The smallest star known to potentially form a black hole is thought to be around 25 times the mass of the Sun. This mass limit is known as the Tolman-Oppenheimer-Volkoff (TOV) limit, beyond which a star is expected to collapse into a black hole rather than a neutron star.
When a massive star exhausts its nuclear fuel, it undergoes a supernova explosion, leaving behind either a neutron star or a black hole, depending on its mass. Stars with masses below the TOV limit are believed to collapse into neutron stars, which are incredibly dense remnants composed mainly of neutrons.
However, stars with masses above the TOV limit are expected to continue collapsing under their own gravity, leading to the formation of a black hole. The precise mass at which a star crosses this limit and collapses into a black hole is still an active area of research and is influenced by various factors such as stellar composition, rotation, and metallicity.
It's important to note that the exact lower limit for black hole formation is not yet definitively known. Further observations and theoretical advancements in astrophysics are necessary to gain a deeper understanding of the processes involved in the formation of black holes.