There are two things that affect the escape velocity – the mass of object and the distance to the center of that object. For example, a rocket must accelerate to 11.2 km/s in order to escape Earth's gravity. If, instead, that rocket was on a planet with the same mass as Earth but half the diameter, the escape velocity would be 15.8 km/s. Even though the mass is the same, the escape velocity is greater, because the object is smaller (and more dense). What if we made the size of the object even smaller? If we squished the Earth's mass into a sphere with a radius of 9 mm, the escape velocity would be the speed of light. Just a wee-bit smaller, and the escape velocity is greater than the speed of light. But the speed of light is the cosmic speed limit, so it would be impossible to escape that tiny sphere, if you got close enough. The radius at which a mass has an escape velocity equal to the speed of light is called the schwarzschild radius. Any object that is smaller than its Schwarzschild radius is a black hole – in other words, anything with an escape velocity greater than the speed of light is a black hole. For something the mass of our sun would need to be squeezed into a volume with a radius of about 3 km.