Aprotostar forms once the nebular cloud condenses and the core begins

to orbit
to cool
to heat
nuclear fusion

A protostar forms once the nebular cloud condenses and the core begins nuclear fusion. When you say nuclear fusions it refers to the action that the power or the energy starts to combine with strong energy reaction that makes the power nuclear to react and create fusion.

The answer is joseph is observing the colour of the reaction mixture to see whether proteins are present in the given solution.

Organism, organ system, organs, tissue, cells