The mass of the star needs to be sufficiently great for the mass to collapse within the region of this sphere. For example, the mass of the Earth is too small to create a black hole – it would need to be packed into a sphere with a radius of just 1 centimetre. Our Sun is also not massive enough: the radius of its event horizon would be just 3 kilometres. But if the mass of the star is 1.4 times greater than the mass of our Sun, the inward pressure of gravity will counter any outward pressure caused by the high momentum of the trapped matter and it will collapse inside its event horizon.
1.4 times
