply a nearby supernova enriched the cloud shortly before or during the solar system's formation. That points to the Sun having formed in a fairly crowded neighbourhood.

In the nineteenth century, nobody knew how the Sun produced its energy. If it were a lump of burning coal, chemical reactions could keep it going for only a few thousand years. Physicists including Lord Kelvin proposed a better idea: the Sun was slowly shrinking, turning gravitational energy into heat, much as a protostar does. That gave an age of tens of millions of years. Geologists and biologists, including Charles Darwin, were unhappy, because rock layers and evolution seemed to need far longer. The physics and the fossils simply did not agree.

The answer came from the new physics of the atom. In 1920 Arthur Eddington suggested that stars might be powered by turning hydrogen into helium, since a helium nucleus has slightly less mass than the four hydrogen nuclei that form it, and that missing mass is released as energy. In 1939 Hans Bethe worked out the detailed nuclear reactions, work that later earned him a Nobel Prize. Fusion could keep the Sun shining for billions of years, finally matching the age of the Earth that geologists and radioactive dating were pointing to.

A collapsing cloud faces a problem that gravity alone cannot solve. Even a slow rotation becomes extremely fast as material shrinks, and if a young star kept all the spin of the gas that formed it, it would rotate so quickly that it could not hold itself together. Something must carry the spin away. Our own solar system shows how thoroughly this happened. The Sun holds about 99.8 percent of the system's mass but only a tiny fraction of its angular momentum, the measure of rotational motion. Most of that is carried by the orbits of the planets, especially Jupiter.

Astronomers think several processes share the work. Within the disc, turbulence and magnetic forces let inner material pass its spin outwards, so gas can drift inwards onto the star while the outer disc spreads. Magnetic fields linking a young star to its disc act as a brake on the star itself. Narrow jets fired from the poles remove spin as they leave. Where these jets strike surrounding gas they create glowing knots called Herbig-Haro objects, and radio telescopes such as ALMA have imaged discs around young stars like HL Tauri, revealing rings and gaps that may mark forming planets.

Today's star-forming clouds contain dust and molecules such as carbon monoxide, which radiate heat away as infrared light. That cooling keeps clouds cold, so gravity can break them into many small clumps. The first stars, which formed a few hundred million years after the big bang, had no such help. The universe then contained almost nothing but hydrogen and helium, because heavier elements are made inside stars. Gas could cool mainly through small amounts of molecular hydrogen, which is far less effective, so collapsing clumps stayed warmer. Warmer gas resists collapse more strongly, and calculations suggest the fragments were therefore much more massive.

Many models suggest these first stars, called Population III stars, were often tens or even hundreds of times the mass of the Sun. Massive stars live fast, so they would have burnt out within a few million years, scattering the first heavier elements into space as they exploded. No Population III star has yet been confirmed by observation, so the details remain uncertain. Astronomers search in two ways: by looking for the faint light of very distant early galaxies with telescopes such as the James Webb Space Telescope, and by studying ancient stars in our galaxy with extremely little iron, which may preserve chemical traces of the first supernovae.

Stars form when part of a cold gas cloud collapses under gravity, heats up as it falls inward, and finally ignites hydrogen fusion in its core, with the star's entire future determined mostly by how much mass it collected.