Stars form when interstellar gas clouds collapse until the core becomes dense and hot enough to ignite nuclear fusion. The young star is now a T Tauri or protostar -- an unsteadily flickering youngster struggling for a balanced state. At this time, planets begin to take shape. In the HRD, these young stars are to find some way above the main sequence. This T Tauri stage lasts a few dozen million years until the star reaches the main sequence.

The main sequence stage is the glory days of a star's life. How long this lasts, depends on the mass of the star: our sun has been on the main sequence for 4500 million years now and is expected to do so for further 5000 million years. Small red stars are likely to stay up to 50000 million years on the main sequence, while bright blue stars have a main-sequence lifetime of only a few million years. This means that only medium and small stars stay long enough on the main sequence to give life on their planets enough time to evolve.

But finally, the end is near. When the star has used up all the hydrogen in its core, nuclear fusion comes to a halt. The core cools down and collapses, and, by that, heats itself up again until the centre is hot and dense enough to allow fusion of helium into carbon. The core temperature now exceeds 100 million kelvins (main sequence stars have core temperatures between 5 and 30 million kelvins). Hydrogen fusion ignites in a shell around the core, and the immense heat inflates the star. It grows up to a few hundred times in size. Now the star is a giant star, one or two orders of magnitudes brighter, but also much redder than before: being far away from the core, the surface is now cooler than it was during main sequence times. The star inflates until it reaches a new stable state. But it will never be as stable as the main sequence stage. Many giants pulsate (e.g. Delta Cephei and Mira stars), and last only some million, if not a few thousand years. Again, the smaller the star, the longer it remains stable. However, the giant stage is far too short and far too unstable to let life evolve on outer planets which now have comfortable temperature (while the inner planets are calcinated or even evaporated).

What happens next, depends on the star's mass. When all the helium is used up, the core collapses again. A final upheaval of nuclear fusion blows off the star's outer hull, creating a planetary nebula. If the star has less than 1.5 solar masses, its core shrinks and becomes a white dwarf. A white dwarf is a dead star without nuclear fusion going on in the inside, eradiating only its remaining heat until it finally glows out and becomes a dark, compact object, a black dwarf. At a mass of more than 1.5 solar masses, the white dwarf undergoes a further collapse. This ignites further nuclear reactions. Carbon turns to oxygen, neon and up the periodic table until silicon. These reactions run faster and faster until, in a final flash of glory, silicon gets fused to iron. This releases so much energy that the star blows up: a supernova. The remaining core is pressed furtherly together until the electrons crash into the nuclei and the nuclei merge together. This is a neutron star, effectively a giant atomic nucleus of stellar mass and 20 kilometers diameter! However, the most massive neutron stars collapse furthermore until the escape velocity at the surface exceeds the speed of light. This means that nothing can escape from this kind of object (except, perhaps, an FTL starship, though this is very likely to be destroyed by tidal stretch long before it reaches the `event horizon'). The star is now a black hole.

(However, a black hole might irradiate energy and lose mass, though. Stephen Hawking has explained how: in the vicinity of the black hole, the high energy density leads to the frequent formation of virtual particle/antiparticle pairs. Now, one of the two particles might cross the event horizon, while the other evades this trap. Now we have a particle with mass and kinetic energy coming out of nowhere. But this energy must have come from somewhere, of course! According to Hawking, it is drawn from the black hole, which loses the corresponding amount of mass.)