Nebulae | Protostar | Accretion Disk | Stellar Nuclearsynthesis | Main Sequence

The time it takes for a nebula to collapse into a star and ignite (start the nuclear burning of its hydrogen into helium) depends on the size of the star. For a star the size of the Sun, it takes about ten billion years but for bigger stars, it is a shorter time - perhaps as low as a hundred thousand years for the largest. Our Sun is approximately in the middle of its life as it was born 4.6 billion years ago and is expected to burn for another 5 billion.

This article looks at stellar nurseries and the formation of a "protostar" before examining two scenarios: If the protostar has sufficient mass there will eventually be enough energy and heat at the centre for nuclear fusion to begin and for the star to ignite. However, if it is of small mass (say, less than 8% of the Sun’s) it becomes a failed star or “brown dwarf” - a large, not-very-luminous celestial body.