What is the primary factor that determines whether a protostar will form a star like the Sun, a brown dwarf, or a massive star?

The primary factor is the mass of the collapsing region in the nebula. That mass determines how much material gathers into the forming object and whether its core can become hot enough for sustained hydrogen fusion: lower-mass objects may become brown dwarfs, while sufficiently massive ones become stars, including Sun-like or massive stars. The amount of material in a surrounding disk can affect planet formation, but it is not the main factor that sets the protostar’s category.

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