How a star is born

From a cold, dark cloud to a spinning core, a disk with jets, planets forming and fusion switching on

Step by step

A cold, dark cloud

What is measured and what is drawn: the temperatures come from the radio light of the gas and the glow of the dust. The cloud here is drawn: its lumps come from noise, not from a map of a real cloud, and the stars behind it are the app's own sky.

Something squeezes it

So whatever squeezes the gas makes stars. A supernova's blast wave, the winds of hot young stars, clouds colliding or the slow crowding of gas passing through a spiral arm can all push parts of a cloud over the line. Short-lived radioactive atoms in the oldest meteorites hint that a nearby supernova may have helped start the Sun's own birth.

What is measured and what is drawn: the Jeans mass is worked out with its formula from measured temperatures and densities. The shock is drawn as a pale sheet; a real one would be invisible to the eye. Magnetic fields and turbulence also hold clouds up, so real cores often need more than the Jeans mass.

The core falls in

While the gas is thin, the heat of the fall escapes as infrared light and the core stays cold. Once the middle is dense enough to trap it, the heat builds up, the pressure rises and a hot core forms: the seed of a protostar. The fall is drawn here in seconds rather than a hundred thousand years.

A protostar and its disk

The spinning gas flattens into a disk around a newborn protostar. Still buried in a cocoon of dust, it can be seen only in infrared and radio light, and it grows by feeding from the disk.

At this stage, which astronomers call Class 0, the protostar is less than about a hundred thousand years old and has gathered only part of its final mass. It shines from the energy of the gas crashing onto it, but its cocoon of dust soaks up that light and glows in infrared. Webb's infrared cameras and radio telescopes like ALMA see through to it.

What is measured and what is drawn: protostars, their disks and their cocoons are seen in infrared and radio images, and their sizes are measured from them. Here the protostar's glow is drawn far bigger than the star, and the dust is drawn thinner than it is, so that you can see inside.

Jets: Herbig–Haro objects

Where a jet rams the slower gas around it, shocks heat the gas and it glows. George Herbig and Guillermo Haro studied such patches around 1950 without knowing yet what made them; hundreds are known now. Webb's infrared image of HH 211 from 2023 shows hydrogen molecules glowing along the jet where it is shocked.

What is measured and what is drawn: the speeds come from Doppler shifts and from knots that move between images taken years apart. The jets here are drawn shorter than real ones, which can stretch for light-years, and their knots move outwards far faster than they really do.

Shining without fusion

T Tauri stars are named after the first one found, a star in Taurus whose brightness changes. They vary irregularly as clumps of gas from the disk land on them, and they are often brighter than the Sun though cooler, because they are several times bigger. Their winds and their light slowly clear away the gas around them.

Fusion switches on

A star stays on the main sequence, steadily fusing hydrogen, for most of its life: about ten billion years for the Sun. What is drawn: the star is shown far bigger than it would be at this scale, and its brightening, which takes millions of years, is sped up to seconds and changes only smoothly.

Nurseries we can see

The Orion Nebula is the nearest big nursery to us: a glowing cloud lit and blown open by the hot young stars of the Trapezium at its heart. Below Orion's belt it shows to the naked eye as a fuzzy star in the hunter's sword. Hubble found many of its young stars wrapped in disks of their own, the raw material of planets.

What is measured and what is drawn: the pink glow of such nebulae is hydrogen lit by ultraviolet light and the teal comes from oxygen, both measured in their spectra. The nursery here is drawn, not a map of a real one: dense dark columns pointing at the hot stars that light them, as the Pillars do.