EVENT HORIZON AND ACCRETION DISK

A black hole’s mass is concentrated at a single point deep in its heart, and clearly cannot be seen. The “hole” that can, in principle, be seen (although no-one has ever actually seen a black hole directly) is the region of space around the singularity where gravity is so strong that nothing, not even light, the fastest thing in the universe, can escape, and where the time dilation becomes almost infinite.

A black hole is therefore bounded by a well-defined surface or edge known as the “event horizon”, within which nothing can be seen and nothing can escape, because the necessary escape velocity would equal or exceed the speed of light (a physical impossibility). The event horizon acts like a kind of one-way membrane, similar to the "point-of-no-return" a boat experiences when approaching a whirlpool and reaching the point where it is no longer possible to navigate against the flow. Or, to look at it in a different way, within the event horizon, space itself is falling into the black hole at a notional speed greater than the speed of light.

(Click for a larger version) Event horizon, accretion disk and gamma ray jets of a black hole (Source: Internet Encyclopedia of Science: http://www.daviddarling.info/ encyclopedia/E/event_horizon.html - Credit & ©: Astronomy / Roen Kelly)

The event horizon of a black hole from an exploding star with a mass of several times that of our own Sun, would be perhaps a few kilometres across. However, it could then grow over time as it swallowed dust, planets, stars, even other black holes. The black hole at the centre of the Milky Way, for example, is estimated to have a mass equal to about 2,500,000 suns and have an event horizon many millions of kilometres across.

Material, such as gas, dust and other stellar debris that has come close to a black hole but not quite fallen into it, forms a flattened band of spinning matter around the event horizon called the accretion disk (or disc). Although no-one has ever actually seen a black hole or even its event horizon, this accretion disk can be seen, because the spinning particles are accelerated to tremendous speeds by the huge gravity of the black hole, releasing heat and powerful x-rays and gamma rays out into the universe as they smash into each other.

These accretion disks are also known as quasars (quasi-stellar radio sources). Quasars are the oldest known bodies in the universe and the furthest objects we can actually see, as well as being the brightest and most massive, outshining trillions of stars. A quasar is, then, a bright halo of matter surrounding, and being drawn into, a rotating black hole, effectively feeding it with matter. A quasar dims into a normal black hole when there is no matter around it left to eat.

Moments after the creation of a black hole, the heat and the hugely amplified magnetic field of the collapsing star combine to focus a pair of tight beams or jets of radiation, perpendicular to the spinning plane of the accretion disk. These beams focus vast amounts of particles and energy (of the order of a billion billion times the energy output of our Sun) away from the black hole at close to the speed of light. The shock waves of this massively enegetic beam cause gamma rays to be emitted in a phenomenon known as a "gamma ray burst" or "hypernova" event (so named because its energy and brightness dwarfs even that of a supernova). Bursts can last from mere milliseconds to nearly an hour (a typical burst lasts a few seconds), usually followed by a longer-lived “afterglow” emitting at longer wavelengths (x-ray, ultraviolet, visible, infrared and radio), and they are the brightest electromagnetic events occurring in the universe.

Interestingly, it appears to be easier for stars with fewer heavy elements to turn hypernova and generate gamma ray bursts. Therefore, for the reasons explained in the section on Stars, Supernovas and Neutron Stars, stars born earlier in the life of the universe are more likely to produce gamma ray bursts, and the phenomenon is consequently rarer today than it was. It should be remembered that any supernovas or hypernovas we observe today in galaxies, say, 9 billion light years away, actually occurred 9 billion years ago.

A non-rotating black hole would be precisely spherical. However, a rotating black hole (created from the collapse of a rotating star) bulges out at its equator due to centripetal force. A rotating black hole is also surrounded by a region of space-time in which it is impossible to stand still, called the ergosphere. This is due to a process known as frame-dragging, whereby any rotating mass will tend to slightly "drag" along the space-time immediately surrounding it. In fact, space-time in the ergosphere is technically dragged around faster than the speed of light (relative, that is, to other regions of space-time surrounding it). It may be possible for objects in the ergosphere to escape from orbit around the black hole but, once within the ergosphere, they cannot remain stationary.

Also due to the extreme gravity around a black hole, an object in its gravitational field experiences a slowing down of time, known as gravitational time dilation, relative to observers outside the field. From the viewpoint of a distant observer an object falling into a black hole appears to slow down and fade, approaching but never quite reaching the event horizon. Finally, at a point just before it reaches the event horizon, it becomes so dim that it can no longer be seen (all due to the time dilation effect).