047.3 Evolution onto the Main Sequence

Each stage in the evolution of a protostar has a brightness and a surface temperature, so it can be represented by a point on the HR diagram. Here, the sequence of points for a one solar mass protostar is shown as it evolves toward the main sequence.

The size of the original collapsing cloud fragment determines the track of the protostar and where on the main sequence it stops.

Once a star achieves stable nuclear burning, it sits at the same point on the main sequence for a very long time. In the case of a one-solar mass star, for billions of years.

A star at one point along the sequence never evolves into a star at another point. Instead, when the nuclear fuel runs out, the star moves off of the main sequence entirely.