So the cloud starts to collapse under its own weight. This compresses the hydrogen. When you compress a gas, it gets hotter (think of a bicycle pump). But a hotter gas has a greater tendency to expand - that is how explosions occur. So as gravity collapses the cloud, the pressure builds until, eventually, the collapse halts. But the halting is short-lived, because all hot objects radiate energy, so the ball of gas cools, at which point gravity takes over again and the ball collapses some more.

What stops this endless collapse in its tracks is that when the core reaches a temperature of about 15  million kelvins, nuclear fusion is suddenly switched on. This produces enough energy to replenish that being lost by radiation. So the ball of gas no longer cools, and it is able to hold gravity in check. A star is born.

This phase of the star's life lasts, in the case of the Sun, for about 10 billion years. When the core hydrogen is finally exhausted, having all been turned to helium, the surface starts to cool, the pressure can no longer counteract gravity and the collapse begins again. Two things now happen:

• the mantle hydrogen gets into the core and starts fusing  to helium
• the core helium is heated to even higher temperatures and starts a new fusion process, producing carbon, oxygen etc.

Both of these processes generate prodigious quantities of energy, which heats up the outer layers of the star so much that the collapse is actually reversed, and the star has to expand again until the outer layers have cooled off enough for a new gravity/pressure equilibrium to be set up. This enormous, but cooler-on-the-surface star is called a red giant : red-hot objects are cooler than white-hot objects. In the case of the Sun, the star will be so large that the Earth will be swallowed up.

Once the helium has all been used up, gravitational collapse can begin again and a variety of outcomes is possible, depending on how big the original hydrogen cloud was:

For a smallish star, like the Sun, the collapse continues until the atoms (of carbon, oxygen, etc, produced in the red giant phase) are all nestling cheek-by-jowl. The star will now be a white-hot solid rather than a gas and, because of the collapse, it will be relatively small - about the size of a planet. It is known as a white dwarf, and its fate is to cool more and more, becoming dimmer and dimmer until it fades out of sight completely.

Stars with more than 1.4 times the mass of the Sun behave differently. Their atoms are squeezed so strongly by gravity that the electrons collapse into the nuclei, where they combine with the protons to form neutrons. The bunch of neutrons is much much smaller than the original collection of atoms, so this change is accompanied by a sudden catastrophic reduction in volume of the star, with all its material falling in to the centre. Gravitational potential energy is converted into kinetic energy very quickly, and the star suddenly undergoes one final heating-up phase, so violent that the outer layers of the star are blown away in a supernova, which flares briefly giving out as much light per second as the whole of the rest of the galaxy it is in. The remnant left behind is composed entirely of neutrons, is very small (about 10 km across), and is often spinning rapidly, emitting pulses of radio waves like a lighthouse. This is a neutron star or, if we are seeing pulses from it, a pulsar. Quite often a red giant will be accreting matter onto itself from a neighbouring star, and will quite suddenly accumulate sufficient mass to reach the 1.4 solar mass limit. So there will be quite a number of supernovae arising from more or less identical stars, and so all looking the same as each other. Such supernovae are called Type 1a, and form the basis of measuring galactic distances (by comparing their apparent brightness with their actual brightness).

Bigger stars still, with masses exceeding 8 solar masses, experience such severe gravitational crushing at their centres that even the neutrons cannot withstand the pressure. Under these circumstances the neutrons collapse into a black hole. No one knows whether the collapse might or might not ultimately stop inside a black hole, because light is unable to escape from a black hole (that's why they are black), so telescopes cannot see what is going on inside.

A first-generation star is made just of hydrogen*, but the supernova at the end of its life scatters into the surrounding space carbon, oxygen, silicon, iron, and all the other elements made by fusion in the nuclear furnace of the supernova explosion. So when a second generation star forms, the cloud of hydrogen that starts it all off has mixed in with it all this debris from the first-generation star. It is this space dust that gets left behind in the outer regions of the collapsing star and forms the planets.

*This isn't quite true. Right at the beginning, Hydrogen had started to turn to Helium in the primordial fireball. But the initial 'big bang' was so violent that the gas quickly expanded to such an extent that the temperature cooled to below the temperature needed for the process to continue. So, by the time the state at the top of this panel was reached, the gas out of which future stars would be made consisted of 75% Hydrogen, 25% Helium and a certain amount of Lithium.

Huge ball of cold rarefied gas

Unchecked gravitational collapse. Temperature kept constant because photons absorb energy from the molecules and radiate it into space

Moderate ball of warm gas

At any given moment, the inwards pressure (caused by gravity) is balanced by an outward pressure (caused by heat)

Each time some heat is lost from the surface by radiation, the 'heat' pressure goes down, the 'gravity' pressure wins, the sphere collapses a little and the temperature rises until equilibrium is re-established

Smallish ball of hot gas

(a Main Sequence star)

The heat radiated away from the surface is replenished by energy from the nuclear reactions: gravity cannot gain the upper hand, and the star is stable.

When the Hydrogen has all been turned to Helium, gravitational collapse continues until the centre is hot enough to ignite Helium 'burning'.

Large ball of hot-ish gas with a very hot core (a Red Giant)

The Helium burning produces enough heat to heat up the middle layers, causing them to expand, and the outer layers with them. The outer layers cool as they expand, making the star less white and more red.

When the Helium runs out, gravitational collapse resumes until the atoms are all packed close together (a solid). And then . . . . .

Small stars (e.g. the Sun)

Big stars (> 8 suns)

The final heating blows off the outer layers as one or more planetary nebulae, while the small white-hot core (a white dwarf) gradually fades as it cools.

If the collapsed star is above a critical limit, the atoms themselves collapse and there is a supernova (often of Type 1a), leaving behind a neutron star

If the collapsed star is above a (larger) critical limit, the atoms themselves collapse and there is an even bigger supernova, leaving behind a neutron star which, in turn collapses to an invisible black hole

The final explosion fuses medium-sized nuclei into heavy elements, which then get scattered throughout the surrounding interstellar medium. They eventually get swept up into new 'second generation' stars, like the sun, and find their resting place in planets.