The star formation process is efficient in the sense that it produces stars with masses from a few times the mass of Jupiter up to a few hundred solar masses.
In a stellar cluster there are many low-mass stars and fewer higher-mass stars. If a giant star (like the Sun) meets a small-mass star (like an M-dwarf or a brown dwarf) then they may end up as a binary system, moving along together and rotating around each other, provided that they have a close interaction, meaning that they come within a few tens/hundreds of astronomical units from each other.
Even for high-density stellar clusters, those that contain a large number of stars per unit volume, the probability of having such close interactions is relatively small. Collisions between stars in clusters are even more rare (for a globular cluster this is expected to happen once every 10,000 years). If this unlikely event happens, for example in the centre of a cluster, then the smaller star will in effect be swallowed by the larger star with some energy being released during the impact.
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