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If the mass remained constant but the diameter of the Earth then reduced under the effects of the immense gravity, the region, whose diameter was the original diameter of the Earth and from which nothing could escape, would remain exactly the same size. The surface of the Earth would then become what is termed the event horizon of the black hole. Suppose that the density of the Earth was vastly increased but it was able to retain its present size so the escape velocity just reached the speed of light at the surface and so became a black hole. The object would then be what is termed a black hole. If one naively used this formula into realms where relativistic formula would be needed, one could predict the mass and/or size of an object where the escape velocity would exceed the speed of light and thus nothing, not even light, could escape. (0 is the escape velocity, M the mass of the object, r0 its radius and G the universal constant of gravitation.) If either the density of the Earth was greater (so its mass increases) or its radius smaller (or both) then the escape velocity would increase as Newton's formula for escape velocity shows: If one projected a ball vertically from the equator of the Earth with increasing speed, there comes a point, when the speed reaches 11.2 km/sec, when the ball would not fall back to Earth but escape the Earth's gravitational pull. So, if you are reassured, then perhaps we can consider…. Even if a star were moving towards a massive black hole, it is far more likely to swing past – just like the fact very few comets hit the Sun but fly past to return again. If our Sun were a black hole, we would continue to orbit just as we do now – we just would not have any heat or light.

An astronaut who ventured too close and was sucked into a black hole would be pulled apart by the overpowering gravity.Gresham Lecture, Wednesday 27 October 2010īlack Holes – do not deserve their bad press!īlack holes seem to have a reputation for travelling through the galaxy “hovering up” stars and planets that stray into their path. Objects that fall into black holes are literally stretched to breaking point. Quasars may be hundreds of times brighter than even the largest ordinary galaxies. Supermassive black holes also power active galaxies and ancient galaxies known as quasars. These may be millions or billions of times heavier than our Sun. Most galaxies, including the Milky Way, have supermassive black holes at their centres. What is left of the star – still several solar masses - collapses into an area only a few kilometres across. These 'stellar-mass' black holes form when a heavyweight star, about 10 times heavier than the Sun, ends its life in a supernova explosion. Many of them are only a few times more massive than the Sun. As the discs swirl around them like a whirlpool, they become extremely hot and give off X-rays.īlack holes come in many different sizes. Many of them are surrounded by discs of material. The gravitational pull of this region is so great that nothing can escape – not even light.Īlthough black holes cannot be seen, we know they exist from the way they affect nearby dust, stars and galaxies. This catastrophic collapse results in a huge amount of mass being concentrated in an incredibly small area. Instead, it is a region of space where matter has collapsed in on itself. A black hole does not have a surface, like a planet or star. Black holes are the strangest objects in the Universe.