Why only very larege stars can form black holes?

Answer 1

Although most black holes are believed to be associated with stellar evolution (stellar remnants), strictly speaking there are other theoretical methods that could create a black hole which merit a mention - for instance primordial black holes from quantum fluctuations in the early universe, natural and artificial high energy particle collisions, and possible ongoing accumulation of matter onto lighter stellar remnants.

To understand why only the more massive stars (several solar masses) would create a black hole, consider the forces at work: black holes are created when the outward pressure is insufficient to balance against the inward pull of gravity. If the outward pressure is insufficient, a black hole might form. For most of a star's lifetime, collapse is prevented by thermal pressure from nuclear processes which generate significant amounts of heat. Once a star's fuel is exhausted (and allowing for other mechanisms which throw off some of the mass) this effect can no longer balance against the pull of gravity, and some quantum effects may provide the necessary resistance to further gravitational compression; these are referred to as degeneracy pressures. In the case of a large star which has shrunk and cooled to the white dwarf stage, electron degeneracy pressure holds against further collapse; this pressure is a consequence of the Pauli exclusion principle which prevents electrons from occupying the same states (such as already filled energy levels). However, for masses above roughly 1.4 solar masses (called the Chandrasekhar limit) the white dwarf is too massive to resist further collapse and the remnant may collapse further into a neutron star. In this case, the nuclear protons have captured electrons and become neutrons with further collapse resisted by nucleon degeneracy pressure. The upper limit for a mass of a neutron star is not exactly known (see Tolman-Oppenheimer-Volkoff limit) but above this limit, the degeneracy pressure of neutrons is insufficent to prevent further collapse and an object of even greater density may form (such as a 'quark star'). In the case where the mass is sufficient to overcome all forces resisting further compression, gravity will dominate and a black hole may form, with a singularity of infinite compression or density.

Answer 2

Only the largest stars have enough mass to form black holes. When a star runs out of fuel and dies the core collapses under the force of gravity while the outer layers are shed either gradually (for most stars) or suddenly and violent (for massive stars). There are various forces, however, that can halt the collapse of a dying star's core. In low to medium mass stars collapse is halted by a force called electron degeneracy pressure and the core becomes an extremely dense remnant called a white dwarf, which consists of atomic nuclei in a sea of electrons from crushed electron shells. More massive stars overcome this barrier and fuse electrons and protons into neutrons but stop collapsing at that point to form neutron stars. The neutrons hold back further collapse. Only in the most massive stars can gravity overcome all opposing forces to form a black hole.

Answer 3

What is relevant here is what remains from the star at the end of its life - for example, a lot of matter can blow away in the case of a supernova explosion.If such a remainder is not massive enough, then gravity will not be strong enough to overcome the so-called degenerate pressure which is present in a white dwarf, or in a neutron star.

Answer 4

Because of gravity. Black holes are basically an unavoidable result of a Universe dominated by gravity.

Answer 5

According to leading theories, black holes may have any size, ranging from the subatomic to black holes many times the size of our Sun.

Answer 6

small stars do not have enough gravity to form black holes when they "die"

Answer 7

In simple terms, only the most massive stars have strong enough gravity to form black holes.