Why doesn the protogalactic cloud just clump down into one big star?
It can’t become one big star because there’s a maximum mass that a star can have, which is something around 100 solar masses. The reason for this maximum mass is that a larger mass that collapses just gets so hot (from conversion of gravitational to thermal energy) as it collapses that the pressure from the radiation it generates counteracts the gravitational attraction – it effectively blows itself apart. In order to achieve a stable fusion/gravitational equilibrium (i.e. a star), the collapsing chunk must be small enough that it’s not unstable, but large enough to get hot enough to achieve fusion eventually. There’s also a natural process of fragmentation- stars will tend to form around local centers of randomly higher density.
