- Education and Science
The Event Horizon and Black Holes in Space
Black Hole Powered Spiral Galaxy
A black hole is a place in space where the gravitational field is so powerful that nothing, not even light, can escape. It's interior is invisible. The Hubble telescope in space took the pictures you find here. What is visible are the movements of the stars, spinning into the vortex that a black hole creates.
To give you an idea: a black hole's escape velocity exceeds the speed of light.
- 299,792 kilometers per second -or-
- 186,282 miles per second
That's how fast you would have to go in the direction of "UP!" in order to escape the gravitational pull of a black hole.
The earth's escape velocity is:
- 11 kilometers per second -or-
- 7 miles per second
One of the most fascinating aspects of a black hole and maybe its defining feature is THE EVENT HORIZON .
It is a boundary of space-time beyond which events can't affect an outside observer.
To understand this better: The presence of mass deforms space/time in such a way that the paths particles take is towards the mass.
At the event horizon of a black hole, this force, this deformation of the space-time continuum, is so great, so strong, that there are no more paths away from the black hole.
So once something is inside the event horizon of a black hole, moving into the hole is as inevitable as moving forward in time, and is, in some respects, the equivalent of doing so. To a far observer, a clock would appear to tick more and more slowly going towards a black hole. This effect, known as gravitational time dilution, causes the object to appear to slow down as it approaches the event horizon, until it is going infinitely slow. (Or, rather, time is going infinitely slow, approaching the event horizon of a black hole, because the object approaching the event horizon is approaching the speed of light. It goes faster and faster as the gravitional pull of the black hole becomes stronger and stronger, and as it approaches the speed of light, time appears to slow until it's stopped.)
We think of time as an absolute. So, imagine a clock, an alarm clock sitting on a dresser. This clock has a loud tick, at the rate of one tick per second. This clock decides to go out and explore space. It leaves the earth's gravity going along pretty quickly--at the rate of 7 miles per second. This is one speedy clock. But wait! The clock is approaching the center of the Milky Way Galaxy where there's a black hole! Uh-oh! The clock keeps going faster, and faster, approaching the speed of light, at 186,282 miles per second. It's approaching the event horizon of the black hole at the center of the galaxy. The ticks of the clock get slower, and slower, and slower...as it approaches the event horizon, but the whole clock itself is moving faster and faster, being sucked in by the black hole. We, observing from the outside, appear to see the clock stop.
That's because time is an artificial measurement created by human beings according to the earth's rotation towards, then away from, the sun. In the beginning, there were no hours, minutes, seconds...there were days, nights...changes of seasons, according to the earth's rotation on its axis and revolution around the sun--the giver of light. We measure time by light. When an object approaches the speed of light, time slows...time even appears to stop, at the event horizon of a black hole. The object appears to stop--but it hasn't. It is moving faster than the speed of light into the enormous field of gravity of the black hole.
Hubble Telescope Image of a Black Hole
The center of a black hole is the most amazing feature of all: THE SINGULARITY .
This is where matter is crushed to an infinite density, the pull of gravity is infinitely strong, and space/time has infinite curvature. It means the black hole's mass has become entirely compressed into a region with zero volume. This zero-volume, infinitely compressed, infinitely dense region at the center of a black hole is called a gravitational singularity .
The mass of a black hole is not infinite. It is infinitely dense, infinitely compressed, but there is a determinate amount of mass.
Black holes found at the center of galaxies have a mass equal to several billion of our suns and are called supermassive black holes. The black hole singularity still has no volume, but it has a hugely (infinitely) dense mass that creates a gravitational field that sucks in surrounding stars. There is a black hole at the center of most galaxies, including ours, and there may be a black hole at the center of all galaxies. That fact has yet to be determined.
Stellar black holes are formed when a large star (20 or 30 times as large as our sun) undergoes gravitational collapse. This happens when the internal pressure of the star's core is insufficient to withstand the star's own gravity. The star goes out in a supernova blaze of glory, and what's left is a remnant, a compact star. The supernova state has burned off quite a bit of the mass, and if the remnant of the resulting compact star has a mass of three or four times that of our sun, not even the degeneracy pressure of neurons is sufficient to alter the continuous collapse of the matter of the star into becoming a black hole.
X-Ray Shine From Materials Falling Into a Black Hole
Dust Disk around a black hole at Galaxy NGC 4261
Black holes can grow. Once a black hole is formed, it can continue to grow, absorbing light, radiation, interstellar dust. The supermassive black holes at the centers of most (or all) galaxies were formed from the accumulation of many galactic objects--stars, moons, planets, comets; anything that came within the growing range of the orginal collapsed star's EVENT HORIZON.
Black holes can shrink, at least according to Stephen Hawking. He showed that black holes aren't entirely black. They emit very small amounts of thermal radiation. If Dr. Hawking's theory of black hole radiation is correct, then black holes will lose mass over time, in proportion to the energy released by the radiation. (e=mc(sqared)).
If Dr. Hawking's theory, which is now accepted science, is correct, than a black hole the weight of an automobile (but remember, with NO VOLUME!) would only take a nanosecond to evaporate, during which time it would be 200 times brighter than our sun.
The large accretion disks and gas jets are evidence of the supermassive black holes at the center of the galaxies within our range. The only mass large enough to power these enormous phenomena is a black hole.