A region of a cold molecular cloud becomes gravitationally unstable and collapses. Material falls onto a growing core through a rotating disc, heating as it contracts, until the core reaches around ten million kelvin and hydrogen fusion ignites, halting the collapse.

How long does it take a star to form?

Typically hundreds of thousands to millions of years from the onset of collapse to ignition. More massive stars form faster than less massive ones.

Why do planets form around stars?

Because any collapsing cloud has some rotation, and conservation of angular momentum forces infalling material into a disc, not straight onto the star. Dust in that disc collides and sticks, building planets as a by-product of star formation.

What is the smallest a star can be?

About 8% of the Sun's mass. Below that, the core never becomes hot enough for sustained hydrogen fusion and the object becomes a brown dwarf, not a true star.

How stars are born

Cold gas, gravity, and a threshold of about ten million degrees. I've watched the raw material from a mountain in Taiwan.

4 min read · Published 26 August 2026

First, the answer

Stars form inside cold, dense clouds of mostly hydrogen. A region goes gravitationally unstable, collapses, and heats as it shrinks. Material falls onto a growing core through a spinning disc while jets fire from the poles. When the core hits roughly ten million kelvin, hydrogen fusion starts and the outward pressure stops the collapse. That's a star. The whole process takes hundreds of thousands to millions of years.

The stages

From cloud to star

A molecular cloud. Cold (around 10 to 20 kelvin) and dense enough for hydrogen to exist as molecules. These are the darkest features in the sky, visible as black patches against background stars.
Collapse begins. Something disturbs the cloud (a passing shock wave, a nearby supernova, a spiral arm) and a region exceeds the mass at which gravity overcomes internal pressure.
Fragmentation. The collapsing region breaks into many smaller clumps, so stars form in groups instead of singly.
A protostar with a disc. Conservation of angular momentum flattens infalling material into a rotating disc. The core heats as gravitational energy converts to heat.
Jets and outflows. Protostars fire narrow jets from their poles, clearing away surrounding material and limiting how much mass the star can accumulate.
Ignition. At around ten million kelvin, hydrogen fusion begins. Radiation pressure balances gravity and the collapse stops. The star joins the main sequence.

Why the disc matters

The collapsing cloud spins, however slightly. As it contracts, the spin speeds up. Same effect as a skater pulling in their arms. Material can't fall straight in, so it settles into a disc.

That disc is where planets form. Dust grains collide and stick, building bigger bodies over millions of years. Planet formation isn't a separate event afterwards. It's a by-product of the same collapse.

This is why planets are common. Discs are an unavoidable consequence of the spin any collapsing cloud has. Telescopes have now imaged many young discs directly, some with gaps carved by forming planets.

Mass decides everything about a star. I'd teach that before any of the stages.

Mass decides everything

The single most important thing about a new star is how much mass it gathered, because mass determines its temperature, colour, luminosity and lifespan.

High mass stars burn hot and blue and exhaust their fuel in a few million years. Low mass stars burn cool and red and last for trillions, longer than the current age of the universe, so no red dwarf has ever died.

The relationship is steeply non-linear: luminosity scales roughly as mass to the power of three and a half. A star ten times the Sun's mass is thousands of times brighter and burns out vastly faster.

There are limits at both ends. Below about 8% of the Sun's mass, the core never gets hot enough for sustained hydrogen fusion, and the object becomes a brown dwarf. At the top end, radiation pressure blows away infalling material, capping stellar masses at roughly a hundred to a few hundred solar masses.

Stars form in groups

Because clouds fragment, stars are born in clusters of tens to thousands. The Sun almost certainly formed in one, and its siblings have long since dispersed through the galaxy.

Open clusters like the Pleiades are young groups still travelling together. Over hundreds of millions of years, galactic tides and passing encounters pull them apart.

The most massive stars in a new cluster then shut down further star formation nearby: their intense radiation and winds blow the remaining gas away, ending the process. Star formation is partly self-limiting, the first stars to form prevent many later ones.

Questions people ask

How do stars form?: A region of a cold molecular cloud becomes gravitationally unstable and collapses. Material falls onto a growing core through a rotating disc, heating as it contracts, until the core reaches around ten million kelvin and hydrogen fusion ignites, halting the collapse.
How long does it take a star to form?: Typically hundreds of thousands to millions of years from the onset of collapse to ignition. More massive stars form faster than less massive ones.
Why do planets form around stars?: Because any collapsing cloud has some rotation, and conservation of angular momentum forces infalling material into a disc, not straight onto the star. Dust in that disc collides and sticks, building planets as a by-product of star formation.
What is the smallest a star can be?: About 8% of the Sun's mass. Below that, the core never becomes hot enough for sustained hydrogen fusion and the object becomes a brown dwarf, not a true star.