047.3 Evolution onto the Main Sequence

Consider a cloud fragment similar to the solar nebula that formed our own Sun.

After about 100,000 years of collapse, the growing heat and pressure at the center of the cloud slow the collapse and the surface becomes hotter and brighter.

As the star collapses further, its surface area shrinks and it becomes less bright even though it is still getting hotter. This phase is called the Tau Tauri phase. It is unstable and gives rise to powerful protostellar winds of ejected gas.

After about ten million years, the central temperature rises enough to ignite nuclear fusion reactions. Over the following 30 million years, the star expands somewhat, then shrinks again and settles down for several billion years of life as a normal, main sequence star.