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View submission: Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science
How small are you talking? Stellar-mass black holes (the smallest BHs that we observe) are formed when massive stars go supernova. We haven't observed any BHs smaller than a few solar masses. For stellar-mass black holes formed by the collapse of large stars, the original stars would be more massive than the BH, but not extremely so - a 20 solar mass star will produce a black hole with a mass of about 5 solar masses. We know that big stars like that exist - we see them all the time.
Theoretically, very low-mass black holes *could* form in the very early Universe (as in, before stars formed!) just by sheer chance - if a region of the Universe happened to get dense enough, it could collapse into a black hole. Unlikely, but possible.
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