Describe the four distinct stages in the life track of a solar-mass protostar on

1. The protostar forms from a collapsing dusty molecular cloud. The temperature and luminosity both rise so the cloud moves from the far lower right corner up and to the left, ending at about 3000 K and 10-100 solar luminosities.
2. At 3000 K, the H- ion traps photons so energy cannot escape by radiation. Rather, energy is transported from the interior by convection. As the star collapses, it becomes smaller and therefore less luminous, but its temperature stays the same. In the  H-R diagram, it therefore moves vertically downwards.
3. Eventually, the energy from contraction and the beginnings of nuclear fusion becomes so large that it is release by radiation again. Both the luminosity and surface temperature increase so the star moves to the left and upwards in the H-R diagram.
4. The fusion rate increases to match the energy loss and the star has reached a point of stability. It is on the main sequence and will remain there for billions of years.