Question

A protostar forms once the nebular cloud condenses and the core begins _____.

to orbit
to cool
to heat
nuclear fusion

Answer 1

The correct answer is "to heat".

Step-by-step explanation:

During the first stage of a new star formation the nebula collects enough mass of gas and dust, but the core is not already formed. When he nebular cloud condenses and the core begins to heat a protostar is formed. A protostar is the second stage of a new star formation. Once the new star begins to collapse itself enough to go through the nuclear fusion, the new star formation passes to stage three.

Answer 2

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.